Methodology

• Panel watts = daily Wh ÷ (peak sun hours × 0.75). The 0.75 system derate covers temperature, wiring, inverter and soiling losses; NREL PVWatts uses a comparable default loss.
• Panel count assumes 400 W panels.
• Battery kWh = daily Wh × days ÷ (0.9 depth of discharge × 0.9 inverter efficiency).
• Inverter: at least 1.25 × the running watts of ticked appliances, or 3× the average load when sizing from a bill. Motor loads may need more surge.
• Monthly bill: kWh ÷ 30 = daily kWh.
• Sun hours: state annual averages derived from NREL PVWatts v8 (state table). Winter is lower; off-grid systems should size for the worst month.
• Appliance energy: manufacturer or ENERGY STAR figures, from the same data set as Willrun. Hours of use are labeled assumptions.