RV battery bank size calculator — Wh to Ah with reserve days
Size an RV battery energy budget using daily Wh, autonomy days, usable capacity and system efficiency.
Estimate battery storage before shopping
Use your measured daily watt-hours and a realistic number of no-charge days. The result estimates nameplate storage needed under your assumptions, not a safe wiring design or model recommendation.
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Your estimate
A 700Wh daily load for two days with 80% usable capacity and 90% delivery efficiency needs approximately 1,944Wh of nameplate storage, or 152Ah at a 12.8V assumed nominal voltage. This is an example, not a product capacity claim.
Make a daily energy budget · Plan for consecutive cloudy days.
How many amp-hours do you need for two days?
For 700Wh/day delivered to appliances and two no-charge days, reserve energy is 1,400Wh. An 80% usable window and 90% delivery efficiency require 1,400 ÷ 0.72 = 1,944.4Wh of nominal battery storage. At 12.8V this is 151.9Ah. Those are scenario requirements, not a claim that a nominal 150Ah product meets them after temperature and aging effects.
Compare reserve days at the same daily consumption
| No-charge days | Energy delivered | Nominal Wh at 80% × 90% | Ah at 12.8V |
|---|---|---|---|
| 1 | 700Wh | 972.2 | 76.0 |
| 2 | 1,400Wh | 1,944.4 | 151.9 |
| 3 | 2,100Wh | 2,916.7 | 227.9 |
Ah changes with voltage: 1,944.4Wh is about 76.0Ah at 25.6V. This is the same nominal energy expressed at a different voltage, not twice the runtime. Equipment still has to be compatible with the selected system voltage.
Translate the number into a documented capacity check
Victron's LFP Smart 12.8/100 reference is 1,280Wh at 25°C. Two such nominal ratings total 2,560Wh arithmetically; at the scenario's 72% delivered fraction that is 1,843.2Wh, above the 1,400Wh target. This comparison does not authorize connecting batteries: the maker's BMS, current, balancing and connection rules still govern.
Should you count solar as reserve?
Reserve days describe time without useful charging. Do not subtract a hoped-for sunny day's production from this requirement and still call it two no-charge days. Model charging separately, then test a poor-weather sequence. A positive average week can hide an empty battery before the next sunny day.
Check the worst load and the complete system
Record simultaneous running watts, startup surge, the battery's permitted discharge current, charging compatibility and cold-weather restrictions. For an integrated power station, compare its own capacity and outlet limits rather than assuming a bare-battery discharge window applies. Price the complete system, including required controls and installation, before deciding that more storage is the cheapest fix. Reducing a measured standby load may change the requirement.
Sources and calculation notes
- Victron Lithium Battery Smart datasheet — capacity, cycle-life conditions and temperature limits
- EcoFlow runtime testing guide — capacity, efficiency, measurement points and standby loads
Documentation checked 2026-10-24. Manufacturer specifications describe the named product or range, not every battery of that chemistry. Tables are our arithmetic under stated assumptions; they are not hands-on discharge tests or product recommendations.
What happens to your batteries during several cloudy days?
Build a seven-day reserve scenario from your own appliance loads, then save an equipment decision plan.
Turn your result into an equipment checklist
No fabricated product tests or affiliate claims. Compare verified specifications before buying.