Off-Grid System Size Calculator
Off-Grid Sizing Has No Utility Backstop to Fall Back On
A grid-tied solar system can undersize the array and simply draw the shortfall from the grid; an off-grid system has no such safety net. Both halves of the system — the panels that generate power and the battery that stores it — have to be sized correctly together, with the battery covering not just overnight use but a stretch of cloudy days with reduced solar production.
The Formula
Battery Capacity (Wh) = (Daily Load × Days of Autonomy) / Depth of Discharge
Days of autonomy is how many days the battery bank needs to cover the full load with no solar input at all — a buffer against extended cloudy weather. System derate accounts for real-world losses in the panel-to-load path; battery depth of discharge limits how much of the rated capacity is actually used, protecting battery life.
Where This Is Useful
- Cabin and remote property systems — size a complete standalone system for a property with no grid connection at all.
- RV and marine systems — work out panel and battery requirements for a mobile off-grid setup based on actual appliance loads.
- Disaster resilience planning — size a standalone backup system independent of grid-tied net metering considerations.
Worked Example
A daily load of 6,000 Wh, 5 peak sun hours, 3 days of autonomy, 80% system derate, and 50% battery depth of discharge:
| Daily load | Sun hours | Days of autonomy | Required panel size | Required battery capacity |
|---|---|---|---|---|
| 6,000 Wh | 5 h | 3 | 1,500 W | 36,000 Wh |
How to Use This Calculator
- Enter the Daily Load in watt-hours — total energy the system needs to supply per day.
- Enter the Peak Sun Hours per Day for the site.
- Optionally enter Days of Autonomy — defaults to 2 if left blank.
- Optionally enter System Derate % — defaults to 80% if left blank.
- Optionally enter Battery Depth of Discharge % — defaults to 50% if left blank.
- Select Calculate to get the required panel wattage and battery capacity.
Related Calculations
Check the resulting panel array's real output with the Solar Panel Output Calculator, or price out the battery portion with the Energy Storage Cost Calculator.