Battery capacity calculator
Add your boat's electrical loads to calculate the minimum battery bank size.
Related tools
Electrical loads
Worked example — overnight at anchor
A typical overnight passage with the following loads on a 12V system:
| Load | Current (A) | Hours/day | Ah/day |
|---|---|---|---|
| Fridge (12V compressor) | 4.5A | 12 | 54 Ah |
| Cabin LED lights | 2A | 6 | 12 Ah |
| Chart plotter | 2A | 8 | 16 Ah |
| VHF radio (receive) | 1A | 12 | 12 Ah |
| AIS transponder | 1A | 12 | 12 Ah |
| Total | 106 Ah |
At 50% DoD (lead-acid): minimum bank = 106 ÷ 0.5 = 212 Ah. With a 20% safety margin: aim for 260 Ah.
At 80% DoD (lithium): minimum bank = 106 ÷ 0.8 = 133 Ah — less than half the weight.
How to use this calculator
For each electrical item, enter its current draw in amps and hours per day. The calculator totals daily consumption and works backwards from your depth of discharge setting to find minimum battery capacity.
Required (Ah) = Daily usage ÷ (Depth of discharge ÷ 100)
Add a 20–25% safety margin on top for battery ageing and unexpected loads.
Frequently asked questions
What is depth of discharge (DoD)?
DoD is the percentage of a battery's capacity you can safely use before recharging. Discharging a lead-acid battery below 50% repeatedly shortens its life significantly. Lithium (LiFePO₄) batteries can be discharged to 80–90% without damage — a major advantage for liveaboards.
What is the difference between Ah and Wh?
Amp-hours (Ah) measure charge capacity at a given voltage. Watt-hours (Wh) measure energy, independent of voltage. To convert: Wh = Ah × Voltage. A 100Ah 12V battery holds 1,200 Wh — the same energy as a 50Ah 24V battery.
How many Ah do I need for a weekend on the boat?
A typical two-night liveaboard weekend uses 80–150 Ah/day depending on fridge, lighting and electronics. For two days without charging, size for 160–300 Ah daily total, then apply your DoD factor. Most cruising sailors aim for 200–400 Ah of usable capacity.
Should I add a safety margin?
Always. Batteries lose capacity as they age (typically 20% after several years), and you'll always use more power than planned. A 20–25% margin on top of the calculated minimum is standard practice.
Typical load reference
| Load | Typical draw | Notes |
|---|---|---|
| VHF radio (receive) | 0.5–1.5A | Transmit can be 5–6A briefly |
| Chart plotter (10") | 1–3A | Depends on backlight |
| LED navigation lights | 0.3–0.8A | Per light |
| Autopilot (light use) | 1–3A | Can spike in choppy conditions |
| Fridge (12V compressor) | 3–6A | Runs ~50% of the time |
| AIS transponder | 0.5–2A | Class B typical |