Wh vs W: the mix-up that leads to bad choices
On a portable power station's spec sheet, two figures look alike yet mean completely different things: watts (W) and watt-hours (Wh). Mixing them up is the number one cause of disappointment after buying.
A watt measures power: how much energy is delivered at a given instant. A watt-hour measures energy: the total amount the station can store, a bit like the capacity of a tank. A 1000 Wh station with a 2000 W output can power an appliance drawing 2000 W, but only for half an hour before it's empty.
Plenty of buyers actually search for a '3000W portable power station' thinking capacity, when that figure says nothing about runtime: it's the maximum power the unit can deliver, not what it stores. Two stations both labeled '3000 W' can have a capacity of 1000 Wh or 3000 Wh, with runtime differing threefold.
Why the advertised Wh are never fully available
The watt-hours printed on the box correspond to the nominal capacity of the internal cells, measured in a lab under ideal conditions. In real use, several factors reduce the energy you actually get back:
- DC/AC conversion: when an appliance is plugged into the station's AC outlet, the inverter turns the batteries' direct current into alternating current. That conversion is never perfect and typically dissipates 8 to 15% of the energy as heat.
- BMS safety margin: the battery management system always keeps a small reserve, at both the top and bottom of the charge, to protect the cells. Invisible on the spec sheet, it usually accounts for 3 to 5% of the advertised capacity.
- Self-consumption: the screen, fans and control electronics draw a bit of power continuously, even with nothing plugged in.
- Temperature: in the cold, lithium chemistry delivers less energy than advertised, a phenomenon that matters even more for power stations, which are often used outdoors, in a van or while camping.
Overall, expect roughly 85 to 92% of the advertised capacity to actually be available through the AC outlet, a bit more through the USB and 12V (DC) outputs, which skip the most demanding conversion stage. These orders of magnitude line up with the inverter efficiency figures measured for stationary lithium storage, documented among others by the NREL.
A worked example
Take a 1000 Wh station, a very common size for van life or home backup. Here's what's actually left depending on which output you use:
| Output used | Typical efficiency | Energy actually available |
|---|---|---|
| AC outlet (through the inverter) | ~85 to 90% | ~850 to 900 Wh |
| USB-C / 12V outputs (direct DC) | ~93 to 97% | ~930 to 970 Wh |
In practice, a 50 W electric cooler will run just under 18 hours on the energy actually available through the AC outlet, versus 20 hours in theory on the advertised raw capacity. The gap looks small on paper; it becomes significant once you're planning several days of off-grid autonomy.
How to read a power station spec sheet without getting fooled
Facing a wall of numbers, a few landmarks let you compare two stations seriously:
- Nominal Wh or usable Wh? The most transparent manufacturers sometimes list both. Absent that detail, treat the printed figure as the nominal (raw) capacity, and apply the efficiency range above.
- Cell chemistry: LiFePO4 (LFP) stations, now the market standard, keep a stable usable capacity over far more cycles than older NMC cells, and tolerate repeated deep discharges better.
- Continuous power or surge? A '4000 W' figure can mean a few-second peak, not the power sustained continuously. Check which of the two values is being advertised.
- Published charging efficiency, when available: a good sign of manufacturer transparency, much like what applies to regular power banks.
The trap of inflated Wh figures
As with power banks, energy has weight: once integrated into a station (housing, inverter and electronics included), a lithium iron phosphate cell stores roughly 90 to 120 Wh per kilogram. A station advertised at 2000 Wh but weighing barely 8 kg is therefore suspicious: at that density, it's likely missing a good chunk of its real capacity.
This trap mostly affects models sold outside established brands, on marketplaces where nobody checks the spec sheet. A Wh capacity that's consistent with the advertised weight is a good sign; the opposite should raise a flag.
How to check a station's real capacity
A few reflexes before buying:
- Check whether the manufacturer states a conversion efficiency or a usable output energy figure distinct from the nominal capacity.
- Compare the Wh/kg ratio to the range given above.
- Favour established brands, whose figures are cross-checked by independent reviews and a large body of user feedback.
- For demanding use (home backup, long-term van life), plan a 10 to 15% margin over your calculated Wh needs to absorb these real-world losses.
Our guide on what appliances you can run on a power station helps you calculate your real Wh needs before choosing a model.
Key takeaways
Three ideas to remember before your next purchase:
- Watt and watt-hour are two different things: instantaneous power versus stored energy.
- Expect roughly 85 to 92% of the advertised capacity to actually be available through the AC outlet, a bit more through direct USB/12V outputs.
- A ratio well above 150 Wh/kg for a consumer-grade LFP station should raise a flag.
Check out our pick of the best power stations 2026 and browse our power stations category to choose a model with an honest capacity.
