FYI, technical details here.. but I didnt want to disagree with AaronD as the above is also a good way to estimate solar output. Its a fair way I think. I wanted to compare it to the .2 to .5 times rated power rule of thumb that I actually got from this forum years ago.I've heard that a good rule of thumb is to estimate that you'll get ~5 hours per day of your panel's maximum power, spread over ~12 daylight hours. So a 50w panel might deliver ~250 watt-hours. In reality, it will be much less than 50w in the morning and evening, and only come close to peak output (if you're lucky) around mid-day. Here in the cloudy PNW, I estimate 3.5 hours.
Amp hours is not an actual energy capacity term. To be technically accurate, this should be watt hours (like AaronD and your home utility company use). But 12 volt boat folks like to use amp hours for capacity with the assumption that its a 12 volt system. Why.. Im not sure but I will guess battery charging is not completely efficient and you lose a little. Ie, watt hours in to the battery from the charge source is greater than watt hours out to the load. However, since battery charging generally occurs at a higher voltage (example 13.5 volts) than battery driving a load (example 12.3 volts), using amp hours "kind of" takes into account the loss so that amp hours in is closer to amp hour out compared to using watt hours. Ie, using amp hours somewhat automatically takes into account the battery charging inefficiency. Regardless, you will almost always see amp hours used with the assumption that its a 12 volt system.
So.. for the 50 watt panel producing 250 watt hours per day, lets say this happens at an average voltage of 13 volts. So the equivalent amp hours would be 250/13 = 19.2 amp hours.
So for the rule of thumb I used, 50 watt peak power / 19.2 amp hours = .38
Remember I had a range of .2 to .5. So .38 is in this range. I measured this one time under a particular set of conditions (using a battery monitor and a Genasun MPPT controller) and got .33 for that particular day/ location. So many things cause the solar output to vary (temperature, time of year, clouds, panel pointing, any even small shading, insolation for a particular location. MPPT or PWM).
Anyhow .. didnt want to conflict with what AaronD said as its correct.
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