When the sun does not shine for several days in a row — a common scenario in winter or during an overcast stretch — your battery bank alone carries the load. Days of autonomy tells you the worst-case answer to the question “how many days can I run before the bank is depleted?” This calculator divides usable battery energy by daily consumption and shows the state of charge falling day by day so you can see exactly when you would run out.
How it works
The core calculation involves two ideas: usable energy and daily load.
Usable energy is not the same as nameplate capacity. Most battery chemistries cannot be discharged to zero without damage, so only the fraction the depth-of-discharge (DoD) allows is actually available:
Usable Wh = total Ah × system voltage × DoD fractionFor a lead-acid bank you typically limit DoD to 50%, while lithium iron phosphate (LiFePO4) can safely discharge to 80–90% DoD, roughly doubling the usable energy per nominal amp-hour.
Days of autonomy is usable energy divided by the daily energy demand:
Days of autonomy = Usable Wh ÷ daily load WhThe table the tool produces shows the remaining state of charge (SoC) after each full day of constant load, so you can see how quickly the bank depletes and at what point you would need solar or grid input.
Worked example
A 300 Ah, 48 V lithium bank at 80% DoD:
- Usable Wh = 300 × 48 × 0.80 = 11,520 Wh
- Daily load (including 10% inverter losses on AC appliances) = 3,000 Wh/day
- Days of autonomy = 11,520 ÷ 3,000 = 3.84 days
| Day | SoC remaining |
|---|---|
| Start | 100% |
| Day 1 | 74% |
| Day 2 | 48% |
| Day 3 | 22% |
| Day 3.84 | 0% (depleted) |
If the same bank were lead-acid at 50% DoD, usable energy would fall to 7,200 Wh — only 2.4 days of autonomy for the same load.
Planning guidance
- Size for the worst consecutive cloudy-day stretch in your location, not the average. Three to five days is common for most temperate regions.
- Include inverter losses (typically 5–15%) when calculating daily load for any AC appliances. A 2,700 W AC load through a 90% efficient inverter draws 3,000 Wh from the battery.
- Whole days are your safe limit: treat the fractional remainder as a reserve buffer, not as safe operating time. Running a lead-acid bank below 50% DoD shortens its life significantly with each cycle.
- Lithium chemistry extends autonomy dramatically for the same physical bank size, mainly because the usable DoD is nearly double that of lead-acid.