I just can’t seem to understand how solar panel efficiency works, when they say say a panel is 20% efficient what does what mean? Do I only get 20% of that 5kwh and does that mean I’d need a 20 kwh system to get the 5kwh’s I need?
In:
You buy the panels based on the output you need / what they actually produce, and that’s how they are marketed.
The theoretical efficiency is of interest to people designing the next generation of panels, not to the consumer buying panels right now.
Your Camry would get 100mpg if it was perfectly efficient in internally combusting gasoline and converting it into forward motion. But they aren’t allowed to say it gets 100mpg.
20% efficiency means that converting 100J of light yields 20J of electricity, so 20% of the energy is retained in the conversion.
A 5kW panel will generate up to 5kW of electricity. Due to the sun rising and setting and the weather, you’ll only average about a seventh of that, even if the panels are ideally positioned. So 5kW times 24 hours divided by 7 gives around 17kWh per day, on average. In the summer it could be more than double and in winter less than half; much depends on the climate where you live, orientation of the panels, and any shading from things like trees and houses.
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5kw is what the panels can actually produce at peak efficiency (So yes, 5kw is peak efficiency, or what your panels will produce in perfect conditions). It is important to remember that panels can only produce peak power if they are not dirty, there are no clouds obstructing the sun and the panels are facing the sun. There are some expensive systems you can install that dual axis mounts so the solar panels can be moved around to always face the sun (Turn and tilt).
The peak light on the earth surface is about 1kW/m^2, a 5kW system puts out 5kW under those conditions. If the 5kW system is 20% efficient it means you installed a 25m^2 array.
Also, since it’s calculated that way, you only really get that 5kW for a few minutes of the brightest day at the brightest time of day, you typically get significantly less than 5kW.
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