Solar Panel Sizing Calculator

☀️ Solar Panel Sizing Calculator

Required array size and panel count from your daily energy use.

Daily energy use (kWh/day)

Peak sun hours (h/day)

System losses (%)

Panel wattage (W)

Required Array

—

Number of Panels

—

Performance Ratio

—

Roof Area (≈)

—

⚠️ Array kW = daily kWh ÷ (sun hours × performance ratio). Peak sun hours vary by location/season; performance ratio 0.75–0.8 typical. Confirm with local irradiance data and a shading study. Verify before professional use.

Standards & method

✓ Independently verified 12 July 2026

Governing standard

NEC (NFPA 70) Article 690 · IEC 62548

Clauses applied

NEC 690.8(A)(1) — PV source circuit current at 125% of Isc · 690.7 maximum voltage with low-temperature correction · IEC 62548 array design

Core formula

Array kW = daily kWh / (peak-sun-hours × system losses) · I_design = 1.25 × Isc

Why this matters

PV circuit conductors are sized at 125% of short-circuit current (690.8), and string voltage must be checked at the coldest expected temperature — Voc RISES as temperature falls, and that is what over-volts the inverter.

Independently verified

12 July 2026 — Re-derived from the governing standard and checked numerically against worked reference cases from the standard itself — not merely tested for “returns a number”.

Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.

Solar Array Sizing — Method

Size the array from energy, not power: take your average daily consumption (kWh/day), divide by the location peak sun hours, and divide again by a performance ratio that captures inverter, temperature, wiring and soiling losses (typically 0.75–0.8). That gives the DC array kW. Divide by the panel wattage and round up for the panel count. For grid-tie, also check export limits; for off-grid, pair with battery and inverter sizing.

Frequently Asked Questions

How many solar panels do I need?⌄

Array kW = daily kWh divided by (peak sun hours x performance ratio, ~0.77). Then panels = array watts divided by panel watts, rounded up. For 10 kWh/day at 5 sun hours that is about 2.6 kW, or seven 400 W panels.

What are peak sun hours?⌄

The equivalent number of hours per day at 1000 W per square metre. It varies by location and season; typical annual averages are 3-6 hours. Use local irradiance data for accuracy.

What is a performance ratio?⌄

The fraction of theoretical output actually delivered after inverter, temperature, wiring and soiling losses, typically 0.75-0.8. Lower it for hot climates or shaded/soiled arrays.

Should I oversize the array?⌄

A small oversize (DC/AC ratio 1.1-1.3) captures more energy in poor light and offsets degradation, but check inverter clipping and any export limit.

Does this work for off-grid systems?⌄

It sizes the array for daily energy. Off-grid systems also need battery autonomy and inverter sizing, which you can do with the battery bank and inverter calculators.

Solar Array Sizing Explained

A solar array is sized to replace energy, so the starting point is your daily kWh, not your peak watts. Dividing by peak sun hours converts energy to power, and the performance ratio accounts for the real-world losses between the panel nameplate and useful output.

From energy to panels

Daily kWh ÷ (sun hours × PR) gives the DC kW the array must be. Divide by the module wattage for the count, rounding up. Hotter sites and shaded roofs need a lower PR and therefore more panels.

Next steps

For a complete design, pair this with battery bank sizing, inverter sizing and PV string sizing, plus DC cable sizing.

Related: Battery Bank, Inverter Sizing, PV String.

Solar Panel Sizing Calculator

Solar panel / array sizing — Quick answer