Wh to runtime: how long will a portable power station really run my stuff?
A friend asked me last winter to help pick a power station for ice fishing. He bought a "2 kWh" off-brand unit on Amazon, calculated 12 hours of runtime for his ice auger and a propane heater fan, and watched it die at 8 hours. He thought it was defective. It wasn't — the math just doesn't work the way the box says.
The "capacity ÷ device watts" formula on every product page assumes nothing leaves the system as heat, the battery management system reserves nothing, and every Wh of stored energy reaches the outlet. None of those are true.
"By the time AC reaches the outlet, you're working with roughly 80% of the nameplate watt-hours. Plan around that, not the box."— Will Prowse, DIY Solar Power Forum
The formula that actually works
For modern LiFePO4 units:
runtime_hours = (capacity_Wh × 0.88 × 0.92) / device_watts
The 0.88 is inverter efficiency — battery DC has to be converted to outlet AC, and that conversion sheds heat. The 0.92 is the BMS reserve: most modern packs hold back 5–10% of the cells to extend cycle life and protect against deep discharge. Hobotech's "100% load test" series on YouTube has verified those numbers across dozens of units, and OutdoorGearLab's lab measurements come in within 2–3% of those constants.
For NMC units (mostly older or ultralight models), drop the inverter efficiency to 0.85.
Real-world examples from independent load tests
Hobotech's load test of the EcoFlow Delta 2 (1,024 Wh spec) at sustained 1,750 W output pulled 980 Wh from the wall before cutoff. That's 95.7% delivery — slightly better than my formula predicts because EcoFlow tunes their BMS aggressively. The older Jackery Explorer 500 from 2019? The same test type pulled 465 Wh from a 518 Wh spec, closer to 90%. The cycle life is showing in the wild too: second-hand Explorer 500s on eBay routinely test at 380–420 Wh after 4–5 years of weekend use.
The pattern across modern LiFePO4 units lands consistently in the 88–96% range. Older NMC and budget Amazon brands trail at 80–88%.
Quick reference table
Using the realistic formula for LiFePO4 units:
| Capacity | 5 W (LED) | 50 W (laptop) | 100 W (TV) | 500 W (small fridge) |
|---|---|---|---|---|
| 300 Wh | 49 hr | 4.9 hr | 2.4 hr | 29 min |
| 500 Wh | 81 hr | 8.1 hr | 4 hr | 49 min |
| 1,000 Wh | 162 hr | 16 hr | 8.1 hr | 1.6 hr |
| 2,000 Wh | 324 hr | 32 hr | 16 hr | 3.2 hr |
| 4,000 Wh | 648 hr | 65 hr | 32 hr | 6.5 hr |
The five mistakes I see most often
I've fielded the same handful of misconceptions from neighbors, friends, and forum posters often enough that they deserve a section of their own.
1. Using the wattage label on the device. The "1500W" stamped on a hair dryer or microwave is the maximum draw, not the average. Most modern devices spend a lot of time well below max. A 65 W laptop charger draws 65 W only when the battery is at 5% and screen brightness is at full — typical use is closer to 30 W. Measure with a Kill A Watt meter ($25 on Amazon) and you'll often find your gear pulls 30–50% less than the spec label.
2. Ignoring the standby drain of the power station itself. Most units consume 5–15 W just being "on" with the display lit. Leave a unit running overnight with nothing plugged in and you can lose 100–200 Wh by morning. Use the auto-off feature, or hit the master power button when not actively using it.
3. Confusing Wh with mAh. Power banks list mAh (milliamp-hours) at 3.7V; power stations list Wh (watt-hours). To compare: Wh = mAh × 3.7 / 1000. A "27,000 mAh" power bank is 100 Wh — less than a third of even the smallest power station.
4. Assuming the surge spec covers any motor. A "1800 W continuous, 3600 W surge" inverter handles most refrigerators (1,000–1,200 W spike), but window AC units can spike to 2,500–3,500 W, and well pumps hit 4,000–5,000 W. Surge spec is real but applies only briefly — typically under 1 second. If your appliance has a soft-start kit, surge requirements drop dramatically.
5. Buying for the worst-case scenario you'll never hit. The flip side of "buy bigger" — some buyers spend $3,500 on a Delta Pro 3 because they once read about a multi-day outage. Most blackouts are under 6 hours. Plan for the 80th percentile of your actual life, not the 99th percentile of someone else's hurricane.
What this means for buying decisions
Buy with 30–50% headroom over your calculated minimum. Three reasons it matters: cold weather eats capacity, batteries degrade (LFP holds 80% after ~3,000 cycles or roughly 12 years of daily use, and the curve isn't flat — you notice the loss before then), and you will add devices once you have the unit. Owners who buy a 500 Wh "just for my CPAP" almost always wish they'd gotten a 1 kWh once they realize they can also run the router, lights, and laptop during a blackout.
If you want to skip the arithmetic entirely, plug your specific numbers into the runtime calculator. Or browse the device database — every product page calculates runtimes for 24 specific device-and-scenario combinations using this same formula.