How Much Battery Storage Does a Solar PV System Need?

Blowout Week 70 featured Tesla’s new 7 kWh and 10 kWh lithium-ion battery storage units. Will they allow households with rooftop solar PV systems to store enough surplus solar power to fill domestic demand throughout the year without the need to import grid power when the sun isn’t shining?  It all depends on how much storage is needed and how much it costs, and in this post I present ball-park estimates of storage requirements and costs for domestic rooftop solar installations calculated using the following simplifying assumptions:

  1. Household consumption is 5,000 kWh/year. (About right for Western Europe, low for the US, high for most of the rest of the world.)
  2. Household demand is constant through the year at 13.7 kWh/day, or 0.57 kWh/hour.
  3. The impacts of changes in cloud cover are ignored.
  4. The storage requirements generated by diurnal fluctuations in solar output are insignificant relative to the storage requirements generated by seasonal fluctuations.
  5. The storage batteries are 100% efficient, with no conversion losses and no charge/discharge restrictions.
  6. Solar panels are inclined south at the optimum angle for maximum annual generation.

Four rooftop solar cases are considered: at the Equator, at latitude 20 north, at latitude 40 north and at latitude 60 north. To generate 5,000 kWh in a year we need 3.8 kW of installed PV capacity on the Equator (load factor 15%), 3.4 kW at latitude 20N (load factor 17%), 3.6 kW at latitude 40N (load factor 16%) and 5.7 kW at latitude 60N (load factor 10%). The load factors are from the Estimating global solar PV load factors post.

Figure 1 shows annual solar output in kWh/square meter/day for the four cases using five-day averages (data from PVeducation). It plots module output from fixed solar panels inclined south at the angle that generates maximum annual generation (which is the same as latitude only on the Equator. Optimum panel inclinations are 18 degrees relative to the horizontal at latitude 20N, 35 degrees at 40N and 49 degrees at 60N.) The most notable feature is the seasonal variation in solar output, which ranges from 13% at the Equator and 45% at latitude 20N to a factor of 2.6 at 40N and a factor of 36 at 60N:

Figure 1: Annual solar output by latitude for the four cases considered

Discussing these cases in sequence:

Rooftop solar system on the Equator:

Figure 2 plots annual solar generation against household demand for a rooftop solar PV system on the Equator. The data on this and following Figures are adjusted so that the system generates 5,000 kWh per year (68.5 kWh every five days). The green-shaded areas show periods where the rooftop system generates more power than the household needs and the pink-shaded areas show where it generates less. The amount of battery storage needed to store the surplus power for re-use during deficit periods is given by the total number of kWh in the green (or pink) shaded area:

Figure 2: Annual solar generation versus demand for a solar system on the Equator

A solar PV system on the Equator does not need large amounts of storage because solar generation doesn’t change much through the year and because the solar cycle is only six months rather than a year long. The total storage requirement is 48 kWh, equivalent to 3.5 days of average generation,  and this can be filled with five Tesla 10kWh wall units costing 5 X $3,500 = $17,500, exclusive of installation. This, however, still more than doubles the cost of the system. (Installation costs for the solar panels would be about $15,000 assuming $4,000 per installed kilowatt and 3.8 kW installed).

Rooftop solar system at latitude 20 north

Figure 3 plots the data for this latitude. Even though we are still in the tropics the storage requirement increases significantly. To fill winter demand the household now has to store 285 kWh of surplus summer generation, requiring 29 Tesla 10kWh wall units costing $101,500.

Figure 3: Annual solar generation versus demand for a solar system at latitude 20N

Rooftop solar system at latitude 40 north

Figure 4 plots the data for this latitude. To fill winter demand the batteries now have to store 676 kWh of surplus summer generation, requiring 68 Tesla 10kWh wall units costing $238,000.

Figure 4: Annual solar generation versus demand for a solar system at latitude 40N

Rooftop solar system at latitude 60 north

Figure 5 plots the data. To meet winter demand at this latitude the batteries have to store 1,522 kWh of surplus summer generation, requiring 153 Tesla 10kWh wall units costing $535,500 and weighing 15.3 tons:

Figure 5: Annual solar generation versus demand for a solar system at latitude 60N

Even with these rough numbers it’s hard to see how anyone living any distance from the Equator is going to be able to justify the cost of installing enough batteries to go off-grid with a domestic rooftop solar PV installation.

Finally, Figure 6 plots storage requirements as a percentage of annual generation against latitude for the four cases considered. The percentages will be the same regardless of the size of the installation.

Figure 6: Battery storage requirements versus latitude.

 

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62 Responses to How Much Battery Storage Does a Solar PV System Need?

  1. Dave Rutledge says:

    Hi Roger,

    Great post. If I understand the math, at 60 degrees North, assuming that the Tesla wall units last ten years, the cost per delivered kWh is $35, right? That is dollars, not cents. And that is assuming 0% interest for the capital expense.

    Dave

    • Yvan Dutil says:

      Exactly, Hence, you need thousand of cycle to make some profit. Actually, Musk claimed these unit are used as UPS in critical situation where space is a premium. I read somewhere that their price was lower than the equivalent existing material. Maybe one day, they will make sense for the energy management too, For now, this is probably a way to reduce the cost of EV battery, for which the power density is a must.

    • Hi Dave: ~$35/kWh is right for 60N, except that I haven’t included installation costs and the cost of the inverter(s). But with a half million dollar order Tesla might throw them in for free. 😉

      Incidentally $35/kWh is the cost per delivered kWh independent of latitude. This is because the kWh stored and the costs of the battery storage units are in direct proportion.

    • Reading Willem Post’s comment below reminds me that I also didn’t allow for efficiency losses and battery degredation with time.

      Another thing I didn’t allow for was a reserve margin. Like any other self-contained grid a household working entirely off solar PV would presumably need one. How large should it be? Five percent, ten, twenty, thirty? I guess it depends on how desperate you are to avoid having to install a backup diesel generator.

  2. Willem Post says:

    Roger,

    Here is an example of load shifting using TESLA battery units:

    Some people are claiming one could buy low-cost energy during off-peak hours, store it in a wall-mounted, TESLA battery pack, and use the energy during high-cost, on-peak hours. Here are some calculations.

    At 90% AC to DC inverter efficiency, and allocating half of the 8% DC-to-DC loss to the charging side (the TESLA unit has a round-trip DC-to-DC efficiency of 92%, per spec sheet), it would take 7/(0.9 x 0.96) = 8.10 AC kWh of off-peak grid energy to charge 7 DC kWh into the unit. During on-peak hours, one would get back 7 x 0.96 x 0.90 = 6.05 AC kWh to use in the house. A big loss of energy!!

    The INSTALLED cost of the 7 kWh TESLA unit = $3,000, plus S & H, plus contractor markup of about 10 percent, PLUS $2,000 for an AC to DC inverter, PLUS installation by 2 electricians, say 16 hours @ $60/hr. = $6,500.

    In Southern California, base rates are $0.11, off-peak, and $0.46, on-peak; which likely is THE best-case scenario in the US for demonstrating the economic viability of the TESLA unit.

    The 8.10 AC kWh, off-peak, would cost $0.89. The avoided cost of 6.05 AC kWh, on-peak, would be $2.78, for a profit of $1.89/day, or $691/yr. The simple payback would be about 10 years, not counting the cost of financing.

    But the TESLA warrantee is for only 10 years, and during these 10 years, there would be 3,650 deep discharge cycles, which far exceeds what such batteries are designed for.

    Here are 4 examples of winter-summer solar generation which has greater variation than insolation:

    Based on DPS data, the Ferrisburgh, Vermont, 1,000 kW, south-facing, correctly angled, field-mounted, PV solar system has monthly averages of 4 years of production that show the production ratio of July/December = 161.905/42.601 = 3.80. The 4-yr average CF = 1,323,879 kWh/yr/(8,760 hr/yr x 1,000 kW) = 0.151.

    Because inverters have lower efficiencies at PV solar outputs of less than 20% of inverter capacity (occurring mostly during winter), the monthly energy feed-in ratio is about 1/4 in New England.

    In Southern Germany, further away from the equator, it is about 1/6. See monthly output from 2 monitored solar systems in Munich.
    http://tinypic.com/view.php?pic=ev564y&s=8#.VIosmItU3cN

    Based on DPS data, the South Burlington Vermont solar farm, 2,200 kW, 2-axis tracking units, field-mounted, has monthly averages of 4 years of production that show the monthly energy feed-in ratio of July/December = 4.936, worse than fixed-angle, and a 4-year average CF = 0.167, which is 0.167/0.151 = 10.6% better than fixed-angle, even though such trackers are claimed to be up to 45% better In Vermont.

    The better performance of 2-axis, up to 21% better, occurs mostly during May, June, July and August. Snow would readily slide off the panels at the steep winter angles. Such systems would be about 25% to 30% more costly and require greater O&M expenses, which will reduce any economic advantage.

    • Thanks Willem. It’s interesting to see that you can’t make money out of a Tesla battery even when the prices are rigged four to one in your favor, as they are in S. California.

      Here are 4 examples of winter-summer solar generation which has greater variation than insolation

      I’m working on some stuff which shows that smoothing out seasonal variations requires over 100 times as much storage as smoothing out diurnal variations, which are what I assume you mean by “insolation”.

      • Willem Post says:

        Roger,

        Diurnal variations are one issue, how a PV system output is affected by them is an other. Apart from system issues, there also are site issues that reduce PV system output, by about 15 – 20%. In Germany, etc., shading, aging, ice/snow, not being true south, not being at the right angle, etc., all affect output.

        Some more on the TESLA units:

        It would be good to know (to make economic calculations) just how much in and outflow of energy, kWh, these batteries can do in 10 years, based on daily cycling and loss of capacity as cycles are accumulated.

        My numbers do not account for loss of capacity, and they assume very deep cycling, 7 kWh down and 7 kWh up, and assume the price differential to be there for 365 days.

        It turns out the price differential is there for about 6 months.

        My 10-year SIMPLE payback is more like 25 – 30 years, even longer if financing cost is included.

    • Roberto says:

      Hi,

      You’ve written

      ‘it would take 7/(0.9 x 0.96) = 8.10 AC kWh of off-peak grid energy to charge 7 DC kWh into the unit. During on-peak hours, one would get back 7 x 0.96 x 0.90 = 6.05 AC kWh to use in the house. A big loss of energy!!’

      … and I agree with you about these ridiculous coats… but you can’t use the multiplier 0.9 twice, during charge and discharge, ’cause that is the roundtrip efficiency, you can use it only once (or combined to get the equivalent)
      R.

  3. Bert van den Berg says:

    I have a 10KW system that produces about 20% more than our household consumes. I looked at this question recently, can came up with something in the order of 2 or 3 MWh as the battery that would be needed to store enough power to cover the winter (real world data, including snow days). Something like 60KWh would assure power from February to October, but even a battery of 240KWh would still be insufficient to provide power for about 90 days of winter.

    • Willem Post says:

      Bert,

      Here is an article with an example of a very efficeint (Passivhaus-level), off the grid, 2,000 sq ft house.

      Such a house would have a 10 kW PV solar system, domestic hot water storage system, lead-acid, battery storage system, and a 2 – 3 kW propane-fired generator, mostly for winter use, when PV solar would not be enough to meet basic needs.

      The battery system would have a capacity of 2400 Ah; 24 12 V units @ 100 Ah each, lead-acid, 6 sets of 4 wired for 48 V.

      1 kWh/12 V = 1000 Wh/12 V = 83.3 Ah, thus the 2,400 Ah battery system equals 28.8 kWh. That 28.8 kWh is supplemented by the PV system and the generator. There may be times some judicious rationing is needed.

      http://theenergycollective.com/willem-post/2162036/comparison-grid-connected-and-grid-houses

      • tty says:

        Based on submarine experience, with that much lead-acid battery capacity you would probably need some sort of a ventilating system to avoid hydrogen buildup.

    • It sounds like you are a long way north of the Equator.

  4. Phil Chapman says:

    Roger:
    In your Fig 1, it seems that the PV system on the equator produces c. 65 kWh/day at midwinter. You could increase that to 70 kWh/day by increasing the size of the array by 8%, which would only cost $1200. There would be a surplus all the rest of the year, amounting to 10 kWh.day in midsummer — not enough for A/C, perhaps, but hey, an electric fan is better than nothing.

    Even at 40 deg latitude, doubling the size of the array would provide enough energy at midwinter and would only cost $3,000, much cheaper than the TESLA battery.

    At 60N, a trade is needed between an oversized PV array and the battery.

    The point is that I think you are not giving Elon enough credit, I don’t think he expects anyone to size the battery according to your analysis.

    Diurnal storage is a different issue.

    • markus says:

      Yes, this angle is what I wanted to comment on. It is obvious to me that storing electricity across seasons is extremely expensive and makes therefore no sense.

      In practice we would size the PV array big enough even for winter and use batteries only for covering night and cloudy days. The PV array would quite a bit oversized (vastly oversized at latitude 60) and the batteries still quite large to cover a week or so without sun.

      But the numbers don’t get as surreal as what you calculate.

      • Cloudy days come in “runs” at the worst time of the year.

        I’ve looked at the insolation data from a nearby agricultural station (Medina, Western Australia) and found regular runs of dim days lasting at least 5 days at least once a year; usually around 10 days and up to 19 days in others. (https://contrary2belief.files.wordpress.com/2013/05/dulldays.png)

        It was one of the best “continuous” records that I could find.

        Note that if you have fixed panels optimised for sunny winter days, then you lose out on the larger amount of “white sky radiation” under a cloudy sky at higher latitudes because of the horizon “seen” by the panels. Sensitivity/response depends quite a lot on the type of cells and their encapsulation.

    • matthew_ says:

      Thanks Phil. I agree that no one is expected to size the battery according to seasonal storage. To me it seems that it the goal would be to size the components such that the load demands are met for lowest total cost. Lowest total cost means overbuilding generation, and combining short term storage (Tesla battery?, to meet load peaks and diurnal cycles) with long term storage and alternative generation (firewood, propane, hydrogen?, to fill seasonal gaps). I’ve read elsewhere that for lowest cost off-grid the optimum PV panel angle is not for maximum yearly production, but for maximum winter production. Take care of winter and summer takes care of itself, which Roger does show in the post.

      • Summer takes of itself says:

        Indeed take care of winter and be aware that, far from the equator, most houses need more electricity per day in the winter than in the summer.

    • Phil: Thanks for bringing this up. The economic tradeoff between increasing the size of the solar array versus installing storage batteries is something that needs looking into. I might write a post on it.

  5. Joe Public says:

    Hi Roger

    5,000 kWh pa is a lot for a southern UK house that doesn’t use ‘leccy for heating.

    3,000 kWh is more reasonable for the above.

  6. Jamie says:

    As your analysis notes, it’s simply not possible to live off grid with average appliances, lighting and behaviour without spending vast amounts of money on some unfeasibly oversized storage system. But it’s critical to note that people who aim to live completely off grid will not use the same amount of electricity as average homes – they’ll use radically less. And they can do so with the same level of comfort as a normal home.

    Try running the calculations on, say, 1,000kWh per year, which is a very achievable level of demand with all of the usual comforts and using current technology, and see how it works out then.

    Also rather than adding crazy amounts of storage, add a bit more PV instead. If you’re fully off grid the excess generation in summer can be dumped into a hot water tank and then you just need to size the storage to cover the cloudier days during winter. Using PVGIS the minimum average daily generation from a 4kWp PV installation at London’s latitude is 4kWh/day during December. 1,460kWh per year is plenty for a 3 person home equipped with high efficiency appliances and lighting.

  7. Euan Mearns says:

    Amber Rudd is new Secretary of State for Energy and Climate Change

    http://www.bbc.co.uk/news/election-2015-32683868
    http://amberrudd.co.uk/

  8. Euan Mearns says:

    Roger, I agree with those who argue that upscaling array size makes more sense at low latitude. At high latitude it would be interesting to see where the financial trade off is balanced between adding array and batteries.

    • According to the numbers Willem Post gives above the all-up installation cost of batteries is $6,500/kW and the conversion efficiency is 75%. Installed costs for solar PV panels are $5,000/kWh or less. So on a straight installed cost basis panels beat batteries no matter where you are, although this doesn’t allow for the added flexibility provided by batteries.

  9. It’s an exciting time for renewables with battery technology finally set to arrive in a serious way on the scene. It will be interesting to see what policies pop up as Aussie households start embracing full off-grid systems and cut off from the grid.

    • Normally I wouldn’t approve comments like this one, which looks suspiciously like an ad. The reason I did approve it is that it shows that battery storage is now being touted as a full off-grid option, which it isn’t, not even in Australia. So any policies that might “pop up as Aussie households start embracing full off-grid systems” are likely to be extremely bad ones.

  10. Mark Miller says:

    Roger
    Navigant has a post on the success of Mr. Musk’s marketing efforts- http://www.navigantresearch.com/blog/powerwall-takes-tesla-into-the-energy-cloud?utm
    Their analysis seems to fit in rather well with how we are going to deal with the existing issues of overcapacity leading to curtailment of either RE generation sources of power or FF based ones on the grid. The EDF has been active in supporting the integration of additional RE sources of generation into the grid-

    http://blogs.edf.org/californiadream/2015/04/27/renewable-electricity-is-in-bloom-in-california/?utm_source=feedburner&utm

    The 50%RES plan addresses, make that acknowledges, the grid balancing issues/opportunities/problems in E-3’s https://ethree.com/documents/E3_PATHWAYS_GHG_Scenarios_Updated_April2015.pdf assessment.

    I take it that the 2500 organizations who reserved 100 kWh of PowerPacks hope to leverage how the loading order will be modified to benefit from one of the technology forks in the road- see page 15:

    “Flexible production of hydrogen fuels using 9,000 MW of grid electrolysis can balance 50% re newables, eliminating need for other storage (straight line)
    •Without flexible hydrogen fuel production, ~5,000 MW of long-duration energy storage is needed at 50% renewables in 2030 (high BEV scenario)”

  11. A C Osborn says:

    Can I bring your attention to 2 previous analysis on this subject that points to it being absolutely no improvement over what is already available and basically not a fix for energy storage problems.
    https://chiefio.wordpress.com/2015/05/01/tesla-home-battery-hype/
    and
    http://www.forbes.com/sites/christopherhelman/2015/05/01/why-teslas-powerwall-is-just-another-toy-for-rich-green-people/

    Even Musk’s own company apparently says it doesn’t really work very well with Solar.
    http://www.bloomberg.com/news/articles/2015-05-06/tesla-s-new-battery-doesn-t-work-that-well-with-solar

    • Jamie says:

      That Chiefio article is making the same error that many others have made when comparing Powerwall to lead acids. The Powerwall has close to 7kWh usable capacity but for the equivalent capacity in lead acid batteries you would need 14kWh because a 50% depth of discharge is about as far as you want to go if you want to get a reasonable lifetime out of lead acids (and 50% is really pushing them a bit too hard imo).

      So Tesla have basically managed to get lithium ion battery prices to within spitting distance of lead acid which is a phenomenal achievement. Still some way to go before the economics of domestic scale storage stacks up but it inevitably will in the not too distant future.

  12. A C Osborn says:

    I am going to reproduce one of the comments over at Chefio’s in full as I think it has a strong message about Musk’s advertising ability.
    Tell me how many of you have even heard of this alternative technology?

    janama says:
    7 May 2015 at 7:57 am

    In order to get down the price of the Tesla car they have to reduce the cost of the battery which to replace is currently $45,000! half the cost of the car.
    They have just built a massive new battery factory and this venture will give them the increased scale to bring the cost down.

    this is the competition coming out of Europe.

    http://www.nanoflowcell.com/en

    It’s a battery where the charge is stored in the electrolyte which is salt water! Charge the salt water, pump it into the car battery, remove and replace when discharged. i.e = pull into a servo and recharge with a simple transfer of fluid, just as it is now.
    The car has 4WD using 4 electric drive motors. 0 – 100 km/h: 2.8 S
    top speed: 380 + km/h

    http://www.nanoflowcell.com/quant/quant-e/

    • Euan Mearns says:

      Too good to be true. Lichtenstein based, two years old company, go figure. Gasoline is one of the most energy dense substances known to Man after uranium that is orders of magnitude higher. The performance figures are possible, the range figures not.

    • Roberto says:

      380 km/h????

      That’s plain nonsense….

      R.

  13. Ed says:

    Found the following article looking at the full life cycle analysis of pv that some may find interesting.

    http://www.resilience.org/stories/2015-05-11/how-sustainable-is-pv-solar-power

  14. Donb says:

    To store an electrical charge in salt water, the positive (Na+) and negative (Cl-) ions have to be separated. Energy can then be released and work done by recombining them. BUT, a sizeable amount of energy would go into separating the ions in the first place. That will be expensive salt water.

  15. Hugh Sharman says:

    “The storage battery is, in my opinion, a catchpenny, a sensation, a mechanism for swindling the public by stock companies. The storage battery is one of those peculiar things which appeals to the imagination, and no more perfect thing could be desired by stock swindlers than that very selfsame thing. … Just as soon as a man gets working on the secondary battery it brings out his latent capacity for lying. … Scientifically, storage is all right, but, commercially, as absolute a failure as one can imagine.”

    Thomas Edison, 1883

    The industry, in which I have been playing an active part for almost ten years, remains filled with crooks, liars and fantasists. Especially in the lithium ion field where hype has resulted in multiple spectalular bankruptcies. The Tesla boom has all the signs of a coming bust, not least the hype around Musk’s brilliantly stahe-managed announcement

    Solutions that do not depend on relatively scarce metals such as lithium, cobalt and manganese are emerging. These will also have a better cycle life and will not spontaneously burst into unquenchable flames! And their proponents do not depend on hype but hard and siberly conducted science!

    So Roger, you are right to put Tesla into focus. You are wrong to imply that good solutions cannot be found. As an example

    The lead acid battery business was actually worth $24 billion in sales last year. Soon there will be home storage and grid businesses based on lead (and zinc) that will sweep lithiums out these sectors, leaving lithiums to its perfect niche, hand-held devices. You are just looking at the wrong sector in the battery business!

    • g.g says:

      “Thomas Edison, 1883”

      Now wait a minute…Edison was building, marketing, and selling his own batteries! http://www.nickel-iron-battery.com/edison-nife.jpg http://upload.wikimedia.org/wikipedia/commons/2/2b/Nickel_Iron_Battery_cut-away_drawing.jpeg

      So, Edison is actually a very good argument IN FAVOR OF batteries. It’s just that Edison bet on NiFe and Tesla bets on Li-ion. This could turn into another Tesla vs. Edison war in future, in fact.

    • So Roger, you are right to put Tesla into focus. You are wrong to imply that good solutions cannot be found.

      A technological and economically-feasible solution to the energy storage problem is of course already available. It’s called pumped hydro. But there’s nowhere near enough of it. Pumped hydro is in fact a classic example of a resource-limited technology, one that we would like to employ on a much larger scale but Mother Earth won’t allow us to. Resources are just too scarce.

      And talking of scarcity:

      Solutions that do not depend on relatively scarce metals such as lithium, cobalt and manganese are emerging …. Soon there will be home storage and grid businesses based on lead (and zinc) that will sweep lithiums out these sectors …

      Lithium (33rd), cobalt (32nd) and manganese (12th) are all more abundant in the Earth’s crust than lead (37th).

  16. yt75 says:

    My understanding was that the “marketing storyline” around this batttery is the necessary buffering over a day, not over a year, not the case ?

  17. Rob says:

    Does anyone know many batteries would it take to replace Hinkley C

    Think of the forbes article

    http://www.forbes.com/sites/jeffmcmahon/2015/05/01/did-tesla-just-kill-nuclear-power/

    • Assuming that the nuclear generation from Hinkley C is replaced by solar generation and that this generation is stored for re-use so as to provide constant baseload power you would need about 6 terawatt hours of battery storage, or 60 million 100kWh utility-sized Tesla storage units.

  18. mark4asp says:

    PS: UK latitude varies from 50° to 58½° North with London at 51½°

  19. energymaven says:

    We installed a 4KW system in 2007 at latitude 33N (Coastal Southern California). We generate an average 550 kwh per month. Looking at this past year’s “bills” (the utility actually pays us at the end of the year), we did not have a month where we went into the red. So it seems to me a small unit would be plenty for the few days that we wouldn’t generate enough. Albeit, we have a mild climate here but a small Tesla battery installation may be ideal for some of us.

    • energymaven

      That works out to a load factor (capacity factor in US) of 18.8 percent, which is a little above the global average for latitude 33N (graph from http://euanmearns.com/estimating-global-solar-pv-load-factors/, you are the blue circle)

      According to my data you would need storage equivalent to 10% of your average annual solar generation, or 550 times 12 times 0.1 = 660kWh = 66 Tesla 10kWh units costing $231,000 to smooth out seasonal variations, but this obviously wouldn’t be a very attractive proposition.

  20. The above article is useful however the authors over simplifies things by just referring to latitude. You must also take into consideration climate issues which affect the number of hours or radiant energy hitting the ground and not absorbed by clouds, fog, dust particles and the like. There are standard “insolation” curves and values for regions worldwide Please see our on-line article ‘Going Solar” and scroll down to the boxed highlighted portion of the article titled “System requirements” to learn how to size solar panels. The assumptions provided above, in fact, are far too generous to Tesla, et al. \The real world is much worse. See: http://fusion4freedom.us/going-solar/

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  22. Watson Smith says:

    This calculation, although accurate, is kind of silly. Why would you size the solar system to exactly meet annual energy use and then rely on storage (which is very expensive) to match up to the diurnal and annual cycle rather than using a larger solar system? It would be far cheaper to size of the system so even in winter it makes a little more than the daily use requirements then you only need to store one night worth of energy (or two or three if you need to count for cloud cover/ weather. It wastes capacity, but is far cheaper.

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  24. I have a friend who has a 4kW system in the Yukon (60th parallel). He has a tracker and a large lead acid battery bank that cost about $9000.00
    That total cost for the system was $45,000 and he had an existing diesel generator.
    Prior to this he spent $12,000 per year on diesel and now he only has to run the generator for 12 hours every 5 days for 3 months in the winter. That means that he has reduced his diesel bill to $300 and it will take him about 4 years to break even
    He has a 4000 sq ft off grid home with a deep freeze, fridge, full shop with power tools, electric oven and heat with wood back up.
    My point is that the Tesla battery isn’t financially viable at the moment and lead acid is but its not that sexy.
    Notwithstanding all that…. Renewable energy and storage are the future and we can banter all day long about the economics but it’s going to happen anyway

    • Mark: Thanks for that. Some details on your friend’s system, like installed PV capacity, battery capacity, demand etc. would be interesting. Do you have any?

  25. stewgreen says:

    Why take the risk of spending all that money upfront, when the gains are so low if not huges losses ?
    Even if you estimate that in 20 years you’ll make back a $30,000 upfront cost by generating electricity to use and sell to the grid to cover all costs plus INTEREST, there are still risks from the panels blowing off in a storm after 1 year to them failing early after 10 or 15 years.
    I’d just stick to paying $1000/year for grid electricity, then any problems/uncertaintiews are theirs not mine.
    It seems strange in a day where companies try to stick to their own core businesses and OUTSOURCE everything else that people would get into risky unpredictable areas like power generation.

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  34. MH says:

    Anyone want to run the numbers in the event that solar conversion efficiency doubles? http://oilprice.com/Alternative-Energy/Solar-Energy/Is-This-The-Efficiency-Breakthrough-Solar-Has-Been-Waiting-For.html

  35. MH says:

    What if solar conversion efficiency doubles? Seems it would reduce storage requirements by over 50%.

    http://oilprice.com/Alternative-Energy/Solar-Energy/Is-This-The-Efficiency-Breakthrough-Solar-Has-Been-Waiting-For.html

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