Solar Array Calculator
Size the array from the meter — daily kilowatt-hours divided by the sun you actually get, derated to honest watts, counted out in panels.
Solar Array Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Flat average model — no tilt, azimuth or shade trace
- Battery refill and inverter clipping not modeled
In short: A household burning 30 kWh a day in a place that banks 4 peak sun hours needs about 9.4 kW of nameplate array — 19 of the 500 W panels, roughly 38 square metres of roof. The derate is the honest part: between inverter losses, soiling, heat and wire, only about eighty cents of every nameplate watt reaches the meter. Sizing to the four-hour months overpanels the summer on purpose; sizing to the year's average buys a dark January.
Formula
kW = kWh ÷ (sun hours × derate) · panels = ⌈kW × 1,000 ÷ W⌉
Peak sun hours are the day's solar energy expressed as hours of full noon: five of them on a 1 kW array is 5 kWh of nameplate output, before the system takes its cuts. Dividing by the derate instead of forgetting it is the difference between an array that works in December and one that only exists in the brochure. The panel count rounds up — roofs install whole panels.
Worked Example
- Enter the daily kWh your meter actually shows.
- Set the peak sun hours your worst month banks.
- Count the panels, then measure the roof.
Defaults: 30 kWh a day, 4 peak sun hours, 80% derate, 500 W panels — a 9.375 kW array, 19 panels rounding up to 9.5 kW installed, about 38 m² of roof. Plan on 5 sun hours instead and it relaxes to 15 panels.
Strengths & Limits Of This Model
Where this engine is strong
- Winter-first sun hours — the honest default
- Derate stated, not hidden in the panel count
Where it stops
- Roof geometry and shading live outside
Practical Use Cases
Off-grid builds
array before battery, always
Bill-killers
daily kWh to roof area
Winter-first sizing
the 4-hour month decides
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Solar Array Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What are peak sun hours?
The day's sunshine stated as hours of full strength. Five peak hours might be eight real hours of daylight — mornings and evenings count fractionally. The annual low month is the honest number to size against.
Why derate by twenty percent?
Because nameplate watts are laboratory sunshine at 25 °C. Inverters lose a few points, heat shaves more, dust and wiring take the rest. Eighty cents on the nameplate watt reaching the meter is a fair field average.
Why does the panel count round up?
Roofs install whole panels. Nineteen panels of 500 W make 9.5 kW against a bare 9.375 need — the sliver of surplus is what whole panels cost you, and it is a bargain against a dark day.
How much roof does an array need?
About two square metres per modern panel, so the default nineteen want roughly 38 m² of unshaded, sensibly oriented roof — plus walkway and service clearance the roof line decides.
Should I size to summer or winter?
Winter. The sun hours input is deliberately the annual low: an array sized to the average quits in November, while a winter-first array just earns a surplus you can sell or store.
Where do batteries fit?
After the array. The battery page sizes storage from the same daily kWh; this page sizes what refills it. Arrays and batteries are bought in that order for a reason — generation first.
Do panels still make power when it clouds over?
Yes, at a fraction — bright overcast delivers maybe a fifth to a third of full sun. That is already inside the derate's spirit, but deep monsoon climates need the worst week, not the worst month.
What if my roof is smaller than the answer?
Then the load shrinks or the efficiency rises: every kWh of daily demand retired — a heat-pump water heater, LED everywhere — shrinks the array by more than a panel. Conservation is the cheapest racking.