Solar Array Sizing Calculator

Frequently Asked Questions

How do I size a solar array from my daily kWh use?

Array kW ≈ daily kWh ÷ peak sun hours ÷ system derate (≈0.77 for inverter, wiring, soiling, mismatch). A home using 30 kWh/day at 5 peak sun hours needs 30 ÷ 5 ÷ 0.77 ≈ 7.8 kW DC.

How many panels does that translate to?

Panel count = array watts ÷ panel watts. A 7,800 W array using 400 W modules is 7,800 ÷ 400 = 19.5 → 20 panels, plus a couple extra if you want margin for future EV or heat-pump loads.

Where do I find my peak sun hours?

NREL's PVWatts and the NSRDB give monthly kWh/m²/day for any U.S. location - that number is your peak sun hours. Most of the continental U.S. falls between 3.5 (Pacific NW winter) and 6.5 (desert SW summer).

Why use a 0.77 derate factor?

Real systems lose energy to inverter efficiency (~96%), wiring (~98%), soiling (~95%), temperature (~92%), and mismatch (~98%). Multiplying those together lands near 0.77, which the industry uses as a default unless a detailed loss model is run.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Sizing a solar array is a backwards walk from the kilowatt-hours your loads consume each day to the nameplate kilowatts of panels needed on the roof. Panels are rated in DC watts under laboratory STC conditions (1,000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum) - real installations never see those numbers for long. The location's peak sun hours (PSH) compress a day's irradiance into an equivalent number of hours at 1,000 W/m², and a derate factor (typically 0.75–0.80) accounts for inverter losses, wiring resistance, soiling, panel mismatch, temperature, and module degradation. This tool divides daily kWh demand by PSH and by derate to get array kW, then divides by per-panel watts to give a panel count rounded up to the next whole module.

Formula / Method

Array kW = daily kWh ÷ peak sun hours ÷ derate factor. Array W = array kW × 1000. Panel count = ⌈ array W ÷ per-panel watts ⌉. The 0.77 default derate is the NREL PVWatts industry benchmark: roughly 96% inverter efficiency × 98% DC wiring × 97% AC wiring × 97% mismatch × 95% soiling × 99% availability ≈ 0.77, give or take site specifics. Lower the derate to 0.70 for dirty, hot, or shaded sites; raise it to 0.82 only for very clean, cool, well-ventilated installations. Always size upward to whole panels - you cannot install 8.4 modules.

Worked Example

A home consumes 20 kWh/day in a region with 4.5 peak sun hours and a 0.77 derate. Array kW = 20 ÷ 4.5 ÷ 0.77 ≈ 5.77 kW DC. With 400 W panels: 5,770 ÷ 400 = 14.4, rounded up to 15 panels, giving 6.0 kW installed. Expected production = 6.0 × 4.5 × 0.77 ≈ 20.8 kWh/day - a small headroom over the 20 kWh need, which is sensible because panels degrade roughly 0.5%/year and PSH varies with season. In December at 3.2 PSH that same array still produces 6.0 × 3.2 × 0.77 ≈ 14.8 kWh/day, so winter shortfalls are made up by grid imports or battery autonomy.

Common Mistakes

Using annual average PSH and assuming year-round coverage - winter PSH may be half the average and dominates off-grid sizing.

Forgetting the derate factor entirely - the array will produce 20–25% less than nameplate × PSH suggests.

Confusing AC output with DC nameplate: a 6 kW DC array typically pairs with a 5 kW AC inverter (DC:AC ratio ≈ 1.2).

Ignoring the inverter's minimum-string voltage in cold weather - panel V_oc rises as temperature falls and can exceed inverter limits.

Sizing only for current loads while planning an EV or heat pump that will double consumption within a year.

Tips & FAQ

What PSH does my site get? NREL's PVWatts and the Global Solar Atlas give monthly PSH for any address - expect 3.5–4.5 in the northern US, 5–6 in the Southwest, 4–5 in central Europe. Why 0.77 and not 0.85? PVWatts shipped at 0.77 for over a decade because real field data clustered there; modern inverters earn back maybe 2–3 points but soiling and degradation eat the gain over a 25-year life. Do I need batteries? Grid-tied net-metered systems size for annual kWh because the grid acts as the "battery"; off-grid systems must size for the worst month plus 30–50% reserve and pair with sufficient battery autonomy. Roof area: a 400 W panel occupies about 21 sq ft - a 6 kW array (15 panels) wants ≈ 315 sq ft of clear, south-facing roof. Orientation loss: due-south at latitude tilt is optimal; east/west costs 10–20%, north 40%+. String vs. micro: microinverters cost more but tolerate shading and per-panel failures - consider them where any shade hits the array. Permitting: arrays over 10 kW often trigger utility interconnection studies; many homeowners size at 9.99 kW to stay below the threshold. These figures are planning estimates - commission a site survey and shading study before purchase, and confirm local interconnection rules with your utility.

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