How many Ah do I need?

Short answer

Add up your daily consumption in watt hours, divide by 12.8 V, and multiply by the number of days you need to manage without charging. For lithium, add 25% on top. For lead-acid, you need double. The calculator below does it for you.

Updated 27.10.2026
5 min read
Sources
Calculator ↓

Calculator in this article Calculate for your own use Figures from datasheets and our product data. Formulas and sources are shown with the result. Open the calculator →

Ah, A and W – briefly explained

AAmps: how much current is flowing right now – like litres per minute through a garden hose.

AhAmp hours: how much is in the tank. 5 A for 20 hours is 100 Ah.

WWatts: power. Watts = amps × volts. Shown on the appliance's rating plate.

WhWatt hours: energy. 100 Ah × 12.8 V = 1,280 Wh 2.

and if it's overcast for several days it gets tight having enough power since we have a 12 volt compressor fridge/freezer.

User on the DCU forum (Dansk Camping Union)

Calculate your needs

Add your devices. We've added a cool box as an example – with the manufacturer's own measured figure. Edit or remove it.

The calculator requires JavaScript. The formula: watts × hours = Wh per day. Wh ÷ 12.8 V = Ah per day. Multiply by the number of days without charging and by 1.25 for lithium (2 for lead-acid).

How to calculate it yourself

Wh per day = watts × hours for each device. If it runs on 230 V via an inverter, add 15% (inverter under 1,000 W) or 10% (over 1,000 W) 1.
Ah per day = Wh ÷ 12.8 V. A 12 V lithium battery has a nominal voltage of 12.8 V 2.
Multiply by the days without charging. Victron recommends at least 2 – and 3 on long trips, in bad weather and in winter 1.
Lithium: multiply by 1.25. Lead-acid/AGM: multiply by 2. You can use 70–80% of a lithium battery, but only 50% of a lead-acid battery 1.

Example: The cool box uses 1.03 Ah per hour at 12 V 5. That's approx. 297 Wh per day or 23 Ah at 12.8 V. Two days gives 46 Ah, and with lithium you should choose at least 58 Ah.

Typical devices – with sources

Device	Consumption according to the manufacturer	Note
Compressor cool box, Dometic CFX3 45 5	1.03 Ah per hour	At 32 °C outside and 4 °C inside. Less in cool weather.
Gas heater, Truma Combi 4 6	approx. 1.2 A on average	Up to 6.5 A for short periods.
Water pump, Shurflo Trail King 7 7	max. 3.5 A	Only runs when water is being drawn.
Starlink Mini 8	25–40 W on average	Can run on 12–48 V.
Starlink Standard 9	75–100 W on average	Via a 230 V power supply, i.e. an inverter.
Inverter at idle, Victron Phoenix 12/800 10	6.5 W	1 W in ECO mode. Uses power just by being switched on.
Inverter at idle, Victron Inverter Smart 12/2000 11	8 W	0.6 W in ECO mode.
Absorption fridge, Dometic RMD10.5 12	170 W on 12 V	Normally only cools on 12 V while driving 13. Don't count it on the battery.

If your appliance only states amps, multiply by 12 to get watts.

Cold costs capacity

100 AhVictron 12.8 V/100 Ah at 25 °C 2

80 Ah · 50 AhThe same battery at 0 °C and −20 °C 2

EVE's cell delivers at least 85% at 0 °C and 75% at −20 °C 4. The figures vary from battery to battery, but the direction is the same: If you use the battery in frost, choose one size larger.

Check the current too – not just Ah

Ah tells you how long the battery lasts. The current in amps tells you whether it can run your appliances. Current = watts ÷ volts 3. An inverter delivering 1,600 W therefore draws approx. 133 A from a 12 V battery – a little more once the inverter's losses are included.

Compare this with the battery's maximum continuous current. Our 12V 100Ah, for example, is rated at 150 A continuous and 200 A peak 14. The calculator shows the current if everything runs at once.

When a smaller battery is enough

You charge every day from driving or solar. Then you can count on fewer days in the calculator. However, Victron recommends at least 2 days if solar is your main charging source 1.
You mainly use 12 V equipment and no inverter. That way you avoid the inverter's losses and idle consumption 10.

Limitations

The consumption figures are the manufacturers' own and apply to their measurement conditions. Your actual consumption depends on temperature and habits. It's better to size a little too big than too small – Victron writes: round up if in doubt 1.

Conclusion

Calculate in Wh, and add 25% for lithium

Add up your daily consumption, divide by 12.8 V, multiply by the days without charging – and add 25% for lithium. With lead-acid you need twice as much. The next step is to check what else you need when you switch.

See what you need See 12V lithium batteries

Sources

Victron Energy (2026): How to size an RV battery bank – lithium: choose 125% of the need; lead-acid: no more than 50% discharge, choose double; +15% / +10% for 230 V appliances; at least 2 days without charging.
Victron Energy: Datasheet – Lithium Battery Smart 12.8 V and 25.6 V – 2,500 / 3,000 / 5,000 cycles at 80 / 70 / 50% depth of discharge to at least 80% capacity, 12.8 V nominal voltage, charging from +5 °C, capacity in the cold and requirement for an external BMS.
Victron Energy: Wiring Unlimited – current = power ÷ voltage; fuse in the positive cable and a main fuse; at least one fuse must be able to break the lithium battery's short-circuit current; no more than 2.5% voltage drop.
EVE Energy: Specification LF105, version E (2023) – cell: 4,000 cycles at 25 °C and 2,000 at 45 °C to 80% SOH; max. 3.65 V; charging from −10 to 65 °C with greatly reduced current in the cold; at least 85% capacity at 0 °C and 75% at −20 °C; storage at 15–40%. The document is hosted by a retailer.
Dometic: CFX3 45 – technical data – 1.03 Ah per hour at 12 V, 32 °C outside and 4 °C inside.
Truma: Combi 4 – technical data – at 12 V: approx. 1.2 A on average, briefly up to 6.5 A.
Shurflo: RV catalogue (2017) – Trail King 7: max. 3.5 A at 12 V.
Starlink: Specifications – Mini – average consumption 25–40 W.
Starlink: Specifications – Standard – average consumption 75–100 W.
Victron Energy: Datasheet – Phoenix Inverter 250–1600 VA – 12/800: 6.5 W at idle, 1 W in ECO mode.
Victron Energy: Datasheet – Inverter Smart 1600–5000 VA – 12/2000: 8 W at idle, 0.6 W in ECO mode.
Dometic: Manual – RMD10.5 absorption fridge – 170 W on 12 V, 250 W on 230 V.
Thetford: Installation manual – N4000 – 12 V cooling requires a D+ signal (engine running); bridging the connection can drain the battery.
LithiumPro: Product data for our batteries (e.g. 12V 100Ah) – Ah, max. continuous and peak current, charging current, charging voltage 14.6 V, Bluetooth and warranty are listed on each product page.