From Amp-Hours to Real Runtime: An RV Battery Sizing Guide

From Amp-Hours to Real Runtime

An RV Battery Sizing Guide

By Bilal Siali  |  PowerMatchLab

At PowerMatchLab, I often see RV batteries compared by amp-hours. That number is useful, but it does not tell you how long your equipment will run. Runtime also depends on battery voltage, appliance use, conversion losses, usable capacity, and the time between charges. The practical way to size a battery bank is to calculate one day's energy use in watt-hours, add the necessary margins, and then convert the result back to amp-hours.

The same method works for a weekend camper or a larger off-grid RV. The figures below are examples, so replace them with ratings from your appliance manuals and, where possible, measurements from your own system before buying or modifying equipment.

Why Amp Hours Alone Can Mislead

Amp-hours measure electric charge, not energy. Two batteries with the same amp-hour rating can store very different amounts of energy if their voltages differ. Convert the label to watt-hours before comparing a battery with your appliance list:

Nominal battery energy in Wh = battery voltage x amp-hours

For example, a 100Ah LiFePO4 battery rated at 12.8V stores 1,280Wh nominally. A 100Ah battery in a 24V system stores about twice as much energy. Amp-hour ratings are only comparable when the voltage is the same.

Nominal energy is not the same as usable energy. The manufacturer may specify an allowable depth of discharge, low-voltage cutoff, temperature range, and other operating limits. Age, cold weather, high current, wiring losses, and battery-management-system settings may reduce the energy available to the RV. Use the manufacturer's documented limits instead of assuming that every watt-hour on the label is available.

Build a 24 Hour Load Audit

Start with every device that may draw from the house battery. Record its power in watts and how long it runs on a typical day. If a label shows amps instead of watts, multiply the amps by the operating voltage. For equipment that cycles on and off, estimate the percentage of time it actually runs rather than counting 24 hours at full power.

Continuous loads include routers, some ventilation fans, detectors, and standby electronics.

Cycled loads include compressor refrigerators, water pumps, and thermostatically controlled heaters.

Short high-power loads include microwaves, coffee makers, hair dryers, and induction cooktops.

A plug-in energy meter can measure an AC appliance over several hours. A shunt-based battery monitor can record total DC current and daily energy use at the battery. A full-day measurement is especially helpful for refrigerators because outside temperature, ventilation, thermostat settings, and door openings all affect the duty cycle.

Figure 1. A practical sequence for converting an appliance list into a battery-bank target.

Separate DC Loads From AC Loads

A 12V refrigerator or roof fan draws directly from the DC system. A household appliance plugged into an inverter uses AC power, so the inverter must convert energy from the battery first. Some energy is lost in that conversion. If an AC device uses 320Wh and the inverter is 90 percent efficient at that load, the battery must supply about 356Wh:

Battery energy for an AC load = appliance Wh / inverter efficiency

Use the efficiency figure in the inverter documentation when one is available. Efficiency varies with load, and the inverter may draw power even when the connected appliances are idle. Include that standby consumption if the inverter stays on for long periods.

A Worked One Day RV Example

Consider an RV with a 12.8V LiFePO4 house bank and the daily loads shown below. The calculation uses 90 percent inverter efficiency for the two AC devices.

 

The appliances use 1,285Wh, but the battery must supply about 1,335Wh after inverter losses. Now account for usable battery capacity and a planning margin. If the manufacturer allows 90 percent usable capacity and the owner adds a 20 percent margin for uncertainty:

Required nominal energy = 1,335Wh / 0.90 x 1.20 = 1,780Wh

At 12.8V, 1,780Wh is about 139Ah. Rounding up gives a practical target of roughly 150Ah for this one-day example. Two days without charging would require about 278Ah under the same assumptions, which points to a 300Ah bank unless the daily load is reduced.

This also shows why one 12.8V 100Ah battery may fall short. Its nominal 1,280Wh becomes about 1,152Wh at 90 percent usable capacity. Against a daily battery demand of 1,335Wh, the calculated runtime is about 20.7 hours, even before applying the 20 percent planning margin.

Check Power and Surge Separately

Energy capacity determines runtime, but it does not show whether the system can start or sustain a high-power appliance. The inverter, battery management system, battery bank, cables, fuses, and connections must all handle the required current.

For example, a microwave drawing 1,200W from the AC outlet may require about 104A from a 12.8V battery at 90 percent inverter efficiency, before any other RV loads are counted. That is near or above the continuous limit of some 100A battery management systems. Compressors, pumps, and motors may also draw a brief startup surge above their running power.

Check both continuous and surge ratings in the manuals. Do not size conductors or protection devices from a simple article calculation. Cable length, conductor size, connection quality, fuse interrupt rating, grounding, and the installation instructions all matter. Victron Energy's Wiring Unlimited guide recommends selecting cables with voltage drop in mind and following equipment manuals for cable and fuse sizes.

Treat Solar as Recharging Not Extra Capacity

Solar panels can replace energy during the day, but they do not increase the battery's overnight capacity. The battery must still carry the loads when the panels produce little or no power. Size the array around the energy it needs to replace during the available charging window.

In the worked example, the bank must recover about 1,335Wh after a typical day. With four peak-sun-hours and an illustrative overall solar yield of 75 percent after temperature, controller, wiring, orientation, and other losses, the arithmetic is:

Required array = 1,335Wh / (4 h x 0.75) = 445W

Under those assumptions, a nominal 500W to 600W array leaves more margin, but that range is not a universal recommendation. A flat RV roof, partial shade, winter sun, heat, dirt, limited roof space, and the solar controller's input limits can all reduce production. NREL's PVWatts tool can estimate local monthly solar resource and system output. An RV estimate still needs to account for its actual orientation, shading, and hardware.

A Pretrip Sizing Checklist

Measure or verify the running watts and daily operating time of every essential load.

Apply realistic duty cycles to refrigerators, pumps, fans, and other cycling equipment.

Convert all battery ratings and loads to watt-hours before comparing them.

Add inverter losses and standby consumption only to the loads that actually use the inverter.

Choose the number of days the RV must operate without shore, alternator, generator, or solar charging.

Use the battery manufacturer's permitted usable capacity and temperature limits.

Check continuous current, startup surge, inverter rating, BMS rating, wiring, and fusing separately from energy capacity.

Add a reserve for weather, aging, measurement error, and unexpected use.

Verify that the planned charging sources can replace a typical day's consumption.

Recheck the system with real monitor data after the first trip and update the audit.

Choose Capacity From the Audit

Do not choose an RV battery bank simply because 100Ah or 200Ah is a familiar size. Start with the daily load, conversion losses, days between charges, usable capacity, reserve, and the current required by the largest simultaneous loads. With those assumptions written down, you can change one at a time and see how it affects the result.

Before buying equipment, compare the calculation with the manuals for the battery, inverter, charger, and appliances. A qualified RV electrical professional should check the installation and protection plan. The energy audit gives you a sensible capacity range, but equipment-specific wiring and safety requirements still apply.

Sources

Victron Energy, Wiring Unlimited, covering current, cable resistance, voltage drop, battery-bank wiring, fuses, and mobile installations.

National Renewable Energy Laboratory, PVWatts Calculator, for location-specific photovoltaic energy estimates.

About the Author

Bilal Siali researches portable power station specifications and builds transparent energy-calculation tools at PowerMatchLab. His work focuses on turning manufacturer-published data into practical guidance for capacity, runtime, and product comparison. Use the free PowerMatchLab Power Calculator to estimate energy requirements from your own devices and operating time.