To calculate how long a power station will last, divide its usable watt-hour capacity by the total wattage of all connected devices. For example, a 1000Wh power station running a 100W device will last roughly 10 hours, though real-world efficiency losses typically reduce this by 10 to 20 per cent.
One of the most common questions when buying a portable power station is whether it will actually last through a camping trip, power cut or work day. The maths is straightforward once you understand a few key figures. Here is how to work it out accurately.
The Basic Formula
Runtime (hours) = Usable capacity (Wh) / Total device power draw (W)
For example:
- Power station capacity: 1024Wh
- Devices connected: laptop (65W) + phone charger (20W) + LED lamp (10W) = 95W total
- Estimated runtime: 1024 / 95 = 10.8 hours
In practice, apply an efficiency factor of around 85% to account for inverter conversion losses and battery internal resistance. So: 1024 x 0.85 / 95 = 9.2 hours.
Finding Your Device's Power Draw
Check the Label or Manual
Most appliances have a wattage printed on them or in their specification sheet. Look for "W" (watts) or, if only amps and volts are listed, multiply them together: Watts = Volts x Amps.
Use a Plug-In Power Meter
For devices with variable loads (refrigerators, laptops, televisions), a plug-in energy monitor gives a real-world average over time. This is far more accurate than nameplate ratings, which often show maximum rather than typical draw.
Common Device Wattages
- Laptop: 45 to 90W
- Smartphone charging: 15 to 25W
- Mini fridge (12V compressor): 30 to 60W average
- CPAP machine: 30 to 60W
- LED TV (32 inch): 30 to 50W
- Electric blanket: 60 to 150W
- Hair dryer: 1000 to 2000W (not suitable for most portable stations)
Understanding Usable Capacity
The watt-hour figure on a power station is the total stored energy. Lithium iron phosphate (LiFePO4) batteries are typically usable to about 95% of their rated capacity. Standard lithium-ion cells in most power stations are safely usable to around 80 to 90%. The manufacturer's runtime estimates usually reflect this usable figure rather than the total.
Factors That Reduce Runtime
Inverter Efficiency
Running AC devices through the built-in inverter uses more energy than running DC devices directly via USB or 12V outputs. High-quality inverters run at 90 to 93% efficiency; cheaper units may be 80 to 85%. For AC loads, multiply your expected runtime by 0.88 as a reasonable average.
Temperature
Cold weather reduces lithium battery capacity noticeably. At 0°C, expect roughly 80% of the rated capacity. At -10°C, this can fall to 60 to 70%. EcoFlow power stations have built-in battery management systems that protect against over-discharge in cold conditions.
Age and Cycle Count
Batteries degrade over time. After 500 full charge cycles, most lithium-ion cells retain around 80% of their original capacity. LiFePO4 cells are rated for 3000 to 3500 cycles at 80% capacity retention, making them far more durable for regular use.
Planning for Multiple Days
If you need power over several days without a mains connection, calculate your total daily energy use first, then check whether solar charging can top the station up between uses. As a rough guide, a 100W solar panel in the UK generates around 250 to 400Wh per day in summer, depending on location and panel angle.
For a daily usage of 500Wh, a 200W solar input (two 100W panels) would typically recharge a station overnight in good conditions. Pair this with a power station of at least 1000Wh to provide a comfortable buffer.
A Practical Planning Table
| Power Station Capacity | 100W Load | 200W Load | 500W Load |
|---|---|---|---|
| 256Wh | 2.2 hrs | 1.1 hrs | 26 mins |
| 512Wh | 4.3 hrs | 2.2 hrs | 52 mins |
| 1024Wh | 8.7 hrs | 4.3 hrs | 1.7 hrs |
| 2048Wh | 17.4 hrs | 8.7 hrs | 3.5 hrs |
Table assumes 85% system efficiency. Actual runtime will vary.
