How long will a 100Ah battery run a CPAP?

Short answer

At 60 W through a 90% inverter, a 100Ah LiFePO4 battery runs a CPAP for about 15.4 hours — nearly two 8-hour nights.

How to work it out

100 Ah × 12.8 V × 80% usable = 1,024 Wh
60 W ÷ 90% inverter = 66.7 W average
1,024 Wh ÷ 66.7 W = 15.4 hours (15 h 22 min)

Champion’s chart puts a CPAP machine at 30–60 W without heat and 60–120 W with a heated humidifier. This page uses 60 W through an inverter — the top of the no-heat range — so it errs on the short side.

Adjust for your project

More options: DoD · Inverter eff. · Duty …

Usable depth of discharge (optional)

Blank uses 80% for LiFePO4 and 50% for lead-acid. Check your battery’s data sheet.

Inverter efficiency

Victron puts modern inverters above 90%.

Share of the time the load is on

For anything on a thermostat. A 12 V compressor fridge runs roughly 25–50% of the time (Victron’s example); a household fridge about a third (Virginia Cooperative Extension).

Inverter idle (no-load) draw, W (optional)

What the inverter uses just being on, all the time it is on. It matters most for small loads left running 24/7 — a CPAP, Starlink, a fridge.

Coldest battery temperature

°F

Batteries hold less when cold: Victron gives about 80% for LiFePO4 at 32 °F (0 °C) and 50% at −4 °F (−20 °C); lead-acid about 86% at 32 °F. LiFePO4 must not be charged below 41 °F (5 °C) unless it is self-heating.

Runtime15.4 hours

In hours and minutes: 15 h 22 min
Usable energy: 1,024 Wh
Average draw from the battery: 66.7 W
Battery current while running: 5.2 A

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Battery monitor with shuntA shunt battery monitor shows real amp-hours used, so you know how much of this runtime is left without guessing from voltage.

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Runtime at other loads

Runtime at other loads
Load	Runtime	Battery current while running
25	36.9 hours	2.2 A
50	18.4 hours	4.3 A
100	9.2 hours	8.7 A
200	4.6 hours	17.4 A
500	1.8 hours	43.4 A
1000	0.9 hours	86.8 A
2000	0.5 hours	173.6 A

What to know about running a CPAP on a battery

Turn off the heated humidifier and hose heat. On Champion’s chart heat takes the range from 30–60 W to 60–120 W. Some CPAP makers sell DC power cords and give settings for battery use — follow your machine’s manual.
Skip the inverter if you can. A DC power cord made for your model avoids the inverter loss and the inverter’s idle draw, which on a small load like this can be a big share of the total.
Plan on lead-acid giving about 9 hours. A 100Ah lead-acid battery used to 50% has about 600 Wh, so the same 60 W setup is a single night with a little to spare.
Medical equipment needs a backup plan. If you depend on the CPAP, keep a second power source and talk to your equipment supplier about battery options.

Sizing the inverter for it is on what size inverter to run a CPAP. A ready-made battery box instead? How long a power station runs an electric blanket shows the power-station version of a small overnight load. To size the inverter, see what size inverter to run a CPAP.

Different size? Use the full Battery Runtime Calculator.

What you’ll need

12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
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Pure sine wave inverter, 1000 WMid-range price
Enough for a CPAP, laptop, TV and small appliances. Pure sine is the safe choice for anything with a motor or sensitive electronics.
See options on Amazon (opens in a new tab)
Inline DC watt meterBudget price
Sits between a 12 V device and its supply and shows watts, amps and watt-hours used — handy for measuring a 12 V fridge, Starlink Mini or fan.
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As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.

Frequently asked questions

How long will a 100Ah battery run a CPAP with a heated humidifier?

At 120 W, about 7.7 hours — one night.

How long will a 100Ah battery run a CPAP on a DC cord at 40 W?

About 25.6 hours (1.1 days) — about three 8-hour nights.

How long will a 100Ah deep cycle lead-acid battery run a CPAP?

About 8.8 hours at 60 W through an inverter, using half the battery.

Other common sizes

How long a 100Ah battery runs a fridge — 41 hours — a 12 V compressor fridge (50 W, on half the time) on 100Ah LiFePO4
How long a 100Ah battery lasts with a 1000W inverter — 55 min at full load (12.8 V LiFePO4)
How long a 200Ah battery lasts with a 2000W inverter — 55 min at full load (12.8 V LiFePO4)
How long a 100Ah battery lasts with a 200W load — 4.6 hours (LiFePO4, via inverter)

Sources

Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).