How big a battery bank, and how long it lasts
Add your bank’s rated amp-hours to see how long it lasts.
The example is Victron Energy’s boat at anchor: a 50 W refrigerator running half the time, a 25 W anchor light for 8 hours, a 1 A radio for 3 hours and 200 W of cabin lights for 0.6 hours. For a trolling motor, enter its amps as one load and the hours you run it.
To size a boat battery bank, add up the amp-hours your loads use between charges and divide by the share of the battery you can use: half of a lead-acid battery, so twice what you use between charges, or 80% of a lithium iron phosphate (LiFePO4) battery. A boat at anchor that uses 79.7 amp-hours a day needs a lead-acid bank of about 159 Ah, or a 100 Ah LiFePO4 bank. The calculator above works out the daily use from up to four loads in amps or watts, the bank you need for the days between charges, and how long your own bank lasts, which also answers how long a battery runs a trolling motor.
Amps = watts ÷ volts · Amp-hours a day = amps × hours a day, added up for every load · Bank needed = amp-hours a day × days between charges ÷ usable share · Runtime = rated amp-hours × usable share ÷ amps.
How to Calculate Your Daily Amp-Hours
List what runs from the batteries, with each load’s current in amps and the hours a day it is on. If a device gives its power in watts, divide by the system voltage: a 50 W refrigerator draws 50 ÷ 12 = 4.2 A on a 12-volt boat and 2.1 A on a 24-volt one. Multiply each current by its hours and add them up. For a refrigerator, count only the hours its compressor runs. Victron Energy’s book Energy Unlimited works through 24 hours at anchor for a boat with a 12-volt system:
| Load | Power or current | Hours a day | Amp-hours a day, 12 V |
|---|---|---|---|
| Refrigerator, compressor on half the time | 50 W | 12 | 50.0 |
| Masthead (anchor) light | 25 W | 8 | 16.7 |
| Radio | 1 A | 3 | 3.0 |
| Cabin lighting, ten 20 W lights | 200 W | 0.6 | 10.0 |
The four loads come to 79.7 amp-hours a day, or 956 watt-hours (amp-hours × volts). Victron adds 5 Ah for small loads, for 85 Ah, and puts the minimum battery at 170 Ah, twice the daily use, for a battery recharged twice a day.
How Much of a Battery You Can Use
A battery’s rating is in amp-hours, quoted for a 20-hour discharge: a 200 Ah battery delivers it at 200 ÷ 20 = 10 A (Victron). Draining it flat every day would wear it out quickly, so plan to use only part of it:
| Battery type | Share used | Basis |
|---|---|---|
| Lead-acid: flooded, AGM or gel | 50% | Concorde (Lifeline AGM): an average depth of discharge no greater than 50% of the 20-hour rating; Victron: at most 50% between recharges for a house battery, and discharging past about 80% ages a battery disproportionately fast |
| Lithium iron phosphate (LiFePO4) | 80% | Victron rates its LiFePO4 batteries for 2,500 cycles at 80% depth of discharge, 3,000 at 70% and 5,000 at 50% |
| Starting battery | not for cycling | Victron: not suitable for cyclic use |
The share is a planning figure you can change. Victron points out that when the batteries are charged only a few hours a day, by the engine’s alternator or a generator, the short recharges bring them back to no more than 80%; with a margin for the unexpected and for an aging battery, its working figure falls to about 32%, a capacity at least three times the discharge between recharges. At 32%, the example boat would need a 249 Ah bank.
Battery Bank Size
The bank you need is the amp-hours used between charges divided by the usable share. With one day between charges, the example needs 159 Ah of lead-acid; with two days, 319 Ah. Victron’s rule of thumb comes to the same figure, a capacity at least twice the daily consumption, but only for a house battery that is recharged twice a day. To find how many hours of a trip the loads run, the Speed, Distance and Time Calculator gives the time under way, and when you work out the boat’s loaded weight with the Boat Weight Calculator, count the batteries in the gear.
Trolling Motor Battery Life
For a trolling motor, enter its current draw as the only load. When a 24-volt or 36-volt motor runs on two or three 12-volt batteries in series, the batteries add their voltages but not their capacity (Battery University; Victron): three 100 Ah batteries make a 36-volt bank of 100 Ah, so enter one battery’s amp-hours as your bank. The runtime is the usable amp-hours divided by the amps: a 100 Ah lead-acid battery used to 50% runs a motor drawing 20 A for 50 ÷ 20 = 2.5 hours, and a 100 Ah LiFePO4 battery used to 80%, 4.0 hours. To run that motor 4 hours a day on lead-acid batteries, you need 160 Ah. A motor’s manual gives its maximum draw, at full speed; it draws less at lower speeds. A lead-acid battery also delivers less of its rating at high currents before its voltage drops too low. Victron’s figures, as a share of the 20-hour rating:
| Battery | At C/5 | At C/2 | At C/1 |
|---|---|---|---|
| VRLA AGM, 230 Ah | 86% | 68% | 62% |
| VRLA gel (Sonnenschein Dryfit A200), 200 Ah | 79% | 60% | 50% |
C/5 is a current of a fifth of the rating, 20 A from a 100 Ah battery, and C/1 a current equal to it, 100 A. Planning to use half the rating stays within the AGM figures even at C/1; the closer you plan to run a battery to flat, the more the current matters. On the fuel side, the Boat Fuel Tank Calculator works out how many gallons a tank holds.
Frequently Asked Questions
How do I calculate the battery size for my boat?
Add up the amp-hours your loads use between charges (amps × hours for each load) and divide by the share of the battery you plan to use: 50% for lead-acid, so twice what you use between charges, or 80% for lithium iron phosphate. A boat using 79.7 Ah a day needs about 159 Ah of lead-acid batteries.
How long will a 100Ah battery run a trolling motor?
Divide the usable amp-hours by the motor’s current. A 100 Ah lead-acid battery used to 50% runs a 20 A motor for about 2.5 hours, and a 100 Ah LiFePO4 battery used to 80% runs it for about 4.0 hours. The motor’s manual gives its maximum draw; at lower speeds it draws less.
How many amp hours do I need for a day on the boat?
It depends on what you run. In Victron Energy’s example of a boat at anchor, a refrigerator, an anchor light, a radio, cabin lights and small loads use 85 Ah in 24 hours, and Victron puts the minimum battery at 170 Ah for a battery recharged twice a day.
How do I convert watts to amps?
Divide the watts by the system voltage. A 50 W refrigerator draws 50 ÷ 12 = 4.2 amps on a 12-volt system. To go the other way, watts = volts × amps.
How far can I discharge a marine battery?
For lead-acid batteries, Concorde, the maker of Lifeline batteries, recommends an average depth of discharge no greater than 50% of the 20-hour rating, and Victron notes that a battery discharged past about 80% ages disproportionately fast. Victron rates its lithium iron phosphate batteries for 2,500 cycles at 80% depth of discharge.
Can I use a starting battery for house loads?
It is not built for it. In Victron’s table of battery life, starting batteries are not suitable for cyclic use; house loads need deep-cycle batteries.
Sources
- Victron Energy, Energy Unlimited (Reinout Vader, revision 9, June 2011): sect. 2.5.3 (rated capacity for a 20-hour discharge; effective capacity at C/5, C/2 and C/1), sect. 2.5.5 (aging beyond about 80% depth of discharge; cycles until the end of service life; starting batteries not suitable for cyclic use), sect. 8.5.2 (usable capacity at most 50%, 32% with margin and aging; capacity three times the discharge between recharges, or twice the daily consumption if the house battery is recharged twice a day), sect. 9.4 (24 hours at anchor: 85 Ah, minimum battery 170 Ah assuming two recharges a day) (read Oct 22, 2026).
- Victron Energy, 12,8 & 25,6 Volt lithium iron phosphate batteries Smart, datasheet: cycle life to 80% of nominal capacity, 2,500 cycles at 80% depth of discharge, 3,000 at 70%, 5,000 at 50% (read Oct 22, 2026).
- Lifeline Batteries (Concorde Battery), FAQs: an average depth of discharge no greater than 50% of the battery’s 20-hour rating; size the bank to discharge no more than 50% (read Oct 22, 2026).
- Battery University, BU-302: Series and Parallel Battery Configurations (Cadex, updated October 25, 2021): cells added in series raise the voltage while the capacity stays the same (read Oct 22, 2026).
- Victron Energy, Wiring Unlimited: Battery bank wiring: batteries in series increase the voltage; higher-voltage lead-acid banks are built from 12 V batteries in series (read Oct 22, 2026).