Was going 420w @ 12v but opted for 280w @ 12v...

Re: Was going 420w @ 12v but opted for 280w @ 12v...

Let's just go over the math again.

280 Watts of panel operating at 77% typical efficiency is 215 Watts effectively.
Over 4 hours of equivalent good sun that would be 860 Watt hours DC per day. About 560 Watt hours AC.

It will supply a potential peak charge current of around 16 Amps with a PWM type controller. (An MPPT controller might squeeze 2 more Amps out on average.) Output of the charge controller should be based on that.

As we discussed before, the biggest problem is with the V-drop from the array to the controller. Your plan to use separate 10 AWG wires per panel over 15 feet should solve that. There would only be a 2% drop this way. Changing to two runs of 8 AWG would reduce this to 1.2% (Anyone who wants to check those calculations feel free to do so.)

Keep in mind this is power loss under maximum load conditions. As the battery charges the current will go down as the Voltage goes up and the power loss will decrease. If the battery is not too large you likely will not see maximum current most of the time. You could use up to 320 Amp hours (5% of which is 16) but would be better off with 160 (10%) to 220 (8%).

The 180 Amp hour battery at 25% DOD will provide about 540 Watt hours DC, slightly less for AC (around 486 depending on the conversion efficiency of the inverter).

I know this is all confusing, but it's because one thing effects another. For instance temperature changes will alter the numbers everywhere: cold panels put out more, hot panels less. Atmospheric conditions change panel efficiency. But more so, how you use the system will alter its performance. Battery capacity is dependent on the rate of draw at the time, so it effectively goes up and down as you use it irrespective of charge. Once the batteries are charged you have "surplus" power available from the panels which can add to your daily supply providing you can make use of it at the time. How much will depend on (same old song) a number of factors. You can only approximate performance; you can't carve it in stone.

Over-all the only thing I'd alter to your plan is to go for the 220 Amp hour 6 Volt batteries.

But let's see what other people's opinions are.

Re: Was going 420w @ 12v but opted for 280w @ 12v...

Ah, kind of mixing the formulas.

Panel efficiency rating is based on "nameplate rating" * "typical efficiency of panels & controller"
That's how we get 280 Watts * 77% = 215 Watts.

This doesn't apply to PWM charging or daily Watt hour "harvest" but is key to understanding panels' "real" output and determining conditions for MPPT controllers. I probably confused you by flinging tons of numbers and formuli at you. Sorry. We were discussing many possibilities including MPPT and even I get confused.

A PWM controller is basically a switch and will only pass the peak current available from the panels irrespective of Voltage. So 7.82 * 2 = 15.64 or 16 Amps. The panels on their own can not produce more current than this except going into a short circuit. The panel Voltage will be "pulled down" to battery Voltage + losses from wiring/controller/heat. This is why "12 Volt" panels have a Vmp around 17-18 Volts: after going through wiring there is hopefully enough Voltage left over to be above the battery Voltage and thus provide charge current. The MPPT controllers can sneak a bit more out if the wiring is efficient enough and can take advantage of cooler panel temps (which increase Voltage).

One of these days we've got to post the whole lengthy formula with all the variables. Maybe in time for Halloween.