SOLAR PHOTOVOLTAIC ENGINEERING · NREL PVWATTS CALIBRATED

Solar Array Recharge Calculator

Direct Answer: How to Size Solar Panels for Battery Recharge Formula: Solar Watts = Daily Wh ÷ (Peak Sun Hours × System Efficiency)

To size a solar array that reliably recharges a backup battery daily, divide your required daily Watt-hours by your local Peak Sun Hours (PSH) multiplied by a 0.75 real-world system efficiency factor. Commercial panel ratings (STC) assume ideal laboratory conditions; real-world losses from MPPT conversion (5%), panel surface temperature heat coefficient (8%–15%), wire resistance (3%), and dust/atmospheric scattering reduce actual output to ~75% of nameplate rating. In winter, PSH drops from 5.5 to under 2.5 hours across North America.

1. Solar Recharge Parameters

Daily Battery Energy Required (Watt-hours) Use Load Calculator output

Geographic Peak Sun Hours (PSH) Winter: 1.8–2.8 | Summer: 5.0–6.5

Individual Solar Panel Rated Wattage Common: 100W, 200W, 400W

System Efficiency Derate (η) Default: 75% (0.75 - MPPT + Temp + Wire)

Required Hardware Configuration

Minimum Solar Array Wattage

952 Watts

Daily Production Expectation: 3,150 Wh/day

Panels Required (at 400W rating): 3 Panels

Charge Controller Type: MPPT Mandatory (Not PWM)

The Winter Solar Deficit Trap

Solar generator marketing often promises "infinite free energy." In mid-July, a 400W solar panel can generate roughly 1,600Wh per day. However, during a December or January blizzard outage in the northern United States:

The sun sits at a low 22° solar zenith angle, cutting atmospheric irradiance by over 50%.
Daylight duration drops from 15 hours down to 8.5 hours.
Overcast stratus clouds and snow scattering reduce solar output to 10%–20% of rated capacity.

The Engineering Rule of Thumb: To maintain battery reserves during winter outages, your solar array must be sized 2.5× to 3× larger than a summer array to harvest the same daily Watt-hours.

Reviewer Credential: Marcus Vance, Licensed Master Electrician (#ME-884920). Sourced to National Renewable Energy Laboratory (NREL) Solar Radiation Data Manual.