Why sizing matters
Buying a CPAP battery without knowing your power draw is like buying a gas can without knowing your tank size. Too small and you wake up at 3 AM with no therapy. Too big and you overpay for capacity you'll never use.
This guide walks you through the exact math so you buy the right battery the first time.
Step 1: Find your CPAP's power draw
Check three places for your machine's wattage:
- The power supply label — flip over your AC adapter and look for "Output" in watts
- The machine's clinical menu — some machines (like ResMed AirSense 11) show real-time power draw
- The manufacturer spec sheet — search your model + "specifications"
Typical power draws by configuration
| Setup | Power draw | Notes |
|---|---|---|
| CPAP only | 15–30 W | Pressure-dependent |
| CPAP + humidifier (low) | 30–45 W | Humidity level 1–3 |
| CPAP + humidifier (high) | 45–65 W | Humidity level 4–6 |
| CPAP + humidifier + heated tube | 50–80 W | Maximum comfort |
| BiPAP | 25–50 W | Higher pressures = more draw |
| BiPAP + humidifier | 50–90 W | Varies widely by model |
Step 2: Calculate watt-hours needed
The formula:
Wh needed = Power draw (W) × Hours of sleep
Examples for an 8-hour night:
- 20 W × 8 hrs = 160 Wh
- 35 W × 8 hrs = 280 Wh
- 55 W × 8 hrs = 440 Wh
- 75 W × 8 hrs = 600 Wh
Step 3: Add your buffer
Real-world batteries don't deliver 100% of their rated capacity. Add a buffer:
- DC-direct battery: Add 10% (very efficient, minimal loss)
- AC power station: Add 20% (inverter conversion losses)
- Cold weather use: Add 15–25% (lithium batteries lose capacity in cold)
Adjusted formula: Wh needed × buffer multiplier = Battery size to buy
Example: 280 Wh × 1.20 = 336 Wh minimum (for AC power station use)
Step 4: Match to available batteries
| Your calculated Wh | Recommended battery tier |
|---|---|
| Under 100 Wh | Purpose-built CPAP battery (Pilot-24 Lite, Freedom V²) |
| 100–250 Wh | Small power station (Bluetti AC2A, Jackery 240) |
| 250–500 Wh | Mid-range power station (EcoFlow RIVER 2, Jackery 500) |
| 500+ Wh | Large power station (EcoFlow DELTA 2) or multi-battery setup |
Pro tips to reduce power draw
If you want to stretch your battery further, these changes make the biggest difference:
- Turn off the heated humidifier — saves 15–40 W; use an HME filter instead
- Turn off the heated tube — saves 10–25 W; use a fleece hose cover for warmth
- Lower your humidity setting — each step down saves 3–8 W
- Use EPR on ResMed machines — the "off" setting draws slightly less power than EPR level 3
- Use DC-direct power — skip the AC inverter and save 10–15% of battery capacity
Real-world example
Setup: ResMed AirSense 10 AutoSet, pressure 12 cmH2O, humidifier off, DC power cable
- Power draw: ~22 W
- Sleep time: 8 hours
- Wh needed: 22 × 8 = 176 Wh
- Buffer (10% for DC): 176 × 1.10 = 194 Wh
- Recommendation: Jackery Explorer 240 v2 (241 Wh) — one full night with room to spare
With humidifier on (level 4):
- Power draw: ~48 W
- Wh needed: 48 × 8 = 384 Wh
- Buffer (20% for AC): 384 × 1.20 = 461 Wh
- Recommendation: Jackery Explorer 500 (518 Wh) — one full night with humidifier
What about multi-night trips?
Multiply your single-night Wh by the number of nights. For a 3-night camping trip at 194 Wh/night (DC-direct, no humidifier), you need about 582 Wh.
Options:
- One large battery: EcoFlow DELTA 2 (1,024 Wh) handles 5+ nights
- Battery + solar panel: A 100 W panel recharges ~300 Wh during a sunny day
- Two smaller batteries: Bring two Jackery 240s and alternate nights
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