What size solar system do I need?
Array watts, battery kWh, and inverter/charge-controller pairing — sized for off-grid reality or grid-tie economics.
The whole system, not just panels
A solar system is four numbers, and they're sized in this order:
- Daily energy need (Wh/day) — list each load: watts × hours per day. This drives everything else.
- Array size (W) = daily Wh ÷ worst-month sun hours ÷ 0.86 (NREL PVWatts' default 14% system losses). See the panel-count math for the breakdown.
- Battery bank (Wh) = daily Wh × autonomy days ÷ the depth of discharge selected in the calculator (80% or 50%).
- Inverter (W) — the largest simultaneous load, not the daily total. The inverter tab multiplies continuous watts by 1.25 and applies a surge factor of 1, 3, 4, or 2.
Grid-tie skips the battery row entirely: array = annual kWh ÷ 365 ÷ sun hours ÷ 0.86, and the grid is your "battery."
A monthly bill worked as its own question: what size solar system for 1,000 kWh per month.
Worked example: full-time off-grid cabin, 6 kWh/day, 4.0 winter sun hours
| Component | Math | Result |
|---|---|---|
| Array | 6,000 ÷ 4.0 ÷ 0.86 | ~1,750 W (5×400 W) |
| Battery bank | 6,000 × 2 days ÷ 0.80 | 15,000 Wh |
| Inverter | largest simultaneous load, then × 1.25 | the load you measure |
| Charge controller | 1,750 ÷ 48 ÷ 0.85 | ~43 A → 60 A MPPT |
The same house grid-tied: that annual total averages back to 6,000 Wh/day; ÷ 4.8 average sun hours ÷ 0.86 ≈ 1,450 W of array, no battery, no inverter beyond a grid-tie unit — about 4 panels. Off-grid costs more because it must survive December; grid-tie only needs to win the year.
Off-grid vs grid-tie: what changes
- Sizing target: off-grid = worst month; grid-tie = annual average or bill offset.
- Battery: off-grid uses the autonomy days you type in. Grid-tie can skip the bank.
- Inverter: off-grid uses the surge factors in the inverter tab (1×, 3×, 4×, or 2×). Grid-tie has to cover the array's AC output.
- Backup: off-grid systems should plan a generator input for long clouds — see GeneratorSizer for that half of the equation.
Inverter & charge-controller pairing
Check the controller's maximum PV voltage against the array's cold open-circuit voltage, and its maximum charge current against array watts ÷ bank volts ÷ 0.85, rounded up to the next listed size. Keep array watts within the inverter's maximum PV input. The cold-voltage factor for one module family is the Canadian Solar manual's Table 1, walked through on how to fuse a solar panel array.
Common sizing mistakes
- Using annual-average sun hours for an off-grid build (December eats the difference).
- Sizing the inverter to daily kWh instead of simultaneous watts.
- Forgetting inverter idle draw. Read that wattage on the inverter's own spec sheet and add it to the daily load.
- Buying panels before the battery bank is sized; the bank sets charge current limits the array must respect.
The cell and the inverter, before the watt-hour formula, are on how solar panels work. What a median degradation rate does to an array that landed on an exact wattage is on solar panel degradation. Turning the bank watt-hours into a battery count is on how many solar batteries do I need.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- NREL PVWatts Version 5 Manual, A. Dobos, September 4, 2014 — Table 6, 14% default system losses; 1 − 0.14 = 0.86. Accessed Oct 3, 2026.
- SolarSizer calculator, assets/app.js (sizeController) — array watts ÷ bank volts ÷ 0.85, then the first of 10, 20, 30, 40, 60, 80, and 100 A at or above that current. The row 1,750 ÷ 48 ÷ 0.85 is about 42.9 A, so the listed size is 60 A. Accessed Oct 3, 2026.
The array rows use the 0.86 derate: 6,000 ÷ 4.0 ÷ 0.86 = 1,744.2 W, printed as ~1,750 W, and 6,000 ÷ 4.8 ÷ 0.86 = 1,453.5 W, printed as ≈ 1,450 W. The bank row uses the calculator's 80% option: 6,000 × 2 ÷ 0.8 = 15,000 Wh. The 6,000 Wh day is this page's input to that function.