Winter battery planning
Estimate how much 12V battery the furnace blower uses overnight, how much propane the burner consumes, and which winter-camping limit shows up first.
Quick winter read
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.
First limiter
2.5 nights battery / 7.0 nights propane
Nightly battery draw
65.3Ah at 12V including other overnight loads
Furnace propane
34.0 lb usable after reserve
Low voltage, restricted ducts, sail-switch trouble, weak grounds, and regulator issues can still stop heat even when the planning math looks comfortable.
Start over
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Winter furnace calculator
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
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 risk check
First thing to check
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
Why this exists
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.
Run the propane runtime calculatorBattery sizing calculator
Use this when the full rig load list matters, not just the furnace blower overnight.
Run the RV battery calculatorCold-weather lithium guide
Use this before charging lithium batteries around freezing temperatures.
Read the cold-weather lithium guideStay length calculator
Use this to compare winter power and propane limits against water and waste tanks.
Run the stay-length calculatorTool notes
The output is a winter-planning estimate. It does not approve a furnace installation, diagnose voltage problems, or replace manufacturer service guidance.
Blower watts are multiplied by the overnight heating window and furnace duty cycle, because the fan only runs when the furnace cycles.
Nightly blower and other overnight watt-hours are compared against the usable battery window between starting SOC and stop SOC.
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
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.
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.
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 the relevant manufacturer guidance, safety limits, installation requirements, and local rules before changing the rig.See assumptions
Gear to compare after the math
These handoffs match the calculator family, not a one-click prescription. Verify fit, specs, clearances, and install limits before buying.
SOK SK12V100P 12V 100Ah LiFePO4 Battery
Best for
Budget lithium builds after a smaller bank result
A common 100Ah building block when the calculator result still fits a straightforward 12V lithium bank.
Current listing
SOK SK12V100P 12V 100Ah LiFePO4 Battery at SOK.
Victron SmartShunt 500A/50mV
Best for
Making battery runtime visible instead of guessed
A shunt-based monitor is the usual next buy when the calculator result depends on real state-of-charge tracking.
Current listing
Victron SmartShunt 500A/50mV at Victron.
Victron Orion XS 12/12-50A DC-DC Battery Charger
Best for
Improving recharge support for larger daily loads
Use this class of DC-DC charger when the battery answer only works if drive-day recovery is part of the plan.
Current listing
Victron Orion XS 12/12-50A DC-DC Battery Charger at Victron.
Frequently asked
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 range for many RV forced-air furnaces, but your model can differ.
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.
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.
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.