Recommendations for new system

Cariboocoot

Re: Recommendations for new system

Why I calculate the way I do:

You need the maximum charging current when the battery Voltage is at its minimum. This should be system nominal, as that assures the batteries do not go below 50% SOC. You do not really need full current at Absorb level Voltage.

Thus:

100 Amp hour 12 Volt battery @ 10% charge rate = 10 Amps * 12 Volts = 120 Watts / 0.77 (typical efficiency derating) = 156 Watt array.

It works. Repeatedly it works. Adjustments need to be made for local conditions, and of course you size the battery and round up to what's available before calculating array size. You also round the array size to what is available, preferably upwards too. No one has yet complained about having too much PV.

The 13% upper limit used to be an economic factor. With PV so cheap it no longer is. However there is another reason to avoid going too high in charge rate: charge acceptance. The higher the rate goes, the more energy is turned into heat rather than actual charging. There's no point in doing this, and if it is carried to the extreme the batteries will suffer greatly. Note that AGM's can take higher current than FLA's.

Working the formula 'the other way' we see that an 80 Amp controller on 12 Volts would take an array of (80 * 12 / 0.77) 1246 Watts. Bill calculates this for 80 Amps @ Absorb Voltage, but again that isn't actually necessary. Particularly not when you are 'maxing' a controller "to be on the safe side".