Off-Grid Solar Calculators

Solar panels

Solar Array Sizer

Find how many solar panels cover a daily watt-hour load. Harvest per panel = panel watts × peak sun hours × derate (typically 0.75). Peak sun hours are equivalent hours at 1,000 W/m², not clock hours of daylight. Use winter PSH for year-round cabins.

Inputs

Wh
W
h

Winter-equivalent PSH is safer for year-round off-grid sizing.

Results

Panels needed
4
Array nameplate
800W
Estimated daily harvest
2,400Wh

Recommended gear

Engineering methodology

Harvest per panel = panel watts × peak sun hours × system derate. Panel count = ceil(daily watt-hours ÷ harvest per panel). Array watts = panel count × panel watts. Peak sun hours are equivalent hours at 1,000 W/m², not clock hours of daylight. A 0.75–0.80 derate covers soiling, temperature, wiring, controller and angle versus STC.

Peak sun hours are not daylight hours

Peak sun hours (PSH) are equivalent hours at 1,000 W/m². A 14-hour summer day can still be 4 PSH. Harvest per panel = panel watts × PSH × derate (often 0.75). Use a winter PSH if the cabin must run year-round.

From daily watt-hours to panel count

Panel count is ceil(daily Wh ÷ (panel W × PSH × derate)). A 200 W module at 4 PSH and 0.75 derate harvests 600 Wh, so 2,000 Wh/day needs four modules. Then size the MPPT from array watts ÷ battery volts.

Frequently asked questions

What are peak sun hours if I have 12 hours of daylight?

Peak sun hours (PSH) are not daylight hours. They are the equivalent number of hours at 1,000 W/m² irradiance. A 5 PSH day might still have 12 hours of weak light. Use winter-equivalent PSH for year-round off-grid sizing.

Why derate a 400 W panel to 75%?

STC ratings are 25 °C cell temperature and 1,000 W/m². Heat, dirt, wiring, mismatch and charge-controller losses typically leave about 70–80% of nameplate over a day. 0.75 is a common off-grid planning factor, not a lab measurement of your roof.

Can I use this instead of a shading study?

No. The formula assumes the entered PSH already includes climate. Trees, vents and seasonal sun angles need a separate shade assessment.

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