Battery Backup for a CPAP Machine: The Humidifier Setting Decides Your Nights

ResMed publishes a bench-measured power table for its Air10 machines — average watt-hours per hour of therapy, by configuration — and as far as I can tell, nobody ranking for this query uses it. Every runtime number below is computed from that document, from ResMed's battery guide, and from the battery spec registry behind this site's calculator. No sleep lab, no watt-meter on my nightstand.

One boundary before any math, because a CPAP is a medical device: this page is power arithmetic, not medical advice. I can tell you what a night of therapy costs in watt-hours. Whether and how you run your machine — pressure, humidification, what to do if a battery dies at 3 a.m. — is a conversation for your doctor and your equipment provider (DME), not for a spreadsheet.

Short version: the blower is not the load — the humidifier is. ResMed's bench numbers for an AirSense 10 AutoSet: about 7 W average with humidification off, 22 W with the heated humidifier, 34 W with humidifier plus heated tube. That one settings screen is a roughly 5× runtime lever, the difference between a small power station holding 41 nights of sleep and holding 8.

What a CPAP actually draws — nameplate vs. measured

The confusion on every retailer page starts with the power brick. An AirSense 10 ships with a 90 W supply, and ResMed's specifications list 53 W typical and 104 W peak consumption — which is how "CPAPs draw 30-60 W" became the internet's planning number. Those are electrical ratings: what the supply must be able to deliver when the humidifier plate is heating hard and the blower is at pressure. A battery does not drain at the peak rating. It drains at average energy per hour, and ResMed measured exactly that on a breathing simulator:

Test conditions, because honest numbers carry them: AutoSet mode at 8-15 cm H2O, humidity level 6, tube temperature 28 °C, on a simulated patient. Bilevel machines in the same document land slightly higher — 9-10 W device-only. Your pressure, your climate, and your machine will move these single digits, but they will not move the ratio, and the ratio is the story. For sizing I use ResMed's bench numbers straight, with one flag: ResMed's own battery tables add a 50% safety margin on top of measured draw. If you want that margin — and for medical equipment I would — take a third off every night-count below.

The humidifier is the whole ballgame

Divide and the lever jumps out: 34 W fully heated against 7 W dry is a 4.9× difference in how fast the same battery empties. No purchase decision on this page matters as much as that one settings screen. The physics is unglamorous — heating water is expensive, moving air is nearly free — which is how a machine that ships with a 90 W brick sips 56 Wh a night with the tank cold.

What I will not do is tell you to turn it off. Humidification is part of how therapy is configured, and running dry is intolerable for some people. The useful move is to have the conversation with your DME before an outage: how should this machine be set up when it is on battery? Many setups can run the water chamber unheated as a passover, and your DME knows what yours supports. The battery math just prices the options: the same box on your shelf is either a week of storms or a month of them, and the setting picks which.

Nights per battery, computed

Nights = usable watt-hours ÷ (average draw × 8 hours). Usable is 85% of nameplate for the plug-in systems — a documented assumption, since no plug-in vendor publishes usable capacity — and 100% for the Powerwall 3, which Tesla rates AC-side. Base units only; expansion modules scale the count linearly.

The honest reading: for a device-only CPAP, everything on this page is overkill — the smallest system holds 41 nights, so giving a slice of it to a phone, a lamp, and the router changes nothing that matters. Fully heated is a different machine: 8 nights from the Apex 300 class, 12 from the F3800 Plus. And the moment the same battery also owns the refrigerator, the CPAP becomes the rounding error: a fridge-day costs about 5,500 Wh of usable battery against 272 Wh for your worst-case night. The fridge owns the budget — that math lives in battery backup for a refrigerator.

One bedside habit worth adopting: run the station as a pass-through, wall power feeding the station and the station feeding the machine, so a 3 a.m. outage is a switchover rather than an interruption. The four systems here market this as a UPS or EPS mode with a switchover time in milliseconds — check that line on the spec sheet of whatever you buy, and ask your DME whether a momentary dropout matters for your machine.

DC beats AC: the converter question

ResMed's battery guide does not hedge: it strongly recommends powering its machines from a battery through a DC-to-DC converter rather than a DC-to-AC inverter. The reason is conversion losses, and ResMed quantified it in its own sizing tables — the same S9 machine, at the same pressure, for the same 8-hour night, needs a 7 Ah battery through the DC converter and 12 Ah through an inverter. Roughly 70% more battery to buy the identical night of sleep, with both figures carrying the same 50% margin.

The principle transfers to power stations. If your station has a regulated 12 V DC port and your machine has a converter — for Air10 machines that is ResMed part 37297 — the DC route stretches every number in the table above, because you skip paying the inverter to reconstruct wall power all night for a single-digit load. Most people use the AC outlet anyway, because it is one cable and zero extra parts, and on a multi-kilowatt-hour station the loss is affordable. If that is you, ResMed's inverter guidance is pure sine wave, 300 W continuous / 500 W peak — every system on this page is pure sine and clears those ratings by an order of magnitude. One warning from the same document: a few of ResMed's legacy heated humidifiers are banned from inverter use outright, so if your machine predates the Air10 series, check the guide before plugging in.

Flying with it: the 100 watt-hour line

None of the systems above boards an airplane. FAA rules for lithium batteries in carry-on baggage: up to 100 Wh travels without approval, 101-160 Wh needs airline approval, and larger batteries are forbidden on passenger aircraft — and spares ride in the cabin, never in checked bags. The smallest battery on this page is 27 times over the line.

That limit is why the dedicated travel CPAP battery class exists — units built to the 95-100 Wh ceiling, flight-legal without paperwork. This site's registry does not carry them, so no recommendations, but the arithmetic above is the spec sheet decoder: at ResMed's 7 W device-only figure, 100 Wh is about a night and a half of therapy on paper. After conversion losses and any margin, plan one honest night per charge, humidifier off — and have the DME conversation about dry running before you are seat 23F finding out the hard way.

Where I land

For a CPAP alone, buy nights, not kilowatts. The smallest system here is already a month of device-only sleep, so the right unit is the cheapest one that runs your machine the way you actually sleep — humidifier configuration included, settled with your DME, with ResMed's 50% margin on top of my table. That is the Apex 300 class at around $1,600, and it covers a fully heated week. If storms are the reason you are shopping, stop sizing for the nightstand: the first night of a real outage puts the fridge, the furnace fan, and the internet on the same battery, and those loads — not the 7 W blower — decide the purchase. The comparison work for that decision is in best portable power station for home backup and battery backup vs. generator. Run yours through the calculator.