The thing that empties your battery on the first cold night is the furnace, and it is not the propane you should be watching. Suburban’s own NT-SEQ product page lists the NT-16SEQ at 16,000 BTU/h input, 12 volts, and an amp draw of 2.7. Its SF-Q page lists 20,000 BTU/h input, 12 volts, and an amp draw of 5.1. Both read August 27, 2026.
Look at that pair again. The bigger unit buys you 25 percent more heat and costs you 89 percent more current.
The math for one cold night
Say the burner runs six hours across a long October night. That is not a measurement of your rig, it is a round number to work with, and the duty cycle is the one variable nobody can hand you off a spec sheet.
At 2.7 amps: 2.7 x 6 = 16.2 amp hours. At 5.1 amps: 5.1 x 6 = 30.6 amp hours.
Now the propane side. The EIA puts a gallon of propane at 91,452 Btu. Divide that by the 16,000 BTU/h rating and you get about 5.7 hours of burner time per gallon. On the 20,000 BTU/h unit, about 4.6 hours.
So six hours of heat costs you roughly a gallon of propane and 16 to 31 amp hours of battery. One of those you can top up at any gas station on the highway. The other one needs a sunny day, and in October, in the mountains, at a site with trees, that is the harder ask.
Why this catches people
Power budgets get built around the fridge, because the fridge is the load everyone knows about. We put ours at a real number in 12V fridge amp hour draw. The furnace then shows up in the fall and quietly doubles the overnight draw, at exactly the season when the solar day is shortest and the panels are at their worst angle.
The lead acid side of this is worse than the arithmetic looks, because you do not get to use the whole bank. We worked that through in the 20 hour rate and usable capacity. Against a realistic usable figure rather than the number on the sticker, 30 amp hours is a serious share of a single 12V lead acid battery.
The unhelpful part
We have not put a clamp meter on either of these furnaces. Everything above is the manufacturer’s published rating and a federal energy conversion factor, run through arithmetic. Real draw varies with the blower, the board, the state of the battery and how cold it actually gets, and startup is not the same as steady running.
More to the point, the number that decides your night is the duty cycle, and that is a function of your insulation, your thermostat setting and the outside temperature. Nobody can publish it for you.
What to do about it
Two moves, in order. Look up the model plate on your own furnace and find its rated amp draw, because a 2.7 and a 5.1 are the same appliance to most people and they are not the same problem. Then add that draw, times your honest guess at burner hours, into the load side of the solar calculator before you go anywhere cold, and read our power budget guide for the rest of the loads.
If the answer comes out ugly, the cheap fixes are insulation and a lower thermostat setting, not a bigger furnace. A bigger furnace makes the electrical problem worse.