Solar & Battery Bank Calculator
Size an off-grid solar array and battery bank based on your daily energy use and days of autonomy.
Your system size
- Daily energy use
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- Battery bank needed (50% depth of discharge)
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- Solar array needed
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How this calculator works
Daily energy use is your connected load in watts times the hours you actually run it (Wh/day). The battery bank is sized to cover that usage for your chosen number of cloudy/no-sun days, then inflated by 2x because lead-acid and most off-grid battery setups shouldn't be discharged past 50% depth of discharge regularly without shortening their lifespan — that "usable half" is why the bank is sized at double the raw energy need. The solar array is sized to fully recharge one day's usage within your area's peak sun hours, with a 20% derate for real-world losses (wiring, temperature, panel angle, dust).
Where these numbers come from
The 50% depth-of-discharge convention is standard off-grid battery-bank sizing practice for lead-acid batteries (lithium batteries can often go deeper, which would change this figure). The 20% solar derate factor is a commonly used real-world efficiency loss estimate. Peak sun hours vary significantly by location and season — use your area's winter average for a conservative, worst-case system size, not the annual average.
Frequently asked questions
Why size the battery for 50% depth of discharge?
Regularly draining a lead-acid battery past 50% significantly shortens its lifespan, so off-grid systems conventionally size the bank at double the raw energy need, treating only the top half as usable capacity.
Does this apply to lithium batteries too?
Lithium (LiFePO4) batteries typically tolerate deeper discharge (80-100%) without the same lifespan penalty, so a lithium-based system could use a smaller bank than this 50%-DoD estimate — check your specific battery's spec sheet.
Should I use average or winter sun hours?
Winter (or your area's lowest-sun season) peak sun hours give a conservative system size that won't leave you short during the worst months — using the annual average risks undersizing for winter.
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