How much extra solar power do I have?

Yes, we can use a battery monitor to see how much power is going into the battery bank. And we can look at the solar charge controller to see how much power it is making.

But how can we tell how much untapped power does our solar have in reserve? Put another way: how much more stuff could we be running off the solar?

• it is easy to test whether or not you can run a specific load
• it takes a lot of “seat time” with the system to predict how much excess power you have at a given moment
• it requires some math to predict whether or not you can run a load on average at a given place/time

about these summaries

Plug it in and see if the solar can support it without affecting the battery bank. Easy peasy.

The downside is this is a YES/NO answer; it tells you a specific load will run but does not tell you about how much power is actually available. If you want to see the Bigger Picture then read on…

Hours of Full Sun Equivalent (FSE) is a convenient way to think of the amount of sunlight reaching the ground over the entire day. 4.5 hours of FSE means 4.5kWh of power reaching the ground per square meter.

To predict daily harvest we will use

• FSE
• your panel wattage
• and your system's overall efficiency

Real-time harvest is a bit easier since there is no time component. We are working in Watts (W) rather than Watt-hours (Wh). What we are doing is comparing the strength of sunlight reaching the ground (perpendicular) vs the cosine of the angle at which it actually strikes the ground, or strikes a flat-mounted panel.

This cosine is calculable for a given time and place.

Example: if you have 200w of panels and calculated cosine is .70 then 140w of direct light is landing on your panels in clear skies.

For various reasons your solar setup will not be able to harvest all the power landing on the panels. For systems with PWM controllers the percent harvested will be something like 70%. For MPPT it might be 83%. These constants will be placeholders you can use until you figure out your specific system's overall efficiency.

You can see our system's overall efficiency at a given time: actual harvest / theoretical harvest. The numbers will jump around but over time you will start to see a pattern. Maybe yours makes 80% or 86% over time. Use that number to predict how your system will behave in another time/place:

  panel wattage x system efficiency x cosine

…if your mppt controller typically yields 83% after derating, and the calculated cosine is .70 then you might expect ~116w in clear conditions at that time in that location. (200 x .83 x .7 = 116.2).