Your Questions About Solar Panel Efficiency

Mandy asks…

Solar Panel Efficiency Project?

I would like to test the efficiency of a solar panel by connecting it to a lamp and a battery. The solar panel would exclusively get its solar energy from the lamp and the battery will be connected so that it serves as the initial source of energy but would eventually run out. Then the lamp would get the energy only from the solar panel.
How can I set up such a series?

ssadmin answers:

It is a non starter.
The circuit described can be easily wired. However, it constitutes a perpetual motion machine of the second kind.
There are two efficiencies involved.
One, the efficiency of the battery and two, that of the solar panel and its output circuitry. The overall efficiency will be the product of the two which will be negligible.

Betty asks…

How do I keep solar panel glass clean?

Any thoughts on the best way to keep solar panels clean ? If they are up on the roof washing them is not that practical but you will all be aware how dirty windows can get if left uncleaned and this I am absolutely sure will reduce the efficiency of the panels over a relatively short time .
Had anyone used any of these car windscreen treatments that claim to ‘ bead' rainwater making it run off quicker ? Or any ‘old wife's treatments that I could apply to the panels before they get put up ?
Any advice or input or comment very much appreciated
Thank you

ssadmin answers:

I understood from the installers that did mine that they are self cleaning, a bit like the glass used in some of the hard to get at windows. Suppose you could always spray with some of the shower cleaning sprays that repel water but I would check first to see if it is safe to use.

Sharon asks…

would plexi glass coating over solar panel…affect it's efficiency?

are there any other ways of protecting solar panels from rain?

ssadmin answers:

Yes. In short, plexiglas will absorb some light at some wavelengths. If that light would have gone to generating electricity, then yes, you're losing efficiency.

I'd worry more about scratches and dirt accumulating on the plexiglas, making even less light reach the PV panel.

Why do you want to keep rain from getting on the glass panel? If anything it will help to clean it off. Glass is also very scratch resistant and smooth, so it will last a long time unless you have sandstorms.

Helen asks…

How many watts can a solar panel produce in one hour if it is one square foot?

I think an answer to this question will give people a better understanding of the efficiency of solar technology… including me

ssadmin answers:

It depends on the type of solar technology. Silicon panels are up to 20% efficient, thin film can be around 8%, but are generally less expensive. Silicon panels output about 14W per sqft (at best) and thin film about 6W per sqft.

Watts is a rate, watt hours is a quantity. Just like miles per hour is a rate, and distance is a quantity. So if a car drives 60mph (rate) for 2 hours (time), it went 120 miles.

A panel that is rated for 100W will generate 100 watt hours (or wh) in one hour (in sunlight equal to a standard test condition).

A much more efficient use of the sun is solar water heating. That's about 85% efficient. Depending on where you live, 40 sq ft to 80 sq ft can provide 75% of your hot water.

Joseph asks…

efficiency of solar panels in the winter?

How detrimental is winter to the efficiency of… a 6kW system?

ssadmin answers:

The efficiency of a solar panel actually goes up in the winter, silicon loves to be cold, hates heat. The voltage can be as much as 25% higher in -25 degrees F than in 100 degrees F. However, the available sun hours drops in the winter, as much as in half or more, depending on where you are. So during the hours that there is sun, the output is great, it's just going to do it for fewer hours, so the overall total output in a day is less.

You can see the data for major US cities of how many sun hours are available in the winter, summer, and annual average. Http://

So if we look at a 6kW system in NYC, the high sun hours is 4.97 in summer, low is 3.03 in winter. So summer expected output is 6kW x 4.97 hours = 29.8kWh a day, minus about 30% for losses = about 20kwh a day. Compare that with winter, 6kW x 3.03 hours = 18.1kwh – 30% = 12.7kWh a day.

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