Hours of use = battery watt-hours × 0.85 ÷ the laptop's watts. That is the formula Anker and EcoFlow both publish for their power stations, and it is what the calculator above runs. EcoFlow's own example: a 256Wh station and a 65W laptop give (256 Wh × 0.85) ÷ 65W, which is 3.3 hours.
Watts, watt-hours and where 0.85 comes from
A watt measures how fast a device uses energy at one moment. A watt-hour measures an amount of energy. In Anker's words, one watt-hour equals one watt used for one hour, so a 60W laptop used for a five-hour stretch consumes 300Wh. The capacity printed on a power station or power bank is a watt-hour figure.
The 0.85 is a loss allowance. A power station stores direct current and your laptop charger wants alternating current from the wall socket, so the station's inverter converts one to the other. Anker says energy escapes as heat during that step, and EcoFlow says the same. Both makers use 0.85 as the planning factor, and Anker calls it the standard efficiency factor for power conversion.
Treat it as an average, not a promise.
Anker also says actual runtime is usually shorter because inverters, temperature, battery management and connected equipment all cause losses. The makers' own numbers show the size of the effect. Anker says a 1,024Wh unit typically delivers 13-15 hours of continuous laptop use, and gives 8.5 hours for a 50W laptop on a 500Wh unit.
Finding your laptop's real watts
This is where most estimates go wrong. The number printed on the charger is its maximum output. EcoFlow puts it plainly: charger wattage is the most the adapter can put out, not what the computer is pulling at any given moment.
EcoFlow gives rough figures by laptop type: 30 to 45W for lightweight laptops, 60 to 70W for productivity laptops and 90 to 100W for higher-performance ones. A laptop doing light work often draws less than that, and the draw changes from minute to minute.
The only reliable way to know is to measure. A plug-in power meter between the charger and the wall shows the actual draw while you do your usual work. Then enter that figure in the calculator.
Heavy jobs push the number up. EcoFlow notes that AI tasks, 3D programs and video rendering keep the chip and graphics card busy for long stretches. We have not measured how many watts a local AI model draws on our test Mac, so if you plan to run one on battery, use a higher figure than you would for browsing and check it with a meter. Our guide to running AI offline on a Mac covers the setup.
Getting more hours from the same battery
EcoFlow points out that the AC route converts twice: the station turns its direct current into AC, and the laptop's charger turns it back. Charging through a high-wattage USB-C port is a DC-to-DC transfer, which EcoFlow says is much more efficient and can give slightly longer runtime. Anker's laptop example also names USB-C charging as the better route.
The calculator keeps 0.85 for every case, so a USB-C setup will usually do a little better than its answer.
A laptop's own battery counts as a power source too. Apple lists a 52.6-watt-hour battery in the MacBook Air (M2, 2022), and it is one of the examples in the tool. A power station only extends that, and it is worth knowing how much the laptop already holds before you buy anything bigger.
We're building Waystone, an offline assistant that is still in development, to run on a laptop, so the power question comes up for us as well. For a longer outage you will also want to know how long your fridge and freezer will keep food safe.