guide · energy
CPAP Battery Size Chart: Nights per Watt-Hour
Updated November 5, 2026 · 7 min read
direct answer
A 1,000 Wh lithium pack is 3.1 nights for a 34 W heated-tube machine on a direct 12 V input, and 2.3 nights for the same pack through an AC inverter. One night of that machine takes 272 Wh out of the battery, so the nameplate you need is 302 Wh of LiFePO4 or 320 Wh of li-ion — before any converter loss. The 96 Wh airline bank at the top of the chart is one night of a 10 W travel unit and roughly a third of a night of a heated machine.
Every column below is the same arithmetic: the pack's usable watt-hours divided by what the machine and the path between them actually pull. Change the watts, the chemistry or the temperature and the pattern holds — which is the point of reading a chart instead of a spec sheet.
How to read the chart
Three numbers produce every figure in these tables, and product pages publish only the first:
- Usable watt-hours = nameplate capacity × the depth of discharge the chemistry allows. The chart uses 85% for lithium, which is the figure most power-station makers use; LiFePO4 supports 90% and lead-acid or AGM 50%.
- Draw at the battery = the machine's watts ÷ the path's efficiency, plus whatever the converter or inverter burns sitting idle. A 34 W machine is 34 W on a DC input, 40.1 W through a 12-to-24 V converter, and 46 W through an inverter.
- Nights = usable watt-hours ÷ that draw ÷ hours slept, here eight.
The two tables are the same batteries with a different path. Read Table A if your machine takes 12 V DC directly — most travel units and many CPAP DC adapters do. Read Table B if you are plugging the machine's own wall brick into an AC outlet.
Table A: nights on a direct 12 V input
No converter, no inverter, no standby burn — the pack feeds the machine straight through. This is the best case, and it is the case a 12 V CPAP cable gives you.
| Pack (Wh) | Usable at 85% | 10 W travel unit | 22 W humidifier | 34 W heated tube | 55 W bi-level |
|---|---|---|---|---|---|
| 96 | 81.6 | 1.02 | 0.46 | 0.30 | 0.19 |
| 150 | 127.5 | 1.59 | 0.72 | 0.47 | 0.29 |
| 300 | 255 | 3.19 | 1.45 | 0.94 | 0.58 |
| 500 | 425 | 5.31 | 2.41 | 1.56 | 0.97 |
| 1,000 | 850 | 10.63 | 4.83 | 3.13 | 1.93 |
| 1,500 | 1,275 | 15.94 | 7.24 | 4.69 | 2.90 |
| 2,400 | 2,040 | 25.50 | 11.59 | 7.50 | 4.64 |
| 3,600 | 3,060 | 38.25 | 17.39 | 11.25 | 6.95 |
Figures are nights of eight hours' sleep on a lithium pack at 85% usable, warm, with no conversion loss. These are the same capacities the CPAP battery calculator offers as quick chips.
Table B: nights through an AC inverter
The same packs, with a pure-sine inverter in the middle at 85% efficiency and a 6 W no-load draw. That idle figure is what makes small packs collapse: 6 W on top of a 10 W machine is a 60% overhead before the conversion loss is counted.
| Pack (Wh) | Usable at 85% | 10 W travel unit | 22 W humidifier | 34 W heated tube | 55 W bi-level |
|---|---|---|---|---|---|
| 96 | 81.6 | 0.57 | 0.32 | 0.22 | 0.14 |
| 150 | 127.5 | 0.90 | 0.50 | 0.35 | 0.23 |
| 300 | 255 | 1.79 | 1.00 | 0.69 | 0.45 |
| 500 | 425 | 2.99 | 1.67 | 1.16 | 0.75 |
| 1,000 | 850 | 5.98 | 3.33 | 2.31 | 1.50 |
| 1,500 | 1,275 | 8.97 | 5.00 | 3.46 | 2.25 |
| 2,400 | 2,040 | 14.36 | 8.00 | 5.54 | 3.61 |
| 3,600 | 3,060 | 21.53 | 12.00 | 8.32 | 5.41 |
Each night here costs the pack 142.1 Wh at 10 W, 255.1 Wh at 22 W, 368.0 Wh at 34 W and 565.5 Wh at 55 W — the machine's own watt-hours plus the inverter's conversion loss and its 6 W idle burn.
Compare the 1,000 Wh row across both tables: 3.13 nights becomes 2.31, and 10.63 becomes 5.98. At 10 W the inverter costs you nearly half the pack; at 55 W it costs about a fifth. The smaller the machine, the more the path matters.
The chemistry column
Nameplate capacity is the least trustworthy number on a battery, and depth of discharge is the reason. The same 1,000 Wh box with a 34 W machine on DC:
| Chemistry | Usable share | Usable Wh from 1,000 | Nights at 34 W | Nameplate for 3 nights |
|---|---|---|---|---|
| LiFePO4 | 90% | 900 | 3.31 | 907 Wh |
| Li-ion NMC / power bank | 85% | 850 | 3.13 | 960 Wh |
| Lead-acid or AGM | 50% | 500 | 1.84 | 1,632 Wh |
The AGM row is the one that surprises people. A 100 Ah AGM leisure battery in a van is 1,200 Wh on paper and 600 Wh in the world, because discharging lead-acid past half its capacity is how you kill it in a few dozen cycles. Lithium does not have that limit, which is the actual argument for buying it for a CPAP: not weight, but the fact that you can use the whole tank.
The cold column
A cold pack delivers less of its rated energy, and a CPAP in a cold room draws more. Both effects point the same way, so a winter figure is never a summer figure divided by one number. For a lithium pack, approximate capacity retention:
| Ambient | Capacity kept | Nights at 34 W on DC | Hours at 34 W on DC |
|---|---|---|---|
| Above 20 C / 68 F | 100% | 3.13 | 25.0 |
| Near 0 C / 32 F | about 80% | 2.50 | 20.0 |
| −20 C / −4 F | about 55% | 1.72 | 13.8 |
LiFePO4 has a second cold problem the derate does not capture: it will not accept a charge below freezing, and forcing one damages the cells. A pack brought in from a cold car and put on a charger can refuse to fill even though it reads empty. Bring it inside, let it warm, then charge it.
When the battery is sold in amp-hours
Camper and marine batteries quote amp-hours, and the chart is in watt-hours. Multiply by the pack's voltage and the two are the same thing: 100 Ah at 12 V is 1,200 Wh, and 100 Ah at 24 V is 2,400 Wh.
| 12 V battery | Nameplate Wh | Nights at 34 W, lithium | Nights at 34 W, AGM |
|---|---|---|---|
| 30 Ah | 360 | 1.13 | 0.66 |
| 50 Ah | 600 | 1.88 | 1.10 |
| 100 Ah | 1,200 | 3.75 | 2.21 |
| 150 Ah | 1,800 | 5.63 | 3.31 |
| 200 Ah | 2,400 | 7.50 | 4.41 |
A 40 Ah or 100 Ah LiFePO4 "slimline" is labelled at 12.8 V or 25.6 V nominal rather than 12 V, and its real figure is printed on the case — 40 Ah at 25.6 V is 1,024 Wh, not 480. Check the label before you divide.
The one formula behind every row
→ 1,000 Wh × 0.85 ÷ (34 ÷ 0.85 + 6) = 18.5 h = 2.3 nights through an inverter
Or run it backwards when you are shopping, which is the more useful direction. Three nights at 34 W is 816 Wh delivered, and 816 ÷ 0.85 = 960 Wh of li-ion nameplate. Then add the path: 18% on top for a DC-DC converter, 35% for an AC inverter — 960 Wh becomes 1,133 Wh or 1,296 Wh.
What these rows mean for a purchase
| Situation | Machine | Wh needed per night | Nights | LiFePO4 nameplate | AGM nameplate |
|---|---|---|---|---|---|
| Overnight flight, carry-on | 10 W travel unit | 80 | 1 | 89 Wh | 160 Wh |
| Weekend trip | 10 W travel unit | 80 | 2 | 178 Wh | 320 Wh |
| One night at home, heated tube | 34 W | 272 | 1 | 302 Wh | 544 Wh |
| Blackout kit | 34 W | 272 | 3 | 907 Wh | 1,632 Wh |
| Week off-grid | 34 W | 272 | 7 | 2,116 Wh | 3,808 Wh |
| Heated bi-level, one night | 55 W | 440 | 1 | 489 Wh | 880 Wh |
Two caveats the table cannot carry. The first row and a weekend trip have to fit the airline rules: 160 Wh is the ceiling for a carry-on lithium battery without the airline's agreement, and carriers want 150% of the flight's expected consumption — which is why the 96 Wh bank is the standard answer for flying and 300 Wh packs are checked-baggage territory at best. The second: a week off-grid at 2,116 Wh is a heavy, expensive battery, while the same week with a 200 W panel and a sunny day is a 500 Wh pack. Recharging changes the size you buy, and how to run a CPAP on battery works that through.
Where the watt figures themselves come from — and why the 90 W on your power brick is not your consumption — is covered in how many watts a CPAP uses.
try it with your numbers
CPAP Battery Runtime Calculator
Every row in this chart is one setting in the calculator. Choose your machine, your pack and the path between them and get nights, the size to buy, and the FAA check on the same screen.
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Frequently Asked Questions
The size questions people ask standing in front of a battery shelf.
What size battery do I need for a CPAP?
Nights, watts and hours slept, divided by what the pack can actually give. A 34 W heated machine takes 272 Wh out of the battery each night, so three nights is 816 Wh delivered. Divide by the usable share — 0.90 for LiFePO4, 0.85 for li-ion, 0.50 for AGM — and the nameplate you need is 907 Wh, 960 Wh or 1,632 Wh. Add 18% on top for a DC-DC converter, or 35% for an AC inverter.
How many watt-hours does a CPAP need for one night?
One eight-hour night is 56 Wh with nothing heated, 176 Wh with the humidifier on, 272 Wh with a heated tube and 352 Wh for a heated bi-level. Those are the watts drawn by the machine; the pack has to be bigger, because a 272 Wh night needs 302 Wh of LiFePO4 or 320 Wh of li-ion nameplate before any converter loss is counted.
Can a 100 Ah battery run a CPAP?
Yes, for several nights — if it is a 12 V battery and you respect its depth of discharge. 100 Ah at 12 V is 1,200 Wh, so a 34 W machine on a DC input gets 3.7 nights from a lithium pack at 85% usable and 2.2 nights from an AGM battery at 50%. Put an AC inverter in the middle and the same lithium pack gives 2.8 nights.
Is a power bank enough for CPAP travel?
For one night of a travel unit, yes. A 96 Wh airline-legal bank holds 81.6 Wh usable, which is 8.2 hours at 10 W — a full night — and 2.4 hours at 34 W, which is not. Stacking two 96 Wh banks still leaves a heated machine short of a night, because the arithmetic needed is 272 Wh, not 192 Wh.
How does cold change the chart?
Every figure drops by the same factor. A pack at 0 C gives roughly 80% of its rated energy and one at −20 C about 55%, so the 3.1 nights a 1,000 Wh li-ion bank provides at 34 W becomes 2.5 nights in a 0 C garage and 1.7 nights parked outside in winter. LiFePO4 is worse than the derate suggests: it will not accept a charge below freezing, so a cold tent can leave a pack that reads full unable to refill.