Also in: Disaster Response and Recovery
PV Array Size from a Target Annual Energy
The DC array size needed to offset a target annual production.
Example
You enter
- Target annual energy (kWh/yr) 12000
- Peak-sun-hours (kWh/m2/day) 5
- Performance ratio (0-1) 0.77
You get
- Required DC array size 8.54 kW DC
- Specific yield 1405 kWh/kWp
Checked against published worked examples from: University of Arizona Cooperative Extension.
Details, formula, and sources
ScopeFrom 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 or tablespecific_yield = psh x 365 x perf_ratio; dc_kw = target_annual_kwh / specific_yield; monthly_kwh_avg = target_annual_kwh / 12.
Source and editionNREL 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.
Free accessThe PVWatts energy model is a public NREL relation; PVWatts is free at pvwatts.nrel.gov.
What governsEstimate. AHJ and licensed professional govern.
API field namestarget_annual_kwh, psh, perf_ratio, dc_kw, specific_yield
- Array size dc_kw = target_annual_kwh / (psh x 365 x perf_ratio) (PVWatts inverted)NREL PVWatts
- Performance ratio PR (default 0.77) rolls up soiling, shading, mismatch, wiring, inverter, and availability lossesNREL PVWatts
- Specific yield psh x 365 x PR is the site's kWh per installed kWpfirst principles