Battery Bank Runtime Calculator

How It Works

The linear answer, then the Peukert correction

The first pass treats the bank as a tank:

That is right for lithium and optimistic for lead-acid. A lead-acid Ah rating is quoted at the 20-hour rate, C/20, and the faster you empty it the less you get out. Peukert's law puts a number on that. The tool computes the discharge current at the battery, I = Load ÷ (V × efficiency), and then:

with k = 1.25 for flooded cells, 1.15 for AGM and gel, and 1.02 for LFP, which is close enough to linear to be a formality. At exactly the C/20 rate the bracket is 1 and the two answers agree; the correction is under 3% down to about C/20 and past 15% beyond C/8, which is where the tool starts flagging it.

The low-voltage cutoff, which the tool does not model and you should

Every inverter has a low-voltage disconnect, and the inverter sizing calculator defaults it to 87.5% of nominal: 10.5 V on a 12 V bank, 21 V at 24 V, 42 V at 48 V. Under a heavy load the terminal voltage of a lead-acid bank sags well below its resting voltage, so the inverter can trip on the cutoff with charge still in the cells, then the voltage recovers and it looks like a fault. Lithium holds its voltage almost flat until the end and then falls off a cliff, so on LFP the DoD limit is what you hit; on lead-acid under load it is often the cutoff. Either way, a runtime that ends at the cutoff is shorter than the one above.

Worked example: the same 200 Ah label, two different banks

200 Ah lithium LFP at 48 V, 0.8 DoD, 92% efficiency, carrying a steady 500 W:

• Usable energy = 200 × 48 × 0.8 × 0.92 = 7,066 Wh
• Linear runtime = 7,066 ÷ 500 = 14.1 hours
• Discharge current = 500 ÷ (48 × 0.92) = 11.3 A, a C/17.7 rate, so Peukert at k = 1.02 takes it to 14.1 hours still: a 0.2% correction

200 Ah flooded lead-acid at 12 V, 0.5 DoD, 92% efficiency, the same 500 W:

• Usable energy = 200 × 12 × 0.5 × 0.92 = 1,104 Wh
• Linear runtime = 1,104 ÷ 500 = 2.2 hours
• Discharge current = 500 ÷ (12 × 0.92) = 45.3 A, a C/4.4 rate, so Peukert at k = 1.25 gives 1.5 hours: a 31% penalty on top of a bank that was already a quarter the size

Same number on the label, 14 hours against an hour and a half. The voltage accounts for a factor of four, the depth of discharge for another 1.6, and the discharge rate for the rest. Drop the load on that flooded bank to 100 W and the current falls to 9.1 A, C/22, slower than the rating condition, and the Peukert term stops mattering: 11.0 hours linear and the tool shows no correction.

From one night's runtime to days of autonomy

An off-grid bank is sized the other way round: how many days of no useful sun it has to cover, not how many hours one load runs. Daily kWh times days of autonomy, divided by DoD, round-trip efficiency and the cold-weather derate, then divided by the bank voltage for the amp-hours. The battery bank size calculator does that and turns the answer into a count of real batteries; the off-grid load calculator produces the daily kWh it starts from. Because a bank sized on autonomy always discharges slowly on average, Peukert costs you on the peak loads, the kettle and the microwave, not on the average, which is why this tool takes a single load rather than a daily total.