Winter battery planning
RV furnace battery drain calculator
Estimate how much 12V battery the furnace blower uses overnight, how much propane the burner consumes, and which winter-camping limit shows up first.
Winter furnace calculator
Check whether the furnace blower or propane supply ends the cold stay first.
Propane runtime only tells half the story. Most RV furnaces also need 12V battery power for the blower and control board, so cold nights can drain the bank even when there is plenty of propane left.
Furnace battery estimate
Battery is likely the winter-stay limiter
The furnace blower and overnight loads use about 783Wh per night, or 65.3Ah at 12V. Battery reserve lasts roughly 2.5 nights, while furnace propane lasts roughly 7 nights before reserve.
Nightly battery draw
783Wh
65.3Ah at 12V including other overnight loads
Battery nights
2.5 nights
41% of usable reserve per night
Furnace propane
4.86 lb
105.0k BTU of furnace heat per night
Propane nights
7.0 nights
34.0 lb usable after reserve
Winter limiter check
First limiter
Battery
Required Ah
350Ah
Recommended Ah
400Ah
Recommended next move
Add usable battery capacity, lower overnight loads, raise the stop-SOC only if chemistry allows, or plan generator/alternator recharge between cold nights.
Watch-outs
- Furnace blower watts vary by furnace model, duct restriction, battery voltage, fan condition, and how clean the return-air path is.
- Duty cycle changes quickly with outside temperature, wind, insulation, slide-outs, thermostat setting, and how often doors open.
- Low battery voltage can make furnace boards and fans misbehave before the bank looks fully empty on a simple capacity estimate.
- The propane estimate covers furnace heat only. Add fridge, water heater, cooking, and generator propane use with the propane runtime calculator if those loads matter.
Why this exists
A propane furnace is also a battery load.
Cold-weather RV planning often stops at propane cylinder size. The missing piece is the 12V blower, which can quietly drain a small battery bank overnight while the propane gauge still looks comfortable.
Use this with
Propane runtime calculator
Use this to add the fridge, water heater, cooking, and other propane loads after furnace-only fuel math.
Open next stepBattery sizing calculator
Use this when the full rig load list matters, not just the furnace blower overnight.
Open next stepCold-weather lithium guide
Use this before charging lithium batteries around freezing temperatures.
Open next stepStay length calculator
Use this to compare winter power and propane limits against water and waste tanks.
Open next stepTool notes
What the furnace battery estimate is actually saying
The output is a winter-planning estimate. It does not approve a furnace installation, diagnose voltage problems, or replace manufacturer service guidance.
Blower watt-hours
Blower watts are multiplied by the overnight heating window and furnace duty cycle, because the fan only runs when the furnace cycles.
Battery nights
Nightly blower and other overnight watt-hours are compared against the usable battery window between starting SOC and stop SOC.
Furnace propane per night
Furnace BTU per hour is multiplied by heating hours and duty cycle, then divided by roughly 21,600 BTU per pound of propane.
Avoid these traps
Common mistakes before buying
Only checking propane
A forced-air RV furnace can have enough propane and still shut down if the 12V blower pulls the battery bank below the furnace control board's comfort zone.
Using summer battery math
Cold-weather camping usually adds furnace blower draw, more lights, more fan time, and weaker solar recovery. A summer battery plan can look much better than it behaves in January.
Guessing duty cycle from one mild night
Wind, slide-outs, skirting, insulation, and thermostat setting can change furnace duty cycle fast. Use a conservative cold-night estimate if the site has no easy bailout.
Treat the calculator result as a planning range, then verify wiring, clearances, fusing, ventilation, and manufacturer limits before installation.See assumptions
Frequently asked
Questions RVers usually ask next.
How do I find furnace blower watts?
Check the furnace manual or measure DC current while the blower runs, then multiply amps by battery voltage. If you only have a rough starting point, 80W to 130W is a common planning lane for many RV forced-air furnaces, but your model can differ.
Why does this include propane if it is a battery calculator?
Winter heat has two bottlenecks: fuel for the burner and 12V power for the blower and controls. Seeing both prevents the common mistake of carrying enough propane while quietly running the battery bank too low.
Should AGM batteries stop at 50% SOC?
For repeated use, 50% is a practical conservative floor for many lead-acid banks. Some owners go lower in emergencies, but repeated deep discharge shortens battery life and cold weather makes recovery harder.
Does this replace a furnace diagnostic?
No. This is a planning estimate. Weak batteries, low voltage, sail-switch issues, dirty ducts, bad grounds, and propane regulator problems can all cause furnace trouble even when the math looks acceptable.