Here at Plans2Reality World Headquarters, we have been looking at solar panels for electricity. This has provided an interesting and instructive lesson in marketing along the way. I shall christen it:
Bragging About the Wrong Thing
Solar panel makers love to brag about two things: their watt-peak (Wp) rating and their efficiency. Oddly enough, neither one matters. Nope, not a bit. Not one d*mn bit at all.
(Let me interject here that _all_ panel makers are equally involved. In private conversation, many have told me they agree with the analysis I am about to present.)
First, Wp. The peak-watt rating is a measurement of instantaneous power output from the panel, taken under an incident light intensity of 1000 watts per square meter, and at a cell temperature of 25C.
So what's wrong with this? The panel will never actually see these conditions in the real world, and there is poor correlation between these "STC" ratings and real-world performance.
1000W/m2 does approximate the noon-day sun, but most of the time it isn't noon. Furthermore, most solar panels are NOT installed in sun-tracking mounting systems, so at best the panel "sees" full sun only at noon. The oblique angle the rest of the day reduces incident light. (Actually, most smaller arrays are installed so the panels aren't at the optimum angle even at noon - residential rooftops are usually not at the correct angle, but it's easier, and nicer-looking, to mount them flat against the roof.)
So your panel will never generate its Wp rating, due to sun angle. But wait, there's more!
PV panels are quite temperature-sensitive - some more than others, but all are to a fair degree. And when you place an object in the sun, it gets warm - warmer than 25C most of the time.
How much warmer? Well, That Depends.
The exact amount of heat gain depends on the specific PV material. Some panels will get warmer than others. In addition, the degree to which the panel cools off, that is, sheds heat via conduction and convection to the environment, varies substantially among panels. There have been attempts to quanity this (NOCT, PTC) but these are at best compromises.
Net-net of all this: the Wp rating of the panel represents a power output you will never see in real life. Couldn't be much more meaningless than that, huh?
So what specification SHOULD we be talking about?
We should be talking about kilowatt-hours per dollar. Why? The answer is quite simple: Solar panels are installed to generate electrical energy. Electrical energy is measured - and sold - in units of kilowatt-hours. So why not measure our PV installation the same way the electric utility measures the electricity it sells us?
It's just that simple. Really. I am putting up PV panels so I don't have to buy kilowatt-hours from the utility. Great. So tell me how much the kilowatt-hours from the PV panels will cost.
Nothing else matters, folks. Nothing.
Next installment: Efficiency doesn't matter either. ;-)