Solar Panel Calculator
Last updated: September 3, 2026
Quick answer
Panels needed ≈ daily Wh ÷ (sun hours × 0.75 system efficiency). A 100 Ah lithium night that used 60 Ah
(720 Wh) needs ≈ 240 W of panels at 4 sun hours. Nameplate watts are not delivered watts.
= Ah × volts (e.g. 60 Ah × 12 V = 720 Wh)
Panel wattage needed
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How this calculator works
Panel watts = daily Wh ÷ (sun hours × system efficiency). The efficiency factor bundles everything the
nameplate rating ignores: heat derating, controller and wiring losses, charging losses inside the battery, and the fact
that flat-mounted RV panels almost never sit at the ideal angle.
Worked example
Your nightly audit says 720 Wh (fridge 45 Wh/h average… or just run the battery runtime calculator and multiply average amps × 12 V × hours). At 4 sun hours and 75% efficiency: 720 ÷ (4 × 0.75) = 240 W — a common 2×200 W or 3×100 W roof array. Winter in the northern US may demand the 3-hour row: 720 ÷ (3 × 0.75) = 320 W.
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Assumptions & sources
- Sun hours: "peak sun hours" per day, seasonally varying across the US (≈3–4 h winter, 5–6 h summer for most states); NREL solar maps publish per-location values.
- System efficiency 65–85%: composite of panel temperature derating, controller type (PWM worse than MPPT), wiring, and battery charge acceptance; 75% is a widely used planning midpoint.
- Nameplate watts = Standard Test Conditions; real-world flat-mounted panels rarely hit them.
Frequently asked questions
Why do you assume only 75% system efficiency?
Every step between panel and stored battery costs power: panel heat and haze (panels rarely produce nameplate watts), controller losses, wiring, and charging inefficiency inside the battery itself. Multiply them and 70–80% of nameplate is realistic for a healthy RV install; 75% is a fair middle.
How many sun hours should I use?
"Full-sun equivalent hours" per day, not daylight hours. Rough US values: 4 h in winter, 5–6 h in summer for most of the country; less with shading, clouds, or a flat-mounted panel far from tilt-optimal. When in doubt, use 4 — oversizing beats disappointment.
Will solar recharge my batteries on a cloudy day?
Partially. Heavy overcast can cut output to 10–25% of rating. Solar planning works best as a multi-day average: cover your average daily use with panel wattage, and keep the generator or alternator as the backup for the run of grey days.
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