How this instrument works
Sizing a solar system starts from the opposite direction of estimating a fixed array's output: instead of asking 'how much power will this array produce,' it asks 'how big an array do I need to meet a known energy demand.' That demand — daily kilowatt-hours, from a utility bill or an off-grid appliance tally — is the fixed target the array has to be sized around.
The other key input is peak sun hours, a way of compressing a full day's varying sunlight into a single number: the equivalent number of hours the sun would need to shine at full standard-test-condition intensity (1,000 W/m²) to deliver the same total daily energy a location actually receives. A sunny southwestern US location might average 5-6 peak sun hours per day; a cloudier northern location might average 3-4, meaning it needs a proportionally bigger array to collect the same daily energy.
System efficiency accounts for everything between a panel's rated output and what actually reaches a battery or the grid — inverter losses, wiring resistance, and typical soiling and shading — so the required wattage this calculator returns is realistically sized for actual delivered energy, not an idealized number that undersizes the system once real-world losses are accounted for.
- Enter Daily energy needed (kWh) — from a utility bill's daily average, or a tally of appliance wattages times hours of use per day.
- Enter Peak sun hours per day — a location-specific average; solar insolation maps and calculators publish this for most regions.
- Enter System efficiency (0-1) — covers inverter, wiring and soiling losses; 0.75-0.80 is a typical planning estimate.
- Read Solar array wattage needed (W) — the panel capacity required to meet your daily energy target under these conditions.
Worked example — 10 kWh/day at 4.5 peak sun hours
A household needs 10 kWh of energy per day, lives somewhere averaging 4.5 peak sun hours, and plans for a 75% system efficiency to account for inverter and wiring losses. Wattage needed = (10 × 1000) / (4.5 × 0.75) = 10,000 / 3.375 ≈ 2,962.96 W.
That figure, just under 3,000 W of panel capacity, is what this household would need to install to reliably cover its 10 kWh daily usage given its local sun exposure and realistic system losses — installing meaningfully less than that would leave the system unable to fully meet daily demand on an average day.
Questions
What are 'peak sun hours' and how do I find mine?
Peak sun hours compress a full day of varying sunlight intensity into the equivalent number of hours at a standard 1,000 W/m² intensity that would deliver the same total daily solar energy. Government solar resource maps (like NREL's solar resource data in the US) and many solar sizing tools publish average peak sun hours by location, typically ranging from around 3 hours in cloudier northern regions to 6+ hours in sunny desert regions.
Why is the wattage needed always bigger than my daily kWh number times 1000?
Because peak sun hours are almost always fewer than 24, and system efficiency is always less than 1 (100%), both terms in the denominator shrink the result below what daily kWh alone would suggest, meaning the required panel wattage always exceeds what a naive 'daily kWh divided by 24 hours' calculation would give. This is intentional — panels don't produce at full rated wattage for a full 24 hours, only for the equivalent of a few peak sun hours.
How do I calculate my daily energy needed in kWh?
Add up each appliance's wattage multiplied by hours of daily use, divided by 1000 to convert watts to kilowatts, then sum across everything you plan to power — or simply take your average daily usage from a utility bill (monthly kWh divided by roughly 30). Off-grid planning usually benefits from tallying appliances directly, since it reveals which loads dominate demand.
Should I size the system for average or worst-case sun conditions?
This depends on how much reliability you need — sizing for an average peak-sun-hours figure means the system meets demand on a typical day but may fall short on cloudy runs, while sizing for a more conservative (lower) sun-hours figure, or adding battery storage, provides more buffer for consecutive low-sun days. Off-grid systems intended for full independence typically size more conservatively than grid-tied systems that can draw from the grid as backup.
Does this account for battery storage sizing?
No — this calculates only the solar array wattage needed to generate enough energy across a typical day, not how much battery capacity is needed to store and time-shift that energy for use at night or during low-sun periods. Battery sizing is a related but separate calculation that depends on how much of your usage happens outside sunlight hours and how many days of autonomy you want to plan for.