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I was asked to today about the available plans (in Israel) to install photo-voltaic receptors on the roof and sell the energy. This is a legitimate plan and there are several companies that do this. The claim is that the initial investment is repaid after roughly 5 years which sounds too optimistic to me. I want to carry the calculation myself and check the results. Where can I find some data about the light flux that hit the earth in a typical year?

note I'm not sure, since this is not my specific field of expertise, about the commonly used term.

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In Germany, investments of a similar kind repay in 10 to 20 years. – Lagerbaer Aug 7 '11 at 20:14
@Lagerbaer And that already includes heavy subsidies. – Leonidas Aug 7 '11 at 22:42
Those are the figures that sound more reasonable to me. Does that includes any loses (maintenance, excess heat, etc.)? – Yotam Aug 8 '11 at 7:51
up vote 5 down vote accepted

It will vary a lot by site, and on the particular PV technology being proposed.

So, you need to know:

  • is it monocrystalline silicon, is it CdTe thin-film, or something else?
  • what's the efficiency of the inverter?
  • what's the guarantee on the kit (5 years, 10 years, 20 years)?

Using @Martin Beckett's figure of 2000kWh/y, that means, roughly, that your annual production would be approximately 2000 hours x the installed capacity of your PV array. So if you had a 1kW capacity system, you might expect to get 2 000 kWh of electricity in an average year.

That's assuming an equator-facing PV array tilted to the optimum angle for your location; and, crucially, that there aren't any objects that would cast shadows over your PV array. Depending on the particular panels and inverter configuration, a bit of shadowing can reduce generation by 10-80%.

There are various PV calculators around on the web - the good ones should ask you my top two questions above, about panel type and efficiency). The USA NREL PVWATTS calculator has no sites in Israel - the closest is Cairo - but I expect you'll be able to find a reliable one somewhere that will run the numbers for you. The EU PVGIS calculator looks like it covers Israel, so I'd recommend trying that out.

Five years might be a plausible payback time, depending on how much you'd get paid per kWh; how much you'd save off your bills; and how much the installation costs are. Currently, I expect you'd be asked to pay of the order of €3/W of installed capacity (before any subsidies or taxes). So, at the above generation rate of 2kWh/W/y, then a five-year payback would imply that you'd be getting paid (in direct payment + bill savings) about €0.30/kWh (assuming a zero discount rate).

Panel efficiency has already been taken account of, in the above calculation of power output and of payback time. That is to say, once you know the panel's rated capacity, you've already accounted for the panel's efficiency. For more info on PV efficiency itself, see this answer to another question. To calculate the efficiency of any given panel, take its rated power in kW, and divide by its surface area in square metres. For example, a 200 W panel that is 1 square metre has an efficiency of 0.2 / 1.0 = 0.2 = 20%. Typical commercial panels currently are in the range 10% (thin film) - 20% (best monocrystalline).

An annual insolation of 2000 kWh, means that 2000 kWh of sunlight falls on $1m^2$ in an average year. If you want to put panel efficiency in there too, you end up putting it in the numerator and denominator, and it all cancels out, leaving you just with 2000 hours x PV capacity in kW.

Now, that factor of 2000 gets derated by various things: panel shading, inverter efficiency, less-than-optimal tilt or orientation, system down-time, and so on.

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What mostly bother me is the estimated loss. The numbers I know is that less than 15% of the light is converted to electrical energy. I was in a lecture lately which stated that due to a cycle which is much similar to Carnot cycle, the most efficient system will be transfer less than 30% of the light to usable energy. This is why I want to know the raw data, how much light hits the earth in my region and not the calculations... – Yotam Aug 7 '11 at 19:52
@Yotam - 15% is a reasonable efficency for a commercial panel – Martin Beckett Aug 7 '11 at 21:03

For the middle east, typically around 2000 KWh/m^2/year

A good place to start is wiki page for insolation (technical term for sunlight arriving)

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is this number for the amount of energy produced or arriving? What area are they relevant to square meter or square foot? Where did you got this number from? – Yotam Aug 7 '11 at 19:43
It's average incoming sunlight (/m^2 sorry missed that) over a year. It's from the africa map on the wiki page - there is a link to the original source – Martin Beckett Aug 7 '11 at 21:01

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