RV Battery Calculator

How the calculator works

Everything starts from one number: your daily energy use in watt-hours. For each device, the calculator multiplies its watts by its time a day and by how many you have. A device entered as watt-hours a day, such as a fridge measured over 24 hours, counts as it is, without multiplying by hours again.

• Daily use at the battery. DC devices count as they are. AC devices run through the inverter, so their watt-hours are divided by its efficiency, 85 percent unless you change it: inverters are less efficient with small loads than their best datasheet figure. The inverter's idle draw counts only for the hours it sits on with nothing running. Always-on loads such as gas detectors count around the clock, 5 W unless you change it. On a 24V or 48V bank, the 12V devices run through a converter counted at 95 percent.
• Battery bank. Daily use × days without charging × 1.2 (a 20 percent reserve) ÷ the usable share. LiFePO4 is planned at 80 percent usable. AGM and flooded lead-acid are planned at 50 percent, because draining them deeper day after day shortens their life. Dividing by the bank's nominal voltage gives amp-hours: 12.8 V per 12V of LiFePO4 and 12.0 V per 12V of lead-acid.
• Batteries to buy. The calculator suggests identical 12V batteries: 100 Ah or 200 Ah for LiFePO4, 100 Ah for lead-acid. On a 24V bank they go in strings of two in series, on 48V in strings of four, and the strings in parallel. It prefers banks of up to 4 strings in parallel, because many makers allow no more, and says so when that makes it pick a bigger bank. It shows how many days each option, and the next smaller one, covers.
• Batteries you already have. They're counted first. If they cover the days, nothing is added. If they fall short, a LiFePO4 bank gets more of the same battery, and a lead-acid bank, or one of a different type, is replaced rather than mixed with new batteries.
• Solar panels. Daily use × the share solar should cover × 1.2 ÷ (sun-hours × 0.75) gives the panel watts you need. The 0.75 covers the losses NREL's PVWatts counts for a home system (14 percent for dust, wiring, mismatch and the like), plus panel heat on a flat roof, the controller and battery charging. The calculator then counts whole panels of the size you pick, after any panels you already have, and works out the production, the surplus and the days to refill the bank from that array.
• Charge controller. An MPPT controller's amp rating is its charging current: the panel watts divided by the charging voltage, about 14.4 V on a 12V battery. That matches makers' ratings: Victron, for example, rates its SmartSolar 100/50 for 700 W of panels on a 12V battery. When one controller isn't enough, the panels are split into groups of whole panels, each on its own controller.
• Inverter. The running watts of everything that runs at the same moment, plus 25 percent, rounded up to a common continuous rating: 300, 600, 1,000, 1,500, 2,000 or 3,000 W. A motor or compressor also has to start while the rest runs, so the calculator checks the other devices' running watts plus its starting watts, from your device list or, if you enter your daily use as one number, from the start-up you add there.
• Battery current. An inverter draws a steady power, so its current rises as the battery voltage falls. The calculator stores energy at the nominal voltage but checks current near empty: 11.2 V per 12V LiFePO4 battery, the end-of-discharge voltage Victron gives, and 10.5 V per 12V lead-acid battery, where its capacity ratings end. It adds the 12V devices that may run at the same moment and divides by the batteries in parallel. A continuous rating covers running loads; a motor's start needs the battery's peak rating.

Every planning value has a documented source, listed in the table at the bottom of this page. Our guides explain the same steps in more depth: how to do an RV energy audit, how many amp-hours you need, how many watts of solar, charge controller sizing and inverter sizing. The calculator keeps full precision and rounds only what it shows and the parts it suggests, always up to the next size that covers the need.

Worked example: a boondocker with solar

Here is the Boondocker with solar example from step 1, worked through by hand: a travel trailer with a 12V fridge, eight LED lights, a roof vent fan, the water pump, two phones, a laptop and a TV, on a 12V LiFePO4 bank, two days without charging, 4 sun-hours and 200 W panels.

In amp-hours, that is about 142 Ah a day from a 12V LiFePO4 bank: 1,818.9 Wh ÷ 12.8 V.

1. Battery bank: 1,818.9 Wh × 2 days × 1.20 reserve ÷ 0.80 usable = 5,456.8 Wh. At 12.8 V that is 426.3 Ah, so at least 427 Ah (5,460 Wh). 3 × 200 Ah batteries in parallel (600 Ah) cover 2.8 days with the reserve kept; 2 × 200 Ah (400 Ah) would cover 1.8 days, not the full 2.
2. Solar panels: 1,818.9 Wh × 1.20 ÷ (4 sun-hours × 0.75) = 727.6 W. In whole 200 W panels that is 4 × 200 W = 800 W. They put back 800 W × 4 × 0.75 = 2,400 Wh on a sunny day, about 580 Wh more than you use, which refills the bank in about 7 sunny days after the two days without charging.
3. Charge controller: 800 W ÷ 14.4 V = 55.6 A of charging current, so a 60 A MPPT controller rated for at least 800 W of panels on a 12V battery.
4. Inverter: the laptop and TV can run together: 100 W × 1.25 = 125 W, so a 300 W pure sine inverter.
5. Battery current: checked at 11.2 V, the end-of-discharge voltage LiFePO4 makers give, rather than the 12.8 V nominal: an inverter draws more current as the voltage falls. At the inverter's full output: 300 W ÷ (11.2 V × 0.85) = 31.5 A, plus 25.0 A for the 12V devices if they all run at once, about 57 A in all, or 19 A for each of the 3 batteries.

Open this example in the calculator to see the same numbers, then change any row to match your own rig.

What the calculator doesn't cover

• Cables, fuses and breakers. They're safety-critical and depend on current, cable length and the installation. See RV fuse and breaker sizing and how to wire an RV inverter safely.
• Panel voltage against the controller's limits. A string's voltage rises on cold mornings and must stay below the controller's maximum PV voltage. Charge controller sizing walks through that check.
• Charging from a generator or while driving. The calculator sizes the bank for the days with no charging at all. Charging while driving and with solar and whether you need a generator cover the rest.
• Prices and brands. Those change too fast to keep right in a calculator. Our buying guides compare current models.

Questions about sizing an RV battery bank

How many batteries do I need for my RV?

It depends on your daily use and how many days you want to go without charging. The calculator multiplies your daily watt-hours by those days, adds a 20 percent reserve and divides by the share of the battery you plan to use: 80 percent for LiFePO4 and 50 percent for lead-acid. It then suggests a bank of identical 100 Ah or 200 Ah batteries and shows how many days each option covers.

How many amp-hours does my RV use a day?

Add up the watt-hours, then divide by the battery voltage. Each device's watts times its hours a day gives its watt-hours, and AC devices count a little more because of inverter losses. Dividing the total by the bank's nominal voltage, 12.8 V for 12V LiFePO4 or 12.0 V for lead-acid, gives amp-hours. The Boondocker with solar example uses about 1,820 Wh a day, or about 142 Ah from a 12V LiFePO4 bank, and the results show both figures. If your battery monitor counts amp-hours, multiply them by the voltage and enter the watt-hours under “I know my daily use” in step 2.

How long will a 100Ah battery last in my RV?

Divide the energy you can use by your daily use. A 100 Ah LiFePO4 battery stores 100 Ah × 12.8 V = 1,280 Wh. At the 80 percent this calculator plans to use, that's 1,024 Wh: about a day at 1,000 Wh a day. The Weekend camper example uses about 1,440 Wh a day, so a single 100 Ah battery would last about 17 hours. A 100 Ah AGM battery, planned at 50 percent, gives 600 Wh. The days covered in the results also keep the 20 percent reserve, so they come out a little shorter.

Why does an AGM bank need so many more amp-hours than lithium?

This calculator plans lead-acid batteries at 50 percent usable capacity, because routinely draining them deeper shortens their life. LiFePO4 is planned at 80 percent. The same loads therefore need about 1.7 times the amp-hours in AGM: 1.6 times for the smaller usable share, and a little more because lead-acid is counted at 12.0 V rather than 12.8 V.

Does a 24V battery bank last longer than a 12V bank?

No. The same batteries store the same watt-hours whether they're wired for 12V or 24V. A 24V bank halves the current the inverter draws, which eases cables and fuses, but the RV's 12V devices then run through a converter, which this calculator counts at 95 percent efficiency.

How do I find my sun-hours?

Use the free PVWatts calculator: enter your location, set the tilt to match your panels and read the solar radiation figure, in kWh/m² per day, for the month you rely on solar. That number is your sun-hours. Size for the weakest month you camp in, not the sunniest.

What size inverter do I need for a microwave?

Add the microwave's input watts from its rating plate to everything else that runs at the same time, then add 25 percent. A microwave sold on 1,000 W of cooking power can draw about 1,500 W, so with a laptop and a TV running too, plan on a 2,000 W pure sine inverter: 1,500 + 60 + 40 = 1,600 W, plus 25 percent.

Can I run my RV air conditioner on batteries?

Only with a very large system. Dometic lists about 1,700 W for its 13,500 BTU units while cooling, so four hours of compressor time comes to 6,800 Wh at the outlet, or 8,000 Wh from the battery, several times what a typical boondocking day uses. Starting the compressor takes far more than running it, so most inverter setups need a soft starter. Most RVers run the air conditioner on shore power or a generator; the calculator shows the difference when you take it out.

Can I count the batteries and panels I already have?

Yes. Enter them in steps 4 and 5. The calculator counts them first and adds only what's missing: more of the same LiFePO4 battery, or whole panels of the size you pick. A lead-acid bank that falls short is replaced rather than mixed with new batteries, because the older ones drag the new ones down. The results show what you need, what you have and what to buy side by side.

Typical values and their sources

Every typical value in the calculator is a starting point, not a fact about your appliance: replace it with the rating on your own label or a measurement. A maker's rating is the figure printed for one product; a measured daily use comes from a label or a maker's test over a day; a planning example is a figure we chose: within the range a maker gives, or between products when no single one stands for all. Values last checked November 2026.