How to Calculate Battery Backup Runtime: The Definitive Formula
Buying a portable power station based on marketing claims is a gamble. To know exactly how long your CPAP, laptop, or fridge will stay alive during a blackout, you need to understand the “Efficiency Tax” and the math behind Watt-Hours. This guide breaks down the 1,000-word science of runtime estimation.
The Real-World Runtime Equation
Most manufacturers list capacity in Watt-Hours (Wh), but you can’t actually use 100% of that energy. Internal components like inverters and cooling fans consume power just to stay on. We call this Inverter Inefficiency.
The Industry Standard Calculation:
(Watt-Hours × 0.85) / Device Watts = Hours of Runtime
Note: We use 0.85 to account for the typical 15% energy loss during AC conversion.
Common Device Power Consumption
Not all devices are created equal. Some pull a steady stream of power, while others (like refrigerators) cycle on and off. Use these averages for your calculations:
| Device | Typical Watts | Estimated Hours (500Wh Station) |
|---|---|---|
| Smartphone | 5W – 10W | 45+ Recharges |
| Laptop | 50W – 100W | 5 – 8 Hours |
| CPAP Machine | 30W – 60W | 7 – 12 Hours |
| Mini Fridge | 60W (Avg) | 6 – 8 Hours |
⚠️ Beware of Surge Watts
Your battery might have the capacity (Wh) to run a device, but does it have the Inverter Strength (Watts) to start it? Motor-based appliances like sump pumps or power tools can require 3x their running wattage for the first 2 seconds. If your station isn’t rated for the surge, it will trip a safety fuse.
How to “Cheat” the Efficiency Tax
You can actually get closer to 95% efficiency if you avoid the AC inverter entirely. Whenever possible, use the DC (12V) or USB-C ports on your power station. This skips the DC-to-AC conversion process, which is the hungriest part of your battery’s internal brain.
Using a 12V car adapter for your CPAP or a USB-C PD cable for your MacBook can add 2–4 hours of extra runtime on a single charge.
Battery Runtime FAQ
This is normal. Between heat loss in the wires, the power needed to run the LCD screen/BMS, and the inverter’s energy conversion, a 15-20% loss is standard across all brands like Jackery and EcoFlow.
Yes. As lithium cells age (after 500-3000 cycles), their chemical capacity shrinks. An old unit may still show “100%” on the screen, but it’s 100% of a smaller “bucket.”
Yes, but it’s variable. Use: (Input Watts – Output Watts). If you are pulling in 100W from solar and using 60W for a laptop, your battery is actually gaining 40W per hour, giving you “infinite” runtime until the sun sets.
