RV BATTERY RUNTIME GUIDE
The useful answer comes from energy, not the amp-hour number printed on the battery. Convert the battery to watt-hours, decide how much of that energy is realistically usable, then compare it with the loads you actually run.
CALCULATE MY RV LOADS →Amp-hours only make sense when voltage is known. Watt-hours let you compare a battery bank directly with appliance energy use.
A 100Ah 12.8V LiFePO4 battery contains about 1,280Wh of nominal energy. A 200Ah bank is about 2,560Wh. A 300Ah bank is about 3,840Wh.
Nominal capacity is not the same as the amount you should plan to consume. Heeler RV's calculator uses an 80% usable planning allowance for 12.8V LiFePO4, leaving a 20% reserve. That is a planning choice, not a claim that every lithium battery must stop at 20% state of charge.
For flooded lead-acid and AGM banks, a common conservative planning approach is to use roughly 50% of nominal capacity when long battery life matters. High current, temperature, battery age and lead-acid's Peukert effect can reduce practical energy further.
| 12.8V lithium bank | Nominal energy | 80% planning usable | Approx. AC energy after 90% inverter efficiency |
|---|---|---|---|
| 100Ah | 1,280Wh | 1,024Wh | 922Wh |
| 200Ah | 2,560Wh | 2,048Wh | 1,843Wh |
| 300Ah | 3,840Wh | 3,072Wh | 2,765Wh |
Rounded planning numbers. Battery BMS limits, temperature, cable losses, inverter efficiency and actual state of charge all affect real results.
For a load that stays fairly constant, divide usable battery energy by load wattage.
For a 120V AC appliance powered through an inverter, include conversion loss. A simple planning version is:
Using 90% as a rough inverter-efficiency assumption, a 100Ah lithium bank with 1,024Wh of planning energy gives about 922Wh available to AC loads. A steady 100W AC load would therefore be in the neighborhood of nine hours, before allowing for inverter idle consumption or other RV loads.
Most RV equipment does not run continuously. A fridge cycles. A furnace blower runs only when heat is called for. Starlink use changes by trip. For these loads, daily watt-hours are more useful than a single continuous-runtime number.
| Example load | Planning input | Approx. daily energy | What changes it |
|---|---|---|---|
| 12V / RV fridge | 70W × 8 h | 560Wh/day | Ambient temperature, compressor duty cycle, fridge type |
| Starlink / Wi-Fi | 60W × 5 h | 300Wh/day | Model, operating mode, AC vs direct-DC conversion |
| CPAP | 40W × 8 h | 320Wh/day | Humidifier/heated hose, pressure, AC adapter losses |
| Furnace blower | 90W × 3 h | 270Wh/day | Outside temperature, thermostat setting, blower current |
| LED lighting | 40W × 4 h | 160Wh/day | Number of fixtures and dimming |
Those are examples, not universal ratings. The best input is the equipment label, manufacturer data or a measured value. The RV Power Calculator lets you edit both watts and hours for that reason.
Suppose an RV uses about 1,200Wh per day from all 12V and inverter loads combined. With no solar, alternator charging, shore power or generator contribution:
| 12.8V lithium bank | Planning usable Wh | Approx. days at 1,200Wh/day |
|---|---|---|
| 100Ah | 1,024Wh | 0.85 day |
| 200Ah | 2,048Wh | 1.7 days |
| 300Ah | 3,072Wh | 2.6 days |
This is why sizing from “I want a 200Ah battery” can put the cart before the horse. First determine the daily load, then choose how many parked days you want the battery to cover.
For a deeper comparison, see 100Ah vs 200Ah vs 300Ah RV lithium batteries and the specific 100Ah lithium runtime guide.
Daily energy tells you how long the bank can last. It does not prove the battery and inverter can supply a large instantaneous load. Microwaves, coffee makers, induction cooking and air conditioners can require high inverter output and very high DC current even when they run for only a short time.
A 1,500W AC load can pull well over 120A from a nominal 12V battery system after conversion losses. Battery BMS current limits, conductor size, fuse protection, voltage drop and inverter surge capability all have to be checked separately.
Solar does not magically make the battery larger. It replaces some of the energy you use during the day. The useful planning number is the daily deficit after charging.
If the RV uses 1,200Wh/day and solar reliably returns 800Wh/day, the battery is only down roughly 400Wh at the end of that day. But cloud, shade, flat roof mounting, season and panel temperature can change production substantially, so the battery still needs enough reserve for poor solar days and overnight use.
For solar sizing, use How Much Solar Do I Need for My RV?. If you drive frequently, DC-to-DC charging can also materially change the daily balance.
Instead of guessing from battery labels, build the actual load list: fridge, furnace, Starlink, CPAP, lights, pumps, entertainment, laptops and 120V appliances. Then choose the number of parked days you want to cover.
USE THE FREE RV POWER CALCULATOR →A 100Ah 12.8V LiFePO4 battery stores about 1,280Wh nominally. Using the same 80% planning allowance as the Heeler RV calculator leaves about 1,024Wh before inverter losses. Divide that by your measured or estimated load energy to get a practical starting point.
Both are useful, but watt-hours are usually cleaner for mixed RV systems because they let you compare 12V loads, inverter loads and battery storage in one energy unit. Convert back to amp-hours when sizing a 12V battery bank.
Yes. A large inverter left on with very little AC load can consume meaningful energy over a full day. Use the actual inverter specification or measure it when runtime matters.
Not reliably. LiFePO4 voltage stays relatively flat through much of its state-of-charge range. A properly configured shunt monitor is generally more useful for tracking energy in and out of an RV battery bank.