The short answer. Add up what each device draws times the hours it runs a day. That’s your daily amp-hours. Divide by how much of the battery you can really use: lithium lets you use far more of its label than lead-acid does, so the same boat needs a smaller lithium bank.
Add your loads to see the bank size.
Rough sizing, not a design. Your battery maker’s manual has the final say on how deep to discharge and how fast to charge. Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
How the math works
Each device has a draw on its label, in watts or amps. On a 12V system, watts divided by 12 gives amps. Amps times hours gives amp-hours. Add them all up and you have what you take out of the battery in a day.
A battery’s label isn’t what you can use, though. Run a battery flat often and it wears out early, so makers rate their cycle life at a set depth of discharge. The calculator divides your daily use by that usable share:
- Lithium (LiFePO4): Victron rates its Lithium Battery Smart for 2,500 cycles at 80% depth of discharge, so the tool uses 80%.
- Lead-acid (AGM or gel): Victron’s AGM Deep Cycle gets 600 cycles at 50% depth of discharge but only 400 at 80%, so the tool uses 50%.
You can change either number under “Usable depth of discharge” if your battery maker gives a different one. Their manual has the final say.
A worked example
A fridge pulls 4 amps and runs about 12 hours a day out of 24: 48 Ah. Cabin lights use 24 watts for 5 hours: 24 ÷ 12 = 2 amps, times 5 = 10 Ah. That’s 58 Ah a day.
- Lithium: 58 ÷ 0.80 = about 73 Ah. A 100Ah lithium battery covers it with room left.
- Lead-acid: 58 ÷ 0.50 = about 116 Ah. That’s already more than one 100Ah battery.
Same boat, same day. The lead-acid bank is about 60% bigger and a lot heavier. That gap is most of the case for switching, and the Lithium vs lead-acid batteries for boats, vans and RVs covers the rest.
AC loads and the inverter
Anything that plugs into a household outlet through an inverter costs extra, because the inverter loses some power turning 12V DC into AC. Tick “AC?” on those rows and enter your inverter’s efficiency from its datasheet. The tool divides the AC draw by it. If you don’t have an inverter yet, the What size inverter for a boat, van or RV is coming.
Days between charges
If you run two days at anchor before the engine or shore power tops you up, set “Days between charges” to 2. Solar changes this too. Every amp-hour your panels put back during the day is one the bank doesn’t have to hold, so a boat with solar can often get by on a smaller bank. A solar sizing tool is on the way.
Once it’s in, count it
The calculator is an estimate. A shunt battery monitor like the Victron SmartShunt 500A counts every amp in and out, and after a week you’ll know your real daily number. Then refill what you use: a Victron Orion-Tr Smart 12/12-30 isolated DC-DC charges from the engine, and a Victron SmartSolar MPPT 100/30 runs the panels. Or start from the Boat lithium upgrade kit. To size the cables between them, use the Wire gauge calculator.
Bundles for this job: Boat lithium upgrade kit, small, RV battery monitor kit
Quick answers
How many amp-hours do I need for a weekend on the boat? Work out one day with the calculator, then set “Days between charges” to 2. Most of the total usually comes from the fridge, so check its real draw on the label or with a battery monitor.
Why is the lithium answer smaller than the lead-acid one? Because you can use more of a lithium battery’s rating. Victron rates its lithium battery for 2,500 cycles at 80% depth of discharge, while its AGM Deep Cycle gets 400 cycles at that depth and 600 at 50%. Using 80% of a lithium label costs it far less, so the same label gives you more usable amp-hours.
Does a 24V system need fewer amp-hours? For the same loads, a 24V bank needs about half the amp-hours of a 12V bank, because each device draws half the current. The stored energy in watt-hours is about the same.
Should I add extra on top of the result? The tool doesn’t add a margin on its own. Batteries lose some capacity as they age and in the cold, so many owners round up to the next battery size. Your battery maker’s manual says how much capacity to expect over time.
Sources
- Victron Lithium Battery Smart datasheet: https://www.victronenergy.com/upload/documents/Datasheet-12,8-&-25,6-Volt-lithium-iron-phosphate-batteries-Smart-EN.pdf
- Victron Gel and AGM batteries datasheet: https://www.victronenergy.com/upload/documents/Datasheet-GEL-and-AGM-Batteries-EN.pdf
