PV Array Size from a Target Annual Energy
The inverse of the PV energy-yield tile: the DC array size needed to offset a target annual production, from the NREL PVWatts model - DC = target_annual_kWh / (peak-sun-hours x 365 x performance ratio). To offset a 12,000 kWh/yr bill at 5 peak-sun-hours and a 0.77 performance ratio takes an 8.54 kW DC array (specific yield 1,405 kWh/kWp). Enter the annual total (monthly bill x 12) to size from a utility bill. The performance ratio (default 0.77) is the biggest lever. A pre-design estimate, not a bankable model; module count and roof area then follow.
Formula and source
specific_yield = psh x 365 x perf_ratio; dc_kw = target_annual_kwh / specific_yield; monthly_kwh_avg = target_annual_kwh / 12.
NREL PVWatts energy model (E_annual = P_dc x PSH x 365 x PR) solved for the array size, the inverse of the PV energy-yield tile, by name; the relations are public.
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