Solar Sizing Calculator
Size a complete off-grid solar system — panel array, battery bank, charge controller and inverter — from your real daily energy use and location.
1 · Your daily energy use
Tick what you run, adjust watts and hours per day. This drives everything else.
| Appliance | Watts | Hours/day | Wh/day |
|---|
Total daily energy use 0 Wh/day
2 · Your location & system
Rough guide: NZ/AU sunnier regions 4–5, UK/Europe 2.5–3.5, US southwest 5–6.
How many cloudy days your battery should cover.
Everything that might run at once — this sizes the inverter.
Your system
Live sizing — updates as you type.
- Solar array
- — W
- Battery bank
- — Ah
- Charge controller
- — A
- MPPT recommended
- Inverter
- — W
- Pure sine wave, continuous rating
Pro Have a licensed electrician verify any final design before installation.
How this calculator works
Off-grid solar has four sized components — panels, battery bank, charge controller and inverter. Get any one wrong and the system underperforms or dies early. This tool uses the standard engineering approach used by professional solar designers:
- Panel array:
daily Wh ÷ (peak sun hours × 0.75 derate) × 1.25. The 0.75 accounts for real-world losses (panel temperature, angle, wiring, controller inefficiency); the 1.25 is a safety margin. - Battery bank:
daily Wh × autonomy days ÷ (system volts × usable DoD × 0.9). Usable depth of discharge is 50% for lead-acid and 85% for LiFePO₄; 0.9 accounts for inverter and wiring losses. - Charge controller:
array watts ÷ system volts × 1.25, rounded up to a standard rating (30/40/60/80/100 A). - Inverter: your largest simultaneous load × 1.25, rounded up — plus enough surge headroom for motors (fridges, pumps draw 3–5× briefly on start).
Key assumptions & caveats
- Appliance wattages are averages — measure yours with a plug-in watt meter for real accuracy.
- Peak sun hours vary by season; size for your worst month, not the annual average, if you need year-round power.
- Above roughly a 1,500 W array or 2,000 W inverter, step up from 12 V to 24 V or 48 V to keep currents (and cable sizes) sensible.
- Electric heating, cooking and cooling are brutal on batteries — prefer gas, wood or efficient heat-pumps for those loads.
Sources & further reading
- How to Size an Off-Grid Solar System (our full guide)
- Batteries for Off-Grid: chemistry and sizing
- NREL PVWatts for location-specific sun hours (pvwatts.nrel.gov)