How long will an RV battery last?
Watt-hours in the bank, divided by watts going out. The version battery brands print is 100 Ah × 12 V × 80% depth of discharge × 90% inverter efficiency = 864 Wh, so a 100 W load runs 8.6 hours. It's a fine first answer and it's wrong in three ways that matter in a rig: it assumes the bank is full, it ignores the inverter's idle draw, and for lead-acid it ignores that a battery's amp-hour label is only true if you drain it slowly. The tool above handles all three. For a 100 Ah lithium running a 32-inch TV through a 90% inverter with 20 W of idle, the answer is 13.5 h; for the same label in flooded lead-acid it's 7.4 h.
How do you calculate battery runtime with an inverter?
Anything on a wall outlet in the rig is a 120 V load, and the inverter that makes 120 V from 12 V loses a share of everything that passes through it, plus its own idle draw while it's switched on (8–30 W on the data sheets the inverter-size page lists). So the library's 50 W TV pulls 50 ÷ 0.9 = 56 W, plus 20 W idle, = 76 W from the bank, about 5.9 A at 12.8 V. A 12 V load skips both. That's the arithmetic behind the "12 V or 120 V" toggle, and it hits small loads hardest: a compressor fridge averaging 27.5 W on its 12 V cord would cost 51 W through the inverter, 1.8× as much.
Lead-acid batteries age fast below half charge, so the usable window is 100% down to 50%: a "100 Ah" flooded or AGM battery has 50 Ah to give. Lithium iron phosphate is routinely taken to 80% and can go to 100%; this site plans at 80% so the answer still holds on a cold night with a tired bank. Both rules are baked into every number on this page, and into "charge now": a lead-acid bank at 60% has 10 points left, not 60.
Why does a lead-acid battery run out faster than the math says?
Because the label was measured at a 20-hour rate, 5 A for a 100 Ah battery, and lead-acid delivers less the harder you pull. Peukert's law describes it with one exponent per battery. Concorde's Lifeline technical manual recommends 1.12 for its AGM batteries. Flooded batteries are commonly quoted at 1.2–1.3, and this page uses 1.3, the conservative end, so a flooded answer errs on the short side. Lithium's discharge curve is nearly flat, and 1.03 covers it. What a 100 Ah label is worth at each draw:
| Draw | ≈ Load at 12 V | Flooded | AGM | Lithium |
|---|---|---|---|---|
| 5 A | 60 W | 100 Ah | 100 Ah | 100 Ah |
| 10 A | 120 W | 81 Ah | 92 Ah | 98 Ah |
| 25 A | 300 W | 62 Ah | 82 Ah | 95 Ah |
| 50 A | 600 W | 50 Ah | 76 Ah | 93 Ah |
| 80 A | 960 W | 44 Ah | 72 Ah | 92 Ah |
Effective capacity of a 100 Ah (20-hour rate) battery by discharge current, Peukert t = H × (C ÷ IH)ᵏ, with k = 1.3 flooded, 1.12 AGM, 1.03 lithium. The label is never exceeded at slow draws.
Lithium stores about the same energy per label. What it does differently is give you what the label says, and then let you use 80% of it instead of half. At an 80 A draw, about a 1,000 W load at 12 V, a 100 Ah flooded battery is worth 44 Ah before the 50% rule and 22 Ah after it.
How long will a 100 Ah battery run a fridge, furnace, CPAP or TV?
Computed by the engine for one 100 Ah battery from full, 90% inverter, 20 W idle, with each appliance's watts from the Power Budget library:
| Load | Watts | Lithium | AGM | Flooded |
|---|---|---|---|---|
| 12 V compressor fridge | 55 × 50% | 37.2 h | 21.8 h | 21.8 h |
| Furnace blower, running | 100 | 10.1 h | 5.6 h | 5.1 h |
| CPAP (12 V cord) | 40 | 25.6 h | 15 h | 15 h |
| TV, 32 in (120 V) | 50 | 13.5 h | 7.7 h | 7.4 h |
| Laptop (120 V brick) | 60 | 11.7 h | 6.6 h | 6.2 h |
| 1,000 W inverter load | 1,000 | 50 min | 22 min | 13 min |
One 100 Ah battery, 12 V. The fridge line uses a 50% compressor duty; the furnace line is the blower running continuously, so multiply by your duty cycle for a night (the Power Budget does this by season).
A 1,000 W inverter load, a little more than a coffee maker draws while it brews, runs 50 min on a 100 Ah lithium and 13 min on a 100 Ah flooded battery: 3.8× as long. The energy on the two labels is close, 1,280 Wh against 1,200, only 1.07× apart. The gap comes from the flooded battery delivering 41 of its 100 Ah at the 94 A this load pulls through the inverter, and only half of that being safe to use.
What if the bank isn't full?
It usually isn't. After a cloudy day a bank might read 70%, and the question is how long the furnace runs from there. "Charge now" handles it: the usable window runs from 100% down to the chemistry's floor, so a lithium bank at 70% has 50 points to give, and a lead-acid bank at 70% has 20. Read the percentage from a shunt-based monitor if you have one; resting voltage is a poor guide for lithium, whose voltage barely moves between 90% and 20%. For lead-acid, resting voltage after 4–6 hours with nothing running or charging is a fair proxy. By the makers' tables full is about 12.7–12.8 V, half is 12.10 V flooded and 12.2 V AGM, and anything under 12.0 V is already past the floor (the battery-voltage page reads your meter against them). Reading a lead-acid bank while it's under load or right after charging gives a number that's wrong in the flattering direction.
Does cold weather shorten runtime?
Yes, for both chemistries, in different ways. Lead-acid loses capacity as it cools: the Lifeline manual says an AGM battery held at 0 °F gives about 60% of its room-temperature capacity, and a cold battery also charges more slowly. Lithium discharges well in the cold but must not be charged below its maker's limit: Battle Born's BB10012 data sheet allows charging from 25 °F, and Victron's Smart lithium batteries from 41 °F (5 °C). That is why RV lithium batteries carry either a heater or a low-temperature charge cutoff. This page doesn't derate for temperature because the curves vary by make; on a freezing night, treat a lead-acid answer as generous.
What about 24 V and 48 V banks?
Bigger rigs and lithium conversions increasingly wire the bank at 24 or 48 V, because the same watts arrive at half or a quarter of the amps: thinner cables, cooler connections, and an inverter that can be sized in the 5,000 W range without heroic wiring. The runtime doesn't change: 400 Ah at 12 V and 200 Ah at 24 V hold the same 5,120 Wh. What changes is the arithmetic in between, which is why the "System" setting matters. Enter each battery's own amp-hours and count, and the engine works out the bank's capacity at the system voltage: four 100 Ah 12 V batteries in series-parallel on 24 V are a 200 Ah bank, and the chart above will say so. A 12 V-only accessory such as the furnace blower or the water pump then runs through a DC-DC converter with its own few percent of loss, small enough that this page ignores it.
A whole day, not one load
"One load" answers the forum question. "A whole day" answers the camping one: with the daily watt-hours the Power Budget saved to My Rig and the solar on your roof, it gives nights from full, the number that decides whether a second battery or another panel is the better buy.
Sources
- Technical Manual for Lifeline Batteries, Document 6-0101 Rev. F, Concorde Battery Corporation: Appendix C, SOC vs OCV at 77 °F after 4 hours of rest (p. 32); absorption 14.3 ± 0.1 V, float 13.3 ± 0.1 V
- 100 Amp Hour 12 Volt LiFePO4 Deep Cycle Battery, Model BB10012 (data sheet), Battle Born Batteries: 50 A maximum charge (0.5C); absorption and cell balancing at 14.2–14.6 V; charge temperature 25–135 °F; 100 A continuous, 200 A for 30 s; 28 W heater on the BB10012H
- 12,8 & 25,6 Volt Lithium-Iron-Phosphate Batteries Smart, Victron Energy: Charge temperature +5 °C to +50 °C (41–122 °F)