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Solar Panel Size Calculator

How many watts of panel you need from your daily energy and your worst-month sun — with a real-world derate, not lab numbers.

energy balance + derate · free · no account

Your energy

Array you need

Array size
— W
Panels
—
Energy balance: daily Wh ÷ (sun-hours × derate). Winter sun is far lower than summer — size for your worst month.
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Panels are one piece.

The designer sizes the array against your loads and climate, matches the MPPT, battery and cabling, and draws the wiring.

Design my system — free

No account needed to design. Same engine, same standards as this calculator.

How array sizing works

Required watts = daily Wh ÷ (peak sun-hours × system derate). The derate (~0.72) accounts for real-world losses lab ratings ignore. Crucially, size for your worst month: winter sun can be a third of summer.

Questions

What is a derate?

A factor (~0.72) for real-world losses — heat, wiring, MPPT and charge efficiency — that panel nameplate ratings ignore. Using it stops you under-sizing.

Design your whole system — free

About a minute. No account, no email.

Planning aid — not a substitute for a licensed electrician. Wattonomy sizes to ABYC E-11 / ISO 13297 / BS 7671 / AS/NZS 3008, but every install has factors a calculator can't see. Verify against the hardware datasheets and your local code, and have the work checked by a professional. Improper electrical work can cause fire, injury or death.