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 the solar setup leaves on the table because aren't demanding it? 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

You can get a feel for overall system efficiency by

1. watching your system over time, as described above
2. comparing real-time harvest to theoretical harvest

From the subarticle on this topic:

Examples: if you have 200w of panels, 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).

If observed 94w at that place/time the overall system efficiency would be 94w/116.2w = 80.9%. After you make many comparisons like this you will start to see a patter in your system's ability to harvest power. If it's 81% over time we can start to multiply the solar zenith angle calculations above times 0.81 to predict how much power our particular system could make on average.