A boondocking battery estimate starts with one simple idea: batteries store energy, appliances spend energy, and solar or generator charging puts some energy back. If the spending is higher than the charging, the battery bank trends down until it reaches its usable reserve.
The formula that matters
Use watt-hours for the cleanest estimate:
Battery volts × rated amp-hours × usable percentage = usable watt-hours.
A 100Ah 12V lithium battery is roughly 1,200Wh rated. Planning around 80% usable gives about 960Wh. A 100Ah 12V lead-acid or AGM battery is also roughly 1,200Wh rated, but it is commonly planned closer to 50% usable, or about 600Wh, because deep discharge shortens life and voltage drops under load.
Common RV load ranges
- LED lights: often 10–40Wh for an evening depending on how many are on.
- Roof vent fan: often 150–400Wh overnight depending on speed.
- Water pump: usually small daily energy, but high short bursts.
- 12V refrigerator: commonly hundreds to over 1,000Wh per day depending on heat and cycling.
- CPAP: may be modest without humidifier, but much higher with heated humidifier or heated hose.
- Microwave or coffee maker: high power for short time; inverter and cable sizing matter.
Example: a 200Ah lithium bank
A 200Ah 12V LiFePO4 bank has about 2,400Wh rated. At 80% usable, plan around 1,920Wh. If your daily use is 650Wh, that bank may last about 2.9 days without charging. If your daily use is 1,500Wh because a 12V refrigerator, fan and laptop are running heavily, the same bank is closer to 1.3 days without charging.
Why inverters change the answer
Household-style 120V items run through an inverter. The inverter is useful, but it is not free. A 500Wh AC appliance load may require roughly 550Wh or more from the battery after conversion losses. Inverters also have idle draw, so leaving one on all night can use energy even when nothing obvious is running.
How solar changes boondocking runtime
Solar replaces daily energy. It does not make the battery larger. A 400W array with 4.5 peak sun hours at 75% real-world efficiency might make about 1,350Wh on a good day. In shade, winter, smoke, storms or poor panel angle, the actual number can be much lower. Use the RV solar panel calculator to compare 200W, 400W, 600W and larger systems.
When the estimate is wrong
If your batteries die sooner than expected, check for hidden loads, an inverter left on, a weak battery, poor charging, incorrect lithium settings, cold temperatures or a battery monitor that has drifted out of calibration. If the battery goes dead while plugged into shore power, use the RV battery not charging on shore power guide. If it drains while stored, start with RV battery keeps dying while parked.
Calculate your own runtime
Enter your battery bank, devices and daily charging to see a plain-English estimate.
FAQ
How many batteries do I need for two days of boondocking?
Add up your daily watt-hours, multiply by two, then divide by your usable battery percentage and system voltage. Add extra margin for weather, battery age and unexpected loads.
Should I use amp-hours or watt-hours?
Use watt-hours when comparing different voltages or inverter loads. Amp-hours are useful only when the battery-bank voltage is clear.
Can I run an RV air conditioner from batteries?
Sometimes, but it requires a large lithium bank, serious inverter capacity, heavy cabling and a charging plan. For most RV owners, air conditioning is the load that changes the entire system design.
Sources and review notes
Battery and inverter limits vary by model. Confirm capacity, discharge limits, temperature limits, fuse sizing and charging profiles in the manuals for your exact equipment.