RV Battery Runtime Calculator

How to plan an off-grid RV battery budget

This tool is for the house battery that powers the living area of a camper, travel trailer, or motorhome, not the vehicle's starter battery. It answers two related questions: how many hours your current bank might support a repeating load pattern, and how many nameplate amp-hours that pattern needs for a target stay. All calculations run in your browser; no account or appliance upload is needed.

A watt-hour is an amount of energy; a watt is the rate at which you use it. Amp-hours only become an energy estimate after you multiply by bank voltage. A 100Ah bank at 12.8V stores approximately 1,280Wh at nominal voltage, while a 100Ah bank at 25.6V stores approximately twice as much. Neither bank necessarily allows you to use all of that energy.

Enter bank capacity correctly: series versus parallel

For identical batteries wired in parallel, amp-hours add and voltage stays the same. Two 12.8V, 100Ah batteries in parallel are a 12.8V, 200Ah bank. For identical batteries in series, voltage adds and amp-hours do not. Two of those batteries in series are a 25.6V, 100Ah bank. Both arrangements have the same nominal energy, but they require compatible equipment. Use the actual configured bank values rather than adding all battery labels together indiscriminately.

Health and charge describe different things. A bank with 80% health has about 80% of its original capacity even when fully charged. Starting at 80% state of charge describes how full that reduced capacity is. Reserve is the charge level where this estimate stops. The illustrative lithium reserve is 20%; lead-acid starts at 50%. Those defaults are planning choices, not universal safe discharge limits.

Appliance watts, duty cycles, and inverter losses

Use a device label, energy meter, or battery monitor to estimate running power. A 60W compressor fridge running 35% of the day averages 21W, but its actual cycle depends heavily on temperature, ventilation, door opening, and thermostat setting. A laptop charger labeled 65W may draw much less once the battery is charged. A propane furnace still needs electricity for its blower and controls; include that load during cold-weather trips.

DC appliances draw directly from the bank in this model. AC appliances draw through the inverter, so their average watts are divided by inverter efficiency. At 90% efficiency, an average 90W AC load requires 100W from the battery before additional idle overhead. If the efficiency figure already includes no-load consumption, set idle watts to zero to avoid counting the same loss twice.

The inverter is assumed to stay on throughout the modeled period whenever an AC appliance has a nonzero average load. If you switch it off between uses, use a measured time-averaged overhead instead. If every AC row is off, overhead is zero; to model an inverter left on anyway, add its idle consumption as a separate continuous DC row. The simultaneous AC result sums running watts for active AC rows, but does not calculate compressor surge or verify inverter compatibility.

Formulas and lead-acid discharge assumptions

• Average appliance watts: running watts multiplied by duty cycle / 100.
• Battery draw P: average DC watts + average AC watts / efficiency fraction + additional inverter idle watts.
• Derated capacity C: nameplate bank Ah multiplied by health / 100. Usable charge fraction f = (starting charge - reserve charge) / 100.
• Average current I: P / nominal bank voltage. Usable nominal energy = C multiplied by bank voltage multiplied by f.
• Discharge-rate factor: min(1, (C / (H multiplied by I)) raised to (k - 1)), where H is capacity rating hours and k is the Peukert exponent.
• Runtime: C multiplied by f multiplied by the discharge-rate factor / I. For lithium, k = 1, so there is no Peukert adjustment.

Peukert's law describes how usable lead-acid capacity falls as discharge current rises. The reference current is derated Ah divided by rating hours; for a new 100Ah C20 battery, that is 5A. This estimator caps the factor at 100% below reference current instead of assuming extra energy beyond rated capacity. AGM's 1.15 exponent is an illustrative starting point; flooded lead-acid's 1.25 matches the average discussed in the Victron reference below. Manufacturer discharge curves are preferable to either default.

For required capacity, the calculator solves the same runtime equation in reverse. Let T be target hours. Without a rate penalty, derated Ah = I multiplied by T / f. In the rate-limited branch, derated Ah = (T multiplied by H raised to (k - 1) multiplied by I raised to k / f) raised to (1 / k). Dividing by the health fraction gives nameplate Ah. The displayed requirement rounds up to the next whole Ah; it does not add a design safety margin or choose a commercially available bank size.

Worked example: how long will a 100Ah RV battery last?

With the default 12.8V, 100Ah lithium bank at full health and 100% starting charge, stopping at 20% leaves 1,024Wh. The example fridge averages 21W, lighting 3W, fan 9W, and laptop 8.125W AC. The laptop requires about 9.03W at the battery through a 90% efficient inverter. Adding 8W inverter overhead gives approximately 50.03W average battery draw.

Runtime is approximately 1,024 / 50.03 = 20.47 hours. A repeating 24-hour day needs about 1.20kWh, so the current bank reaches reserve before one full day. The model calls for at least 118Ah at the same voltage, health, and reserve to reach 24 hours. Turning the inverter off and eliminating that AC load can substantially change the result; merely changing the battery label without revisiting usage does not.

For a separate lead-acid example, use a 12V, 100Ah C20 bank, 50% reserve, full health, a 120W continuous DC load, and k = 1.25. Current is 10A, twice the reference current. The rate factor is approximately 84.09%, giving roughly 4.20 hours, compared with 5 hours if you ignore the discharge-rate loss. This illustrates why the simplest Ah divided by amps estimate can be optimistic under heavy loads.

What these estimates do not cover

Averages smooth out real appliance cycles. Applying Peukert's law to a day's average current is not a time-resolved simulation, so alternating high and low loads can produce different real outcomes. Remaining charge is a linear planning estimate within the modeled charge window, not a battery-monitor reading. The sensitivity table scales total battery draw, including overhead, by 25% in either direction; it is a what-if comparison rather than a confidence interval.

Nominal voltage is held constant. Voltage sag, wiring losses, temperature, self-discharge, battery balancing, inverter cutoff, battery-management-system current limits, and aging beyond the entered health figure are not modeled. High-current loads can fail to start even when the energy calculation looks adequate. Do not use this result to size fuses, wiring, or protection equipment. Consult equipment documentation and a qualified installer for electrical changes.

Solar and alternator charging are deliberately excluded because generation depends on time of day, shading, weather, controller behavior, and driving time. The daily energy figure helps you compare measured charging against demand, but it does not establish that a particular solar array can keep up. For trip planning, leave additional margin and compare this estimate with a battery monitor over a representative day before relying on it.

Frequently asked questions

• Can a 100Ah battery run an RV refrigerator overnight? It depends on voltage, chemistry, available charge, other loads, and fridge duty cycle. Enter measured fridge watts and cycling time, then include fans, lights, controls, and inverter overhead instead of treating the fridge as the only load.
• Why does lead-acid show less runtime than lithium? Lead-acid often uses a larger planning reserve and loses effective capacity at high current. This model applies a Peukert penalty to AGM and flooded batteries; it does not apply that penalty to LiFePO4.
• Does changing chemistry also change voltage? No. Chemistry resets reserve and exponent defaults, but bank voltage remains your input. Enter the correct nominal voltage separately when comparing 12V lead-acid with 12.8V lithium.
• Can I model an air conditioner or microwave? You can enter their running watts and average usage, but the tool does not verify startup surge, BMS limits, inverter rating, or voltage sag. A runtime estimate does not mean your equipment can safely run that appliance.
• What happens when all appliances are off? The tool reports no active load rather than an infinite number. Modeled demand and required capacity are zero, but real batteries still self-discharge and may power unlisted electronics.
• Are inputs saved or shared? The calculator does not save your appliance inputs or transmit them to a calculation service. Reset restores the example. The site's standard analytics and advertising scripts still operate under the privacy policy.

Sources and further reading

• Victron Energy: battery capacity and Peukert exponent explains C ratings, rate-dependent capacity, and why a fixed exponent is only an approximation.
• Your battery and inverter datasheets are the primary sources for nominal voltage, capacity rating, discharge limits, efficiency, idle consumption, and operating temperature. Replace the examples with your own equipment's figures.

Related calculators

Budget travel fuel with the gas mileage cost calculator, compare vehicle energy spending with the EV charging cost calculator, or estimate household electricity with the energy bill estimator.

Last updated: 2026-10-06. This is an educational planning estimate, not an equipment specification or electrical safety assessment.