For most people, a power outage is a minor inconvenience that means reading by candlelight or losing internet access for a few hours. But for individuals who rely on essential medical devices—such as CPAP (Continuous Positive Airway Pressure) machines for sleep apnea, BiPAP machines, or oxygen concentrators—a loss of electricity can quickly become a serious health hazard.
Preparing for emergencies means having a reliable battery backup system or portable power station ready to take over when the grid fails. However, buying a battery without knowing its actual capabilities is dangerous. You must be absolutely certain that the backup system can keep your medical device running for the duration of the night or until power is restored.
In this guide, we will explain how to determine the power requirements of common medical devices and how to calculate the exact battery capacity you need. You can also utilize our Battery Runtime Calculator to quickly test different battery sizes against your specific medical equipment.
Determining Your Medical Device's Power Consumption (Watts)
The first step in calculating backup time is figuring out exactly how much power your medical device consumes. This information is usually printed on a sticker on the back or bottom of the device, or on the power supply brick.
You are looking for the maximum power draw in Watts (W). If the label only lists Volts (V) and Amps (A), you can find the Watts by multiplying them together: Watts = Volts × Amps.
CPAP Machines
CPAP machines vary wildly in their power consumption based on one major factor: the heated humidifier and heated tube.
- Without Humidifier: A standard CPAP machine running purely as an air pump is very efficient, usually drawing between 10W to 20W.
- With Heated Humidifier: Heating water to create steam requires a massive amount of energy. Turning on the humidifier can cause the power draw to spike to 70W to 100W.
Crucial Tip: When operating on battery power during an emergency, it is highly recommended to turn off the heated humidifier and heated tubing to drastically extend your battery life.
Oxygen Concentrators
Oxygen concentrators work much harder than CPAP machines, as they have to compress air and filter out nitrogen.
- Portable Oxygen Concentrators (POCs): These are designed to be efficient and typically draw between 40W to 120W, depending on the flow setting.
- Home Oxygen Concentrators: Larger, stationary units require significantly more power, often drawing between 300W and 600W.
Calculating Required Battery Capacity
Once you know the Wattage of your device, you can calculate how long a specific battery will last. Battery capacity is usually measured in Watt-hours (Wh) or Ampere-hours (Ah). For portable power stations (like Jackery, EcoFlow, or Bluetti), the capacity is almost always advertised prominently in Watt-hours (Wh).
The basic formula to find the runtime is:
Runtime (Hours) = (Battery Capacity in Wh × Usable Capacity Percentage × Efficiency) / Device Wattage
Let's look at the variables:
- Usable Capacity (DoD): You cannot use 100% of a battery's advertised capacity. The Battery Management System (BMS) reserves a portion to protect the battery from permanent damage. For modern lithium-ion power stations, you can usually access about 85% to 90% (0.85 - 0.90) of the rated capacity.
- Efficiency (Inverter Loss): If your medical device plugs into a standard AC wall outlet (110V/220V) on the power station, the station has to convert its internal DC battery power into AC power using an inverter. This process loses about 10% to 15% of the energy as heat. We use 85% (0.85) as a safe efficiency factor.
Pro Tip for CPAP Users: Many CPAP machines can run directly on 12V or 24V DC power using a special DC cable provided by the manufacturer (like ResMed or Philips). Bypassing the AC inverter by plugging directly into the DC/car-port of a power station avoids the inverter efficiency loss, giving you roughly 15% to 20% more runtime!
Example 1: CPAP on AC Power with Humidifier OFF
You have a 500Wh portable power station. The grid goes down, and you plug your CPAP (using the standard AC wall plug) into the station. You turn the humidifier OFF, so the CPAP draws 15 Watts.
- Capacity: 500 Wh
- Usable Capacity: 0.90
- Inverter Efficiency: 0.85
- Load: 15 W
Runtime = (500 Wh × 0.90 × 0.85) / 15 WRuntime = 382.5 Wh / 15 WRuntime = 25.5 Hours
With the humidifier off, this small 500Wh battery can easily power your CPAP for three full 8-hour nights of sleep.
Example 2: CPAP on AC Power with Humidifier ON
Using the exact same 500Wh power station, you decide to leave the heated humidifier ON, increasing the power draw to 90 Watts.
Runtime = (500 Wh × 0.90 × 0.85) / 90 WRuntime = 382.5 Wh / 90 WRuntime = 4.25 Hours (4 hours and 15 minutes)
By leaving the heater on, the battery will die in the middle of the first night. This illustrates why managing your load is critical during an outage.
Example 3: Portable Oxygen Concentrator
You have a 1000Wh power station and need to run a portable oxygen concentrator that draws 80 Watts on its current flow setting.
Runtime = (1000 Wh × 0.90 × 0.85) / 80 WRuntime = 765 Wh / 80 WRuntime = 9.56 Hours (approx. 9 hours and 30 minutes)
To test different scenarios and ensure you buy the correct size battery for your needs, run your numbers through our Battery Runtime Calculator.
Frequently Asked Questions (FAQ)
Can I use a computer UPS to back up my CPAP machine?
It is generally not recommended. A standard computer Uninterruptible Power Supply (UPS) is designed to give you 10-15 minutes of power to save your work and safely shut down a PC. Their internal batteries are very small (often under 100Wh) and use heavy lead-acid batteries. They will not power a CPAP through the night. You need a dedicated "Portable Power Station" or a deep-cycle battery setup.
Will a portable power station wake me up when the power goes out?
Most portable power stations do not function exactly like a UPS. If you plug the power station into the wall, and the CPAP into the power station, the station's internal fan might run constantly, and there may be a slight delay (a few milliseconds) when switching to battery power if the grid fails. Some CPAP machines will reboot during this delay. It is best to test this specific setup while you are awake to see how your medical device reacts.
When it comes to medical devices, always err on the side of caution. Calculate your needs carefully, buy a slightly larger battery than the math dictates (for a safety margin), and always test your backup system before an emergency actually strikes.
