Watts and watt-hours are two of the most common units on portable power station spec sheets — and they're constantly confused. Mixing them up leads to either wildly optimistic expectations for what a battery can do, or purchases of units far larger than needed. This short article aims to clear it up.
The water analogy
Think of electricity like water.
- Wattage (W) is the flow rate — how wide the pipe is. A high-wattage device draws a lot of flow at any given moment.
- Watt-hours (Wh) is the volume of the tank. The bigger the tank, the longer the flow can keep running before emptying.
A 1,000W microwave needs a high enough pipe (station rated for at least 1,000W continuous). Running that microwave for an hour uses 1,000 Wh from the tank. Running it for six minutes uses 100 Wh.
The two questions to ask
When sizing a power station, you need to answer two separate questions:
- Can the station deliver enough power at an instant to start and run the device? This is the wattage question. If the device needs 800W to run and the station is rated at 500W continuous, no — it can't run it. The inverter will trip.
- Can the station store enough energy to run the device for as long as you need? This is the watt-hours question. A 500Wh station running a 100W light bulb will run it for about 5 hours. Running a 1,000W microwave on a 500Wh station? Mathematically about 30 minutes, but in practice you'd never use it that long.
Why the labels confuse people
Manufacturers quote capacity in Wh prominently and wattage less prominently. So shoppers fixate on the kWh number and assume a 2,000 Wh unit can power anything for 'a long time.' It can — if the device's running wattage is low. A 2,000 Wh station running a 2,000 W device can run it for about an hour (with inverter losses, ~50 minutes). Running a 100 W device, it runs for about 20 hours.
The wattage rating meanwhile is critical for things with motors. A fridge's compressor might need 800 W for the brief moment it surges on, even though it averages only 150 W after that. If the station's surge rating is below 800 W, the fridge won't start.
How to use both numbers
When comparing stations for your use case:
- List the highest-wattage device you'll plug in, including its surge (start-up) wattage. The station's continuous wattage needs to meet or exceed the running wattage; the station's surge wattage needs to meet or exceed the start-up wattage.
- List how many watt-hours per day each device uses (watts × hours per day of use). Add them up for a daily Wh total, with a 20 percent buffer for inverter losses.
- Buy a station whose wattage rating is comfortably above your highest-wattage device, and whose watt-hour capacity is your daily total × the number of days of autonomy you want.
A worked example
Suppose you want to run a small fridge (150 W running, surge ~600 W), a few LED lights (~30 W total), and a laptop charger (~60 W) during a power outage:
- Wattage check: highest running surge is the fridge's 600 W. Choose a station with at least 600 W continuous (800+ W to be safe). Most stations in this class are 1,000 W+ continuous, so this is easy.
- Wh per day: fridge at ~1,500 Wh (with duty cycle), lights at ~120 Wh (4 hours), laptop at ~240 Wh (4 hours) = ~1,860 Wh/day total.
- Autonomy: for 1 day of outage, that's ~2,000 Wh with the buffer. For 3 days, ~6,000 Wh.
If you live somewhere with frequent short outages, a ~2,000 Wh / 1,500 W station is the right size. If you live somewhere where outages last days, you want significantly more capacity or you want solar recharge — which is where our solar calculator on the home page comes in.
Round-trip losses
One last number to know: every time energy is moved in or out of a battery, some is lost. For LiFePO4 with a good inverter, that's typically 10 to 15 percent round-trip. So a 1,000 Wh battery delivers about 850 to 900 Wh to your devices. Manufacturers sometimes publish the gross capacity (1,000 Wh) and the usable capacity (often ~850 Wh); the usable number is the one that matters for your calculations.
Want to Measure Your Own Usage?
Understanding the difference between watts and watt-hours is useful in theory — but the fastest way to actually see how much power your devices use is to measure it directly. Smart plugs with energy monitoring do exactly that: they sit between your device and the outlet and report real-time watt and watt-hour usage back to an app.
For indoor devices: Minoston Z-Wave Indoor Smart Plug Plugs into any standard indoor outlet and adds energy monitoring to whatever's connected — a space heater, a lamp, an appliance you're curious about. Z-Wave 800 series means reliable smart home integration if you're already running a Z-Wave setup. Price: from $24.64 — Buy on Minoston via Awin
For outdoor devices: Minoston Z-Wave Outdoor Smart Plug Same energy monitoring in an IP65 weatherproof housing — useful for outdoor lighting, holiday displays, or anything plugged in outside where you want to see actual watt-hour draw over time. Price: from $26.35 — Buy on Minoston via Awin
Why this matters: A device's wattage rating tells you its draw at any given moment, but watt-hours tell you what it actually costs you over time. A smart plug bridges that gap — instead of estimating, you can see exactly how many watt-hours a device used over a day, a week, or a month, and use that to spot which devices are actually driving your electricity bill.