A propane RV furnace needs battery power for its blower and controls. In the example below, a full 100 Ah bank with a 50% reserve, a 5.1 A furnace running half the time and 1 A of other draw reaches reserve in about 14.1 hours. Your starting charge and actual blower time change that estimate.
The furnace’s propane supply and its electrical supply are separate limits. A full cylinder cannot keep the blower running once the battery can no longer power it. This guide estimates the battery side for an installed propane furnace on a 12V system.
Calculate your night · Compare battery examples · Check your actual draw
RV furnace battery runtime calculator
PLAN A NIGHT WITHOUT HOOKUPS
Will your battery last the night?
For a propane furnace on a 12V battery bank, with no charging during the night. The filled-in values are an example; change them to match your RV.
Interactive inputs need JavaScript. The example calculation and comparison table below work without it.
Your planned night fits the entered battery budget
4 blower-on hours during 8 elapsed hours. This estimates a battery budget, not a guaranteed period of heat.
Inputs: 100 Ah at 12V; 100% starting charge; 50% reserve; 100% available capacity; 5.1 A furnace; 30 blower min/h; 1 A other draw; 8-hour night.
See the calculation
- Capacity after your allowance: 100 Ah × 100% = 100 Ah.
- Available above reserve: 100 Ah × max(0, 100% − 50%) = 50 Ah.
- Average draw: 5.1 A × 30/60 + 1 A = 3.55 A.
- Planned night: 20.4 Ah furnace + 8 Ah other loads = 28.4 Ah.
- Time to reserve: 50 Ah ÷ 3.55 A = 14.1 h.
A positive margin is not a heating or safety guarantee. Cold, battery wear, voltage drop and the furnace's cutoff can shorten actual runtime. Keep a separate propane supply and cold-weather backup plan.
How long will a 100 Ah battery run an RV furnace?
There is no single runtime for a “100 Ah battery.” You need to know how much charge is available above your reserve, how often the furnace runs and what else draws from the same bank. The table changes the battery budget while keeping the furnace and background load the same.
| Battery and chosen reserve | 15 min/h | 30 min/h | 60 min/h |
|---|---|---|---|
| 100 Ah lead-acid; 50% reserve | 22 h | 14.1 h | 8.2 h |
| 200 Ah lead-acid; 50% reserve | 44 h | 28.2 h | 16.4 h |
| 100 Ah lithium; 20% reserve | 35.2 h | 22.5 h | 13.1 h |
Calculated with this page’s engine: 5.1 A furnace, 1 A other average draw, 100% starting charge and no capacity reduction. Minutes mean blower-on time per elapsed hour. These are illustrative inputs, not measured campsite results.
The lithium reserve is an editorial choice; your battery’s requirements may differ.
The table’s hours are elapsed time, not continuous blower operation. At 30 minutes per hour, the blower runs for half of each elapsed hour. Colder weather can make it run more often, moving your result toward the continuous-running column even before any battery-capacity loss is considered.
How many amps does an RV furnace use?
Look up the exact model’s electrical current, or measure its draw. BTU/h describes heat input or output, not battery current. A fuse rating is a protection rating, not the furnace’s normal consumption.
| Exact listing | Supply | Published draw |
|---|---|---|
| Suburban NT-16SEQ, 2503ABK | 12V DC | 2.7 A |
| Suburban SF-Q 20,000 BTU/h listing, 2552A | 12V DC | 5.1 A |
Source: the linked Suburban specification pages, checked November 2026. The 5.1 A value applies to this listing, not every SF-Q model. Use your furnace’s own rating or measured draw.
For this type of furnace, propane supplies the heat; electricity runs the blower and controls. You do not need to add an inverter loss to a furnace connected directly to the 12V system. An electric space heater powered through an inverter is a different load and is outside this calculator’s scope.
How much of your battery can you actually use?
Start with the bank’s amp-hours at its operating voltage. Two matching 12V, 100 Ah batteries in parallel make a 12V, 200 Ah bank. Two matching 6V, 220 Ah batteries in series make a 12V, 220 Ah bank—not 440 Ah.
These examples describe capacity arithmetic, not wiring instructions.
Victron illustrates the distinction in its battery-bank wiring guide. If you are unsure what is installed, identify the batteries and arrangement before entering a total. Our guide to RV battery types explains the main battery categories.
Reserve means charge left in the battery. A 50% reserve means you plan to stop at half charge. If you start at 70%, only the 20 percentage points between 70% and 50% are available for the night.
The lead-acid preset keeps 50% in reserve. That is a planning choice consistent with Trojan’s recommendation for shallower discharges, not a universal shutdown point. Check your battery’s instructions, especially if you have a different chemistry or a battery designed for a different duty.
The lithium preset keeps 20% in reserve as an editable planning allowance. It does not claim that every lithium battery requires that exact reserve. A battery-management-system cutoff is a protective limit, not a sensible target for an overnight heating plan.
A replacement battery is not automatically the answer to a short estimate. First verify that the existing bank is charging properly and that its usable capacity is understood. If replacement is justified, compare the fit and charging requirements in our RV deep-cycle battery guide and lithium battery guide.
An overnight calculation you can check
Use the calculator’s starting example: a 100 Ah bank, full charge, 50% reserve, a 5.1 A furnace, 30 blower minutes per hour, 1 A of other average draw and an eight-hour night. Assume the battery can deliver its label capacity, with no charging during the night.
- 50 Ah kept in reserve
- 28.4 Ah used overnight
- 21.6 Ah left above reserve
The furnace runs for 4 hours during the eight-hour night. After the furnace and other loads use 28.4 Ah, 71.6 Ah remains: your 50 Ah reserve plus a 21.6 Ah margin.
Illustrative calculation from a full battery; no allowance for cold or age. A positive margin does not guarantee overnight heat.
- Available charge: 100 Ah × (100% − 50%) = 50 Ah.
- Furnace average: 5.1 A × 30/60 = 2.55 A.
- Total average: 2.55 A + 1 A = 3.55 A.
- Time to the chosen reserve: 50 Ah ÷ 3.55 A = about 14.1 hours.
- For this 8-hour night: 28.4 Ah of planned demand, leaving 21.6 Ah above reserve.
If that same bank starts at 70% instead of full, only 20 Ah is available above the 50% reserve. The estimate falls to 5.6 hours. Entering the actual starting charge matters more than assuming every night begins with the label’s full capacity.
The relationship is straightforward: available amp-hours ÷ average amps = estimated elapsed hours to reserve. Using amps for current and amp-hours for capacity keeps the calculation consistent. Do not enter a watt rating in a field asking for amps.
How do you measure the furnace and other battery loads?
A shunt-based battery monitor can show total current leaving the bank. Note the baseline with the furnace idle, then observe the current while its blower runs with other loads unchanged. The increase gives you an estimate of furnace draw.
Repeat the check if a refrigerator or another cycling appliance interferes.
Time the blower through a representative hour, including fan operation before ignition and after the burner stops. Enter those minutes, then try a longer-running scenario as well. A mild evening’s observation may underestimate the draw later on a colder night.
For background consumption, include everything other than the furnace once: refrigerator use, alarm and control circuits, lighting, pump use and any inverter idle draw. Use the average across the period, not just the lowest reading while everything happens to be off.
If you already have a working monitor, use it. Our RV battery-monitor comparison covers alternatives if you need one. The Victron BMV-712 documentation explains how that model measures current and estimates state of charge; correct installation and configuration still matter.
For that specific option, check the Victron BMV-712 on Amazon. It helps replace assumptions with readings; it does not add battery capacity or make an uncertain heating plan reliable by itself.
Why can real runtime be shorter in cold weather?
Cold can affect both sides of the calculation: the RV needs more heat, while the battery may supply less usable capacity. Trojan’s temperature guidance confirms that colder conditions reduce lead-acid capacity. Battery age, discharge rate and voltage drop can also make label-based estimates optimistic.
The calculator does not infer those losses from outdoor temperature. Its available-capacity field starts at 100%, which means no allowance has been made. Use manufacturer data or measured capacity when available; otherwise compare more conservative scenarios and keep a backup plan rather than treating the first result as a promise.
Charging limits need a separate check. For example, Victron’s Lithium Battery Smart instructions allow charging only from 41°F (5°C) for that series. Other batteries have different limits and protections.
Being able to discharge in the cold does not mean a battery can also accept a recharge there.
What should you change if the overnight budget is too short?
| What you find | Useful next step |
|---|---|
| The bank starts partly charged | Check the charging setup before buying more capacity. Our RV converter guide explains the equipment used on shore power. |
| Other loads take a large share | Identify optional loads you can reduce. Keep required alarms and furnace controls operating. |
| The blower runs longer than expected | Recalculate with the observed running time. Address ordinary drafts without covering required ventilation, furnace vents or return-air openings. |
| The bank is adequate for one night, but not the trip | Plan how it will recharge. Our RV solar-sizing guide covers the daylight side; solar does not supply the furnace during a dark night. |
Check the fuel side too: how long propane lasts in an RV covers a separate limit that this battery calculator cannot resolve. More electrical reserve does not refill an empty cylinder.
Follow the furnace manufacturer’s operating instructions, keep required alarms working and never bypass a shutdown or safety control. Check your assumptions while another warm place is available, before depending on an unfamiliar setup overnight.
How this guide was prepared: manufacturer furnace specifications, battery-care instructions and monitor documentation were checked against the calculation shown above. The examples are calculated, not campsite tests. For related equipment, browse our RV power guides.
