Solar Panel Sizing Calculator

Solar panel sizing answers one practical question: how much array wattage—and how many modules—you need to replace the energy your loads use each day. The inputs are daily watt-hours (from your load list or utility data), peak sun hours for your site, and the rated watts of the panels you plan to buy.

Panels rarely power appliances directly in off-grid or hybrid setups. They recharge a battery bank (or feed the grid) through a charge controller or inverter. This calculator estimates array size so that, on an average day with your stated sun hours, generation can cover daily consumption after typical wiring and conversion losses.

Use the result to compare panel counts (e.g. 300 W vs 400 W modules) and to sanity-check a larger off-grid design before you size batteries and inverters.

How this calculator works

1. Enter daily energy in watt-hours (Wh)—the sum of each load’s watts × hours per day, or a whole-home daily total.
2. Set peak sun hours: equivalent full-sun hours for your location and season (often 3–6 in winter, 5–7 in summer in temperate climates).
3. Enter the rated wattage of one panel (e.g. 400 W). The tool uses this to convert required array watts into a module count.
4. The calculator applies a 1.25 loss factor for cable drop, temperature, MPPT efficiency, and dust, then divides by peak sun hours to get required solar watts.
5. Panel count is solar watts ÷ panel watts, rounded up to a whole number of modules.

Core sizing relationships

Solar W = (Daily Wh × 1.25) ÷ peak sun hours | Panel count = ⌈Solar W ÷ panel W⌉

Daily Wh is your energy budget. Multiplying by 1.25 builds in roughly 25% overhead for real-world losses so the array still meets demand on average days. Dividing by peak sun hours converts daily energy into the average power the array must produce during sun hours. Panel count simply splits that total into commercially available module sizes.

Formulas used by this tool

• Solar watts = (daily Wh × 1.25) ÷ peak sun hours.
• 1.25 accounts for system losses (cable, heat, MPPT, etc.).
• Panel count = solar watts ÷ rated panel watts, rounded up.

Worked example: 3 kWh per day

A small cabin uses 3,000 Wh per day, you expect 5 peak sun hours, and you plan to install 400 W panels.

1. Solar watts = (3,000 × 1.25) ÷ 5 = 750 W of array.
2. Panel count = 750 ÷ 400 = 1.875 → round up to 2 panels (800 W nameplate).
3. The extra 50 W of nameplate capacity helps on hazy days and as modules age.
4. If winter sun drops to 3 peak hours, rerun with 3 h—you may need three or four panels for the same daily load.

Practical tips

• Size for the month with the shortest sun and highest loads, not summer peak production alone.
• If loads will grow (EV, heat pump, workshop), add headroom now or plan spare MPPT input capacity.
• Match array voltage to your charge controller’s maximum input and battery bank design.
• Check physical roof or ground space—panel count × dimensions must fit the site.
• For off-grid systems, pair this with battery and inverter sizing; panels alone do not cover night loads.

Frequently asked questions

Related guides

• How Many Solar Panels to Run Your Appliances: A Practical Guide