Every portable power station spec sheet lists watts, watt-hours, and amps. If you do not know what these mean, the numbers blur together and buying decisions become guesswork. This guide explains the three units in plain English, with real examples from devices and power stations in our database.
The Water Tank Analogy
The simplest way to understand electrical units is to think of water flowing from a tank through a faucet.
Watt-hours (Wh) = the size of the tank. A 2,048 Wh power station holds 2,048 watt-hours of energy, just like a 50-gallon tank holds 50 gallons of water. A bigger tank means more total energy available before you run out.
Watts (W) = the flow rate of the faucet. A device that draws 1,500 watts is draining the tank fast, like a wide-open faucet. A device that draws 56 watts is a slow trickle. The flow rate determines how quickly the tank empties.
Amps (A) = the width of the pipe. More amps means more current flowing through the wire. A thicker pipe carries more water per second at the same pressure. In electrical terms: more amps means more current at the same voltage.
Volts (V) = the water pressure. In the US, your wall outlet delivers 120 volts. This is fixed. You do not choose it. It is the pressure that pushes the current through the wire and into your device.
These four units are connected by one formula: Watts = Volts × Amps. At 120 volts (standard US outlet), a 12.5-amp device draws 1,500 watts. That is it. Every other calculation in this guide builds on this one relationship.
Watts: Can It Run This Device?
Watts measure how much power a device needs right now, at this moment. When a product spec sheet says “1,500W,” it means the device draws 1,500 watts of continuous power while operating.
Your power station has a running watts rating (also called continuous watts). This is the maximum power the station can deliver continuously without overheating or shutting down. If your device draws more watts than the station can deliver, the station trips its overload protection and shuts off.
Real examples from our database:
| Device | Running Watts | What This Means |
|---|---|---|
| CPAP machine (ResMed AirSense 11) | 56.1W | Almost nothing. Any power station can run this. |
| French-door refrigerator (LG LMXS28596S) | 207W | Low draw. Most stations 300W and up handle this. |
| Space heater (1,500W standard) | 1,500W | Heavy draw. Needs a station rated 1,500W or higher. |
| Circular saw (DeWalt DWE575, 15A) | 1,800W | Very heavy draw. Needs a station rated 2,000W+ for margin. |
The rule is simple: your power station’s running watts must be higher than the device’s running watts. If the station is rated 2,400W and the device draws 1,500W, it works. If the station is rated 300W and the device draws 1,500W, it does not.
Watt-Hours: How Long Will It Last?
Watts tell you whether the station can run the device. Watt-hours tell you for how long.
A watt-hour is one watt of power delivered for one hour. A 2,048 Wh battery can deliver 2,048 watts for one hour, or 1,024 watts for two hours, or 56 watts for roughly 36 hours. The math works in both directions: more watts means shorter runtime, fewer watts means longer runtime.
In practice, you never get 100% of the rated watt-hours. Inverter losses, battery chemistry, and real-world conditions reduce usable energy by approximately 30%. We use a 0.70 derate factor (70% of rated capacity) for all runtime estimates on this site.
Runtime (hours) = Battery capacity (Wh) × 0.70 / Device running watts
The 0.70 factor accounts for inverter efficiency, battery chemistry losses, and real-world conditions.
Two examples using the same station, different devices:
Station: 2,048 Wh. Device: CPAP at 56.1W.
2,048 × 0.70 / 56.1 = 25.6 hours
More than a full night of sleep therapy, even during a long outage.
Station: 2,048 Wh. Device: Space heater at 1,500W.
2,048 × 0.70 / 1,500 = 0.96 hours (about 57 minutes)
Same battery, same station. Under one hour of runtime because the heater draws 27 times more power than the CPAP.
This is the most important lesson in this guide: battery capacity (Wh) and device power draw (W) together determine runtime. A large battery does not guarantee long runtime if the device draws heavy watts. A small battery can last all night if the device draws almost nothing.
Amps: The Nameplate Number
Most devices do not list watts on their label. They list amps. Converting amps to watts is straightforward in the US:
Watts = Amps × 120 (for any standard 120V outlet)
| Device | Nameplate Amps | × 120V | = Watts |
|---|---|---|---|
| Space heater | 12.5A | × 120 | 1,500W |
| Circular saw (DeWalt DWE575) | 15A | × 120 | 1,800W |
| CPAP machine | 0.47A | × 120 | 56.1W |
Watch the voltage next to the amps. A label that says “5A” means very different things depending on the voltage. At 120V AC (wall outlet), 5 amps is 600 watts. At 12V DC (car adapter or USB), 5 amps is only 60 watts. Ten times less power from the same number of amps. Always check whether the label shows AC or DC voltage before calculating watts.
What about 240V devices? Some large appliances (central AC, well pumps, clothes dryers, water heaters) require 240V circuits. For these: Watts = Amps × 240. A 240V well pump rated at 10A draws 2,400W. Most portable power stations output 120V only. Five models in our database support 240V output, all priced above $3,700. See our 120V vs 240V guide for details.
Surge Watts: The Hidden Fourth Number
Running watts are not the whole story. Many devices draw a brief spike of power when they first start up. This spike is called surge watts (or starting watts, or inrush current). It lasts a fraction of a second to a few seconds, but if your power station cannot handle it, the station shuts down and the device never starts.
Motors are the biggest offenders. A sump pump that runs at 1,127W may surge to 3,381W at startup. That is 3× the running watts. If your power station is rated for 2,400W surge, it cannot start this pump, even though 1,127W is well within its running capacity.
| Device | Running Watts | Surge Watts | Ratio |
|---|---|---|---|
| CPAP machine | 56.1W | 73.2W | 1.3× |
| Refrigerator (LG french-door) | 207W | 414W | 2.0× |
| Sump pump (Wayne 1/2 HP) | 1,127W | 3,381W | 3.0× |
Resistive devices (space heaters, electric blankets, toasters) have no surge. They draw the same watts from the moment you turn them on. Electronic devices (laptops, routers, TVs) have minimal surge. Motor-driven devices (pumps, compressors, saws) have significant surge. Compressor-driven devices (AC units, refrigerators) can surge 3× to 5× their running watts.
Your power station lists both running watts and surge watts. Both must exceed the device’s requirements. For the full explanation of how surge works and how we calculate it, see our surge watts guide.
Putting It All Together
When you look at a power station spec sheet, you now know what the numbers mean:
| Spec | What It Tells You | Example |
|---|---|---|
| Running watts (W) | Can it power your device? | Bluetti AC200L: 2,400W continuous |
| Surge watts (W) | Can it start your device? | Bluetti AC200L: 3,600W surge |
| Capacity (Wh) | How long will it last? | Bluetti AC200L: 2,048 Wh |
The sizing check takes three steps:
- Running watts. Is the station’s continuous output higher than your device’s running watts? If no, the station cannot power the device at all.
- Surge watts. Is the station’s surge rating higher than your device’s startup surge? If no, the device will not start even if the running watts are fine.
- Runtime. Capacity (Wh) × 0.70 / device watts = hours of backup. Is that enough for your needs?
For step-by-step sizing with specific devices, see our how to size a power station guide or use our compatibility calculator to check any device-station pairing in our database.