HesapHanem

Solar Battery Autonomy Calculator

Enter your battery bank size and daily energy use to see how many days of autonomy you have.

How it is calculated

The tool first turns the battery bank into energy: usable Wh = Ah × V × DoD/100 × η/100, where Ah is the bank capacity in amp-hours, V the bank voltage, DoD the share of capacity you allow yourself to drain (percent) and η the efficiency of the battery plus inverter path (percent). Then days = usable Wh ÷ daily load (Wh/day) and hours = days × 24. Both DoD and efficiency must be above 0 and at most 100, and capacity, voltage and load must be positive, otherwise no result is shown. The daily load is a total in watt-hours per day, not the power of a single appliance in watts. Panels are ignored: this is the no-sun case.

Worked example

A 200 Ah, 24 V bank with 80 % depth of discharge and 90 % efficiency, feeding a 1,500 Wh/day load. Nominal energy is 200 × 24 = 4,800 Wh. Allowed discharge: 4,800 × 0.80 = 3,840 Wh. After efficiency losses: 3,840 × 0.90 = 3,456 Wh usable. Autonomy is 3,456 ÷ 1,500 = 2.30 days, which is 55.3 hours with no sun at all.

Try this example →

Common mistakes

  • Entering the load in watts (appliance rating) instead of watt-hours per day; multiply watts by hours of use first, and add up every appliance.
  • Typing the total of several batteries in series as if they were in parallel: series adds voltage, parallel adds amp-hours, so enter the combined bank values, not one battery's.
  • Using 100 % depth of discharge for lead-acid or any battery whose maker limits it; the result then overstates how long the bank really lasts.
  • Leaving out inverter and wiring losses (efficiency 100 %), which makes autonomy look longer than it is.
  • Reading the answer as sunny-day runtime: it assumes zero solar input, so on partly sunny days the bank lasts longer.

About this calculator

Battery autonomy is how many days an off-grid or backup solar system can power a home's loads with zero sun input, calculated as usable battery capacity (Wh) ÷ average daily energy use (Wh/day), where usable capacity already accounts for the battery's depth-of-discharge limit — for example, a 10 kWh lithium bank usable to 90% DoD provides 9,000 Wh of usable energy, not the full 10,000 Wh.

Solar installers and off-grid homeowners size battery banks around this number because autonomy is what protects a home through cloudy stretches, not the panel array — a system with plenty of panel capacity but a small battery bank can still run out of power on the third overcast day in a row, and most off-grid designs target 1 to 3 days of autonomy depending on climate and how critical continuous power is.

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