Using the fridge's rated current for 24 hours a day.
Apply its duty cycle. Most marine fridges run between 35% and 55% depending on climate and insulation.
Sizing
How to Calculate Boat Electrical Loads in Amp-Hours
Multiply each load's current in amps by the hours it genuinely runs in a day, then add them up. A 5 A fridge cycling at 45% duty is 5 × 10.8 = 54 Ah, not 5 × 24.
- Steps
- 6
- Last reviewed
- 4 September 2026
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Equipment and information required
- A list of every DC load on the boat
- Each load's current draw in amps
- A clamp meter, if you want to check rather than trust the label
How to Calculate Boat Electrical Loads in Amp-Hours: steps
List every load, including the ones that are always on
Fridge, freezer, cabin and nav lights, instruments and chartplotter, VHF and AIS on standby, fans, water and shower pumps, and device charging. The standby loads are small individually and are the ones most often left out.
Get each load's current in amps, not watts
If the label gives watts, divide by the system voltage — use 12.8 V for a 12 V lithium bank and 25.6 V for a 24 V one, since that is the nominal voltage LiFePO4 actually sits at. A 60 W fridge on a 12 V bank is about 4.7 A.
Use running hours, not hours present
Anything thermostatic runs at a duty cycle. A fridge at 45% duty over a full day is 10.8 hours of compressor time. Estimating 24 hours will roughly double your bank and your budget.
Multiply and total
Amps × hours for each load gives amp-hours. Add them. This total is your daily draw, and every other number in the system derives from it.
Add the loads you only use sometimes, separately
An inverter for a kettle at 85 A for 18 minutes is about 25 Ah, and a watermaker at 25 A for 90 minutes is about 38 Ah. Keep these as a separate line so you can size for the days you use them and the days you do not.
Halve the amps if you are working in 24 V
The same appliance draws half the current at twice the voltage. If you have listed everything at 12 V and are sizing a 24 V bank, scale the amp figures by 0.5 before totalling.
Common installation and calculation errors
Mixing watts and amps in the same total.
Convert everything to amps at the system's nominal voltage first.
Related equipment and battery guides
More on sizing
How to Size a Boat Lithium House Battery BankTake your daily amp-hour draw, subtract half of what your solar returns on an average day, multiply by the nights you spend away from shore power, then divide by 0.8 because you cycle lithium to 80%. Round up to a bank you can buy.How to Size a Marine DC-DC Battery ChargerTake the smaller of a quarter of your bank's capacity and half your alternator's rating, then round down to a charger you can buy. A 300 Ah bank behind a 90 A alternator gives min(75, 45) = 45, so a 40 A charger.How to Size an MPPT Solar Controller for a BoatDivide your array's watts by the bank's nominal voltage to get charge current, add headroom for a cold bright day, and buy a controller at or above that figure. 400 W on a 12.8 V bank is about 31 A, so a 30 A controller is marginal and a 40 A is not.How Many Solar Panels Does a Boat Need? Sizing StepsDivide the amp-hours you want solar to replace by 4.25 sun-hours and 0.75 for real-world losses, then multiply by the bank's nominal voltage to get watts. Replacing 100 Ah a day on a 12 V bank needs roughly 400 W.