Portable power stations are sold in a wide range of capacities — from a few hundred watt-hours to several kilowatt-hours. Buying too small means the unit can't run what you need; buying too large means paying for capacity you'll never use. This guide walks through how to narrow the choice based on what you're actually trying to power.
Watts vs watt-hours
Two numbers matter most when sizing a portable power station:
- Wattage (W) is the rate at which the unit can deliver power at any instant. This is what determines whether the station can start and run a high-draw device like an air conditioner, a microwave, or a fridge compressor.
- Capacity in watt-hours (Wh) is how much energy the unit stores. This determines how long it can run that device before recharging.
If you're confused about the difference, our article on understanding watt-hours vs watts goes deeper. For sizing, the short version: capacity tells you how long, wattage tells you whether it can run at all.
Step 1: list what you'll plug in
Start by making a list of the devices you actually want powered in an outage or off-grid scenario, with two numbers for each:
- Run wattage (continuous draw while operating)
- Start-up wattage (especially important for anything with a motor — fridges, well pumps, air conditioners)
Start-up wattage is typically 2 to 4 times the running wattage for motorized loads. A 200W fridge might briefly need 800W when the compressor kicks in. If your station can't deliver that peak, it'll trip and the fridge won't start.
You can usually find run wattage on the device label or in the user manual. For start-up wattage, the manufacturer's specs are best; for a ballpark, multiply motor-driven appliances by 3.
Step 2: total the wattage
Add up the running wattages of every device you'd want to run simultaneously. That total is the minimum wattage rating your station needs. Pay attention to peak/start-up: if the station's surge or peak rating isn't at least as high as the largest start-up surge of any device you'll plug in, you'll trip the inverter.
Most modern stations publish both continuous wattage and a surge rating. The surge rating is what matters for motorized loads.
Step 3: estimate run time
For each device on the list, decide how many hours per day you realistically need it to run during an outage. Multiply by the device's running wattage to get watt-hours per day. Add them all up. That's your daily Wh requirement.
A quick example:
- Refrigerator: 150W × 24 hours = 3,600 Wh/day (this assumes a duty cycle — most fridges run the compressor only ~30 to 50 percent of the time, so real consumption is closer to 1,200 to 1,800 Wh/day)
- Phone and laptop charging: ~80 Wh/day total
- A few LED lights: ~50 Wh/day
- A window AC: ~500W × 8 hours = ~4,000 Wh/day
That medium-power-draw scenario totals roughly 5,500 Wh/day.
To convert to 'how big a battery do I need', add a 20 percent buffer and round generously — for the scenario above, you'd want at least a 6,000 to 7,000 Wh station, or you'd want to plan to recharge partway through the day via solar or grid.
Step 4: account for the inverter and chemistry
Two other inputs to confirm before buying:
- Pure sine wave inverter — important for sensitive electronics and motorized loads; modified sine wave stations will cause motors to run hot and can damage compressors.
- LiFePO4 (lithium iron phosphate) chemistry — substantially more cycle life than NMC or older lithium chemistries. Read more in our guide to LiFePO4 vs lead-acid batteries.
Don't oversize just for safety
It's tempting to buy the largest unit available so you never worry about it. Two costs come with that: upfront price, and battery degradation if you don't cycle the capacity. A battery kept topped up at 100 percent for years with no draw ages faster than one that's used.
Match the station to your realistic 80th-percentile use case, not your hypothetical worst-case scenario. Use the calculator on the home page to put real Wh numbers on your system before buying.