Guides

Calculator

Battery Size Calculator

Estimate the nominal battery capacity needed to cover your daily energy use for a chosen number of days without charging. Keep direct DC use separate from AC use powered through an inverter. Start with your device labels or measurements; the Daily Power Consumption Calculator can help estimate Wh/day for each device.

Not sure what to enter? Read the step-by-step guide.

Your inputs

What gets replaced?

Compatible fields are replaced with values from your saved setup, including blanks. Capacity values and units are replaced together where supported. Other fields stay unchanged. You can undo the replacement until you manually change one of the replaced fields. Edit My Setup

Your daily energy use
What does DC daily energy mean?
Energy used each day by devices powered directly from the battery's DC system, measured in watt-hours. Use Daily Power Consumption to build this total. Do not include shore or generator-only loads.

Add the daily Wh for devices powered directly by the battery's DC system. Leave blank if none.

What does AC daily energy mean?
Energy used each day by AC devices powered from the battery through an inverter, before inverter losses. Use Daily Power Consumption for this total. Do not enter the same overall RV total in both fields.

Add the daily Wh for devices powered from this battery through an inverter. Leave blank if none.

Your battery plan
What is meant by days without charging?
How many days the battery must supply the daily loads with no charging. The estimate starts with a fully charged battery. Decimal values such as 1.5 days are fine.
What does minimum battery level mean?
This is the percentage of charge where you choose to stop using the battery. Choose a limit appropriate for your battery and system based on the manufacturer's or system guidance.

The estimate starts at 100% charge. Choose a minimum battery level for your battery and system. The calculator does not choose a default based on battery chemistry.

What does nominal voltage mean?
Use the rated voltage of the whole battery bank from its label or documentation. Do not use a temporary voltage reading taken while charging or under load.

This value is only used to convert the nominal Wh estimate to Ah.

All calculations run in this browser. Enter daily energy at the devices, before inverter losses; do not enter the same device in both groups.

Battery size estimate

The capacity you could need

Based on your daily energy use and battery plan.

—

Enter daily energy, days without charging, a minimum battery level, and nominal battery voltage.

This is an energy estimate, not a battery model recommendation or a guarantee of runtime. It does not account for inverter idle draw, additional wiring and conversion losses, battery aging, temperature, discharge-rate effects, charging, or continuous and surge-power limits.

Edit your inputs
See calculation details, assumptions and sources

Battery Wh/day = DC Wh/day + AC Wh/day ÷ inverter efficiency (as a fraction). Multiply by days without charging for usable Wh, then divide by (100% − minimum battery level) to estimate nominal Wh from a full battery. Divide nominal Wh by nominal battery V to show Ah.

For example, 600 Wh/day DC and 400 Wh/day AC at 80% inverter efficiency draw 1,100 Wh/day from the battery. Two days need 2,200 Wh usable. With a 20% minimum battery level, that means 2,750 Wh nominal, or about 214.84 Ah at 12.8 V.

Daily Power Consumption shows separate direct DC and inverter AC Wh/day totals based on how each device is powered. Once every device has a power source assigned, Daily Power Consumption can pass those two totals directly into this calculator. You can also enter or edit them yourself. Do not use the overall total if it includes shore or generator loads. This estimate assumes the battery starts fully charged and that daily energy use remains the same each day. Select a minimum battery level from your battery guidance and an inverter efficiency relevant to your loads.

References: Victron Phoenix inverter manual on load energy, losses, and Ah and Victron on minimum state of charge. See the site methodology.