How to Evaluate Actual Solar Panel Output

In this video I’m going to share with you how you can evaluate the amount of electricity a Solar Panel will actually produce…in YOUR neck of the woods.

Renogy 100 Watt Panel

I am using this Renogy 100 Watt panel for the evaluation as this is one I own.

Now, first thing you must understand is that while the panel says 100Watts, that is NOT what you can expect it to produce. That 100 Watts is when the sun is directly overhead at noon on a clear day. This is the MAX output. If the sun were ALWAYS overhead at noon with no clouds, ever, you could get 100Watts. That is true.

Max Output vs. Actual Output

News Flash! This is not realistic. The sun does set on occasion. There are clouds, thundrestorms, snowstorms etc. So, to evaluate a solar panel we really need to find out what the average amount of energy the sun shines on a space of 1meter squared.

In Atlanta, the average sunshine on 1M/sq is 180watts. 180 Watts is the energy that falls on us from the sun here in the Atlanta area per meter squared. WIth me so far? Now, we must ask how much of that 180 Watts can a solar panel convert to usable electricity? And this is the second key part of this entire equation.

Efficiency of the Panel

As I explain in the video, this Renogy solar panel is actually only 6.7 sq. feet, so it’s basically 2/3 the size of a sq. meter. Thus we add 50% to its max output stated of 100Watts and get 150 Watts, if we were to convert this to a full meter squared panel. 150 Watts Max Output / 1000 Watts of sunshine at noon directly overhead means this panel is 15% efficient. It can convert 15% of the suns energy to electricity. That’s what the efficiency number means.

Atlanta gets 180 Watts per Sq. Meter from the Sun

In Atlanta we get 180 Watts per sq. meter and this panel converts 15% of that to electricity meaning its output is 27 watts. (180 * 15%). 27 watts over the course of a day is 648 watts hours, or easier to understand, .648 kWh. Ultimately this means that for each average day in Atlanta I should be able to generate .648 kWh of electricity. That is well more than enough electricity to run some of low-load appliances, like LED light bulbs, your computer, cell phone. But it will not be enough to run your fridge or larger appliances. To run those appliances you will have to wire a bunch of these panels together in order to generate the electricity you need.

Use Panels to Charge Batteries, Not Run Appliances

However, the best use of a solar panel is really to charge your batteries. If you have batteries you have a source of power when the utility goes down. Many people assume that solar panels give them “off-grid” coverage. Meaning they’ll have electricity when the power goes out. This is simply incorrect. For solar panels to provide you electricity without being tied to the grid, you’ll need other accessories, deep cycle marine 12volt batteries, golf cart 6 volt batteries and such. You’ll also need inverters. My video on inverters and batteries is linked so you can watch that too to get a gauge how this all works. From a pure efficiency standpoint though solar panels charging batteries are the most efficient way to use them.

What Size Load do you Intend to Use

However, if you’re going to use panels to run your appliances, you will need to know how much of a load you intend to run. Think of a fridge, it may need 600 watts most of the time but when the compressor kicks on it will need another 300 watts or so. Thus you’re really going to need panels to provide around 1000watts. Will 10 of these Renogy panels at 100watts each provide 1000watts though? I mean, 10 panels times 100 watts = 1000 watts, right?

No! Wrong! Again that is MAX capacity with the sun overhead at noon and no clouds. This panel will provide about 27 watts or .65kWh roughly per day. You’re going to need a whole heck of a lot more than this one panel to run your fridge. And this is my whole issue with photovoltaic (PV) panels..They will need a huge amount of space to provide you much electricity.

Rooftop PV panels will provide SOME electricity, this is absolutely correct. But in order to provide you the electricity you need for your daily operations, you’re going to need a huge array of panels. There is no getting around this.

Panels at 30% efficiency??? Nah…

Efficiency’s have been improving. This panel is 15% as we’ve discussed. However, there are some limits to the efficiency scale of solar. You’re not going to see rooftop solar panels at 30% efficiency. Maybe 25%??? Hard to see that en masse. But even at 25% efficiency, what would this panel generate for electricity in Atlanta? I’ll let you figure that one out.


At the end of the day, I like my panels and fully anticipate getting more. But I like them for the reason of varying sources of electricity. Gasoline generators. Wood burning fuel. Natural gas. Battery banks. Solar lighting. All of these give me varying degrees of preparation if power goes out. For me, that’s very comforting. You may have a different desire for your solar panel array. More power to you. Just understand the math behind them before you go all in.


Leave a Comment