Sizing off-grid solar, honestly
An off-grid system is sized from one number: how many watt-hours you use in a day. Everything else follows. List your loads (watts × hours per day), sum them, and the array, battery bank, and inverter all size themselves from that total. The tricky parts are the fudge factors, and this calculator makes each one explicit instead of burying it.
Peak sun hours and the derate factor
Your panels do not produce their rated wattage all day. Peak sun hours is the number of hours of full-strength (1000 W/m²) sun a location gets in an average day — 3 to 6 across most of the US, lower in winter and the far north. Array watts = daily Wh / (sun hours × efficiency). The efficiency factor, around 0.8, accounts for charge-controller losses, wiring, heat, dust, and panel aging — real losses that a naive calculation ignores, leaving you chronically short.
Worked example: 3000 Wh a day at 5 peak sun hours and 0.8 efficiency needs 3000 / (5 × 0.8) = 750 W of panels — two 400 W panels.
Battery bank and depth of discharge
The bank must carry your loads through the sunless stretch — the days of autonomy — and it can only use part of its rated capacity. Lead-acid batteries should not go below 50% charge without dying young; LiFePO4 tolerates 80% or more. So bank size = daily Wh × days / depth-of-discharge. Two days of 3000 Wh at 80% DoD is 7500 Wh — 313 Ah at 24 V. The same load on lead-acid at 50% DoD needs 12,000 Wh, which is why lithium has taken over off-grid.
Inverter sizing
The inverter must handle everything running at once, not the daily average, plus headroom for motor surge (fridges, pumps, and power tools draw several times their running wattage at startup). Size it about 25% above your peak simultaneous load and round up to a standard size.
Related tools: the battery life calculator and battery pack builder for the storage side, and the wire size calculator — low-voltage array and battery runs carry big currents and need heavy wire.