SunReckon → Solar Array Sizing Calculator
Solar Array Sizing Calculator
Find the watts of solar panel — and how many panels — you need to refill your battery bank by sundown, from your daily load, peak sun hours, system derate, and panel wattage.
Solar & off-grid explainer
Run the numbers first, then read the why. Start with the calculator below — the example values are pre-filled so you can see how it behaves — then keep scrolling for the method, a worked example, and the questions that trip people up. Everything runs in your browser; nothing you type is stored.
Calculator
Key takeaways
- Array watts = daily Wh ÷ peak sun hours ÷ (derate ÷ 100).
- Panels = array watts ÷ panel wattage, rounded up to whole modules.
- Use a 75% derate to cover wiring, controller, heat, and battery round-trip losses.
- 3,000 Wh/day at 4.5 sun hours and 75% derate = a ~889 W array (three 400 W panels).
How to size a solar array
Solar array sizing answers one question: how many watts of panel do you need to put back the energy you used each day before the sun goes down? It starts from an honest daily load in watt-hours, then accounts for how much usable sun your site gets (peak sun hours) and the losses every real system carries (the derate factor). Panel wattage only decides how many modules add up to that array size.
The derate factor is the step most beginners skip. Panels are rated under lab conditions, but heat, dust, wiring resistance, charge-controller efficiency, and battery round-trip losses all eat into output. Sizing at roughly 75% efficiency means the array is about a third larger than the raw watt-hour math — enough to actually refill the bank on an average day.
Worked example: 3,000 Wh/day, 4.5 sun hours, 75% derate
Array watts = 3,000 ÷ 4.5 ÷ 0.75 = ~889 W. With 400 W panels, that's 889 ÷ 400 = 2.2, rounded up to three 400 W panels (1,200 W installed, leaving healthy margin). Drop to 100 W panels and the array watts stay the same, but you'd wire in nine modules instead of three.
Array watts for common daily loads
| Daily load | Array watts needed | 400 W panels |
|---|---|---|
| 1 kWh/day | ~296 W | 1 panel |
| 2 kWh/day | ~593 W | 2 panels |
| 3 kWh/day | ~889 W | 3 panels |
| 5 kWh/day | ~1,481 W | 4 panels |
All figures assume 4.5 peak sun hours and a 75% derate. Sites with fewer winter sun hours, or a lower derate, will need more panel watts for the same daily load.
Start with the load, then size the bank
Good array sizing begins with a real number for daily use — total your appliances with the off-grid load calculator before touching panel counts. Once the array is sized for the average refill, make sure the storage can carry you through cloudy days with the battery bank sizing calculator. The array handles the average day; the bank handles the bad ones.
Frequently asked questions
How many solar panels do I need for off-grid?
Daily Wh ÷ peak sun hours ÷ derate = array watts; ÷ panel wattage, rounded up = panels. 3,000 Wh/day at 4.5 sun hours and 75% derate ≈ 889 W, or three 400 W panels.
What are peak sun hours?
Hours per day that sunlight averages 1,000 W/m² — the rating condition for panels. At 4.5 peak sun hours, a 400 W panel makes roughly 1,800 Wh before losses.
Why is there a derate factor?
Real systems lose 20–30% to wiring, controllers, battery round-trip, dust, and heat. A 75% derate sizes the array about a third larger so it still refills the bank.
How does panel wattage change the count?
It only changes module count, not total array watts. An 889 W array is three 400 W panels or nine 100 W panels — same watts, more modules and wiring.
Does this cover cloudy days?
No — it sizes for an average day. Cloudy-day resilience comes from battery days of autonomy or a generator, not from a bigger array alone.
Does AC or DC coupling change the sizing?
The array-watt math is the same; only the derate shifts. DC-coupled off-grid setups often use 70–80%; a tighter AC-coupled system may run a bit higher.
Peak-sun-hours and derate guidance follows the U.S. National Renewable Energy Laboratory's modeling — see NREL PVWatts. The watt-hour, derate, and panel-count arithmetic here is exact.
Last reviewed June 2026