Solar Sizing
Solar Array Sizing Calculator
Size the array and panel count for a target daily energy need
Updated July 12, 2026 · Live
What this tool does
Turns a daily energy target into an array size in kW and a panel count, allowing for your local sun hours and system losses.
Enter the daily kWh you need, your peak sun hours, a losses allowance and the panel wattage; the calculator returns the array size and how many panels it takes.
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Formula Used
How the solar array sizing works
The array size is your daily energy need divided by the usable sun — peak sun hours multiplied by one minus the losses fraction. That gives the kW of panels that would, on an average day, generate the energy you use. Dividing the array watts by a single panel’s wattage and rounding up gives the panel count, and multiplying the count back by the panel wattage shows the capacity you actually install.
What you enter
Enter the daily kWh you need, your peak sun hours, a losses allowance and the panel wattage; the calculator returns the array size and how many panels it takes. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
The array size drives every downstream choice — inverter rating, charge-controller current and roof area. Peak sun hours vary by location — typical daily figures are 2–3 in northern Europe, 4–5 in southern Europe and much of the US, and 5–6 in India and the Middle East. Use your own local value.
Safe by design
This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.
Assumptions and limits
Nothing time-sensitive is baked in. Electricity prices, export tariffs and local peak sun hours all change and differ by location, so the calculator asks you to enter your own figures rather than embedding a database that would silently go stale. Peak sun hours are a long-run average; a run of dull days is covered by battery autonomy, not by the array alone. The losses figure is a single lumped allowance — hot roofs, soiling and long cable runs push it higher.
Using it with the rest of BuildMetricLab
This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Turns a daily energy target into an array size in kW and a panel count, allowing for your local sun hours and system losses. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
What are peak sun hours?
Peak sun hours are the equivalent hours per day of full 1000 W/m² sunlight. In the US they typically range from about 3-4 h/day in the Pacific Northwest and Northeast to 5-6+ h/day in the Southwest (e.g. Arizona, New Mexico). Use your own local value or a solar resource map such as NREL's.
Why round the panel count up?
Panels come in whole units, so any fractional requirement rounds up to the next whole panel. The result shows the installed capacity that whole number gives you.
What losses figure should I use?
A combined 15–25% covers wiring, temperature, soiling and inverter conversion for most roof-mounted systems. Raise it for hot climates, long cable runs or heavy soiling.
Does a bigger array always help?
Only up to the point your battery and inverter can absorb it. Size the array to the load and autonomy you need, then check the inverter and controller tools before buying.
Spotted something off?
Calculations or display — let us know.