The Boat Mill

Battery Bank Runtime Calculator

Estimate how many hours a house battery bank can carry a steady DC load before it reaches the depth of discharge you are willing to use. Pick a chemistry preset or type your own usable percentage.

When to use it

Use it to answer “will the batteries last the night at anchor?”, to size a bank before an upgrade, or to compare lead-acid and lithium for the same space. The load is the average current your boat draws in amps, which a battery monitor shows directly. Without one, add up the nameplate amps of what will be on, scaled by how much of the time each runs (a fridge compressor that runs 40% of the time at 5 A averages 2 A).

How it works

usable Ah = bank Ah × usable % ÷ 100

hours = usable Ah ÷ load amps

The usable percentage is the part of the rated capacity you plan to draw. Lead-acid and AGM batteries are usually taken no lower than 50% to protect their life. Lithium iron phosphate (LiFePO4) batteries tolerate 80 to 90% routinely.

Worked examples

Two 100 Ah lithium batteries, 10 A average load. 200 × 80% = 160 Ah usable; 160 ÷ 10 = 16.00 h.

The same 200 Ah in AGM. 200 × 50% = 100 Ah; 100 ÷ 10 = 10.00 h. To match the lithium bank’s runtime you would need 320 Ah of AGM.

Overnight at anchor. Fridge, anchor light and electronics average 4.5 A. A 400 Ah AGM bank gives 200 Ah usable, so 44.44 h or about 1.85 days before recharging.

Hours of runtime by bank size and load (50% usable)

Bank	2 A	5 A	10 A	20 A
100 Ah	25.00	10.00	5.00	2.50
200 Ah	50.00	20.00	10.00	5.00
300 Ah	75.00	30.00	15.00	7.50
400 Ah	100.00	40.00	20.00	10.00
600 Ah	150.00	60.00	30.00	15.00

For 80% usable, multiply any figure by 1.6.

Why real runtime is often shorter

Peukert effect. Lead-acid batteries deliver less than their rated amp-hours at high current. The rating is usually at a 20-hour rate (5 A for a 100 Ah battery). At 20 A the same battery may give noticeably less. Lithium batteries are much less affected.
Inverter loss. AC loads through an inverter cost roughly 10 to 15% more than their DC equivalent. A 120 W AC load at 12 V is about 10 A plus that loss.
Age and temperature. An older bank may only hold 70 to 80% of its rating; cold reduces available capacity.
Batteries not full. If the bank only reached 90% after the last charge, the runtime starts from there.

Safety

Do not discharge flooded or AGM batteries below about 50% as a habit, and never let a lithium bank’s protection system be the only thing stopping a discharge. Keep engine starting on a separate battery so a flat house bank cannot leave you unable to start.

FAQ

My monitor shows watts. How do I get amps?

Divide watts by the bank voltage: 60 W at 12.8 V is about 4.7 A.

24 V bank?

Use the bank’s Ah rating at its own voltage and the load in amps at that voltage. A 24 V bank draws half the amps of a 12 V bank for the same watts.

Does solar change this?

Yes. Subtract the average charge current from the load during the day. The solar harvest calculator gives daily amp-hours.