Guides
Portable Power Station Sizing: Load by Load, Plus a Buying Checklist
Size a portable power station by listing each appliance's rated watts, startup surge and daily watt-hours, then check what a 120 V unit cannot safely run.
What to take away
- Three numbers decide what a station can docontinuous AC watts, surge watts and stored watt-hours. EcoFlow's DELTA 2 manual lists 1,024 Wh, 1,800 W continuous and 2,700 W surge, a mid-size unit.
- Add up the rated watts of everything that will run at the same moment. That total must stay below the continuous rating, not the surge rating.
- Take each appliance's number from its own label or manual. A ResMed AirSense 11 power supply, for example, is documented at 56.1 W typical and 73.2 W peak.
- The EcoFlow manual states AC output at 120 V. A 240 V appliance needs a different product.
- Never feed a generator into house wiring through a wall outlet. The U.S. Consumer Product Safety Commission calls that backfeeding and warns of electrocution risk to utility workers.
The three ratings on the box
Continuous watts is what the inverter can supply all day. Surge watts is a short-term allowance for the moment a motor or compressor starts. Watt-hours (Wh) is the size of the tank. Our units and labels guide explains how watts, volts and amps relate.
The EcoFlow DELTA 2 user manual_1670906417941.pdf) is a good example of how to read a spec sheet:
- Capacity 1,024 Wh at 51.2 V.
- AC outputpure sine wave, 1,800 W total, surge 2,700 W, 120 V, 50/60 Hz.
- A feature called X-Boost that keeps the unit running when a load exceeds the rated output. The manual says X-Boost is not available for all appliances, is not suited to devices with strict voltage requirements, and is better suited to heating devices. It tells owners to test their own devices.
- Maximum AC charging 1,200 W, solar input 11 to 60 V, 500 W maximum, and a 3,000-cycle rating to 80% capacity.
Treat these as one product's numbers, not a standard. The figures come from the 2022 version 1.0 manual, and EcoFlow's DELTA 2 product page now redirects to its DELTA 3 page (checked 25 October 2026), so confirm the specs of whatever model you are actually shopping for. Other brands differ. The point is the method: match your loads to the rated output, not to the marketing headline.
Where to find each appliance's number
Use the nameplate or manual. Look for watts (W). If only volts and amps are printed, multiply them. Editorial suggestion: if the label gives a range or nothing at all, borrow a plug-in watt meter and measure the real draw.
| Item | Verifiable figure | Source |
|---|---|---|
| CPAP (ResMed AirSense 11) | 56.1 W typical, 73.2 W peak for the 65 W power supply | ResMed user guide |
| Phone (iPhone 17) | Fast charging quoted with a 40 W or higher adapter | Apple specifications |
| Laptop (MacBook Air, 13-inch, 2022 or later) | 30 W adapter is the smallest Apple recommends; 14-inch MacBook Pro lists 67, 70 or 96 W | Apple adapter guide |
| Refrigerator | Yearly energy use in kWh, which US refrigerator labels must show (16 CFR 305.14), not instant watts | eCFR: 16 CFR 305.14 |
Two notes on that table. A refrigerator's yearly kWh can be turned into an average wattage (kWh times 1,000, divided by 8,760 hours), but the compressor starts with a surge. The fridge runtime article works through that, so it is not repeated here. CPSC also advises plugging electric space heaters straight into a wall outlet, not a power strip or extension cord. Editorial suggestion: treat heaters as poor candidates for a battery and read the nameplate wattage before trying.
A five-step sizing method
- List what must stay on.Separate must-haves (medical device, router, phone and laptop charging, a lamp, the fridge) from nice-to-haves.
- Write each rated watt figure.Use the label, then a meter if you have one.
- Add the loads that will run together.That sum must sit below the station's continuous rating. If one appliance has a motor or compressor, check the manual for starting current, and compare it with the surge rating.
- Convert to watt-hours.Watts times hours of use equals Wh. A CPAP at 56.1 W for 8 hours is about 449 Wh by arithmetic.
- Add a margin.Editorial suggestion: the inverter wastes some energy as heat, so plan on using no more than about 80% of the label figure. On that rule a 1,024 Wh unit is about an 820 Wh planning number.
Following the CPAP example, the 1,024 Wh unit would cover roughly two nights at that load, assuming nothing else is connected and the figures hold. That calculation only illustrates the method. For any medical device, ask your clinician and the device maker about backup power. Ready.gov advises asking your provider for a plan that covers power-dependent devices and refrigerated medicines.
What a 120 V station should not be asked to run
- 240 V appliances.Check the label for 240 V. The DELTA 2 manual lists 120 V output only.
- Heat-producing loads.Check the label wattage first. The DELTA 2 manual says X-Boost suits heating devices better but also tells owners to test with their own device.
- Anything wired into the house.CPSC's portable generator alert says never to power house wiring by plugging a generator into a wall outlet, because it endangers utility workers and neighbors and bypasses built-in circuit protection. Editorial suggestion: treat a battery unit the same way, and ask a licensed electrician about a transfer switch or interlock if you want it tied to the house. Local code decides what is allowed.
- Medical equipment without clearance.Editorial suggestion: do not rely on a consumer station for life-sustaining equipment unless the device maker and your clinician support it.
If you are still choosing between a battery unit and a gas-powered one, the power station versus gas generator comparison covers fuel, noise and cost. Gas generators carry carbon monoxide risks that a battery unit does not. Ready.gov says to run generators outdoors, at least 20 feet from windows, doors and attached garages.
Buying checklist
- Rated continuous outputabove the sum of your planned loads with room to spare.
- Surge ratingabove the highest startup draw among your motor loads, using each appliance's manual.
- Capacity in Whfrom your step-four total plus the margin.
- Output type.Count the outlets you need and confirm 120 V or 240 V.
- Recharge path.Check maximum AC charging watts and solar input limits. The DELTA 2 manual lists 1,200 W AC and 500 W solar.
- Chemistry and cycle life.The DELTA 2 manual lists LFP cells and 3,000 cycles to 80%. Compare against any other model on the same terms. Battery chemistry basics are in the batteries and charging guide.
- Third-party safety listing.NFPA's tip sheet says to buy products listed by a nationally recognized testing laboratory. The relevant safety standard is UL 2743, "Portable Power Packs," per the Intertek standards notice. Ask the seller for the listing.
- Travel.A 1,024 Wh unit is far above the 100 to 160 Wh limits on the FAA's spare lithium battery rules, so do not plan to fly with one.
Specifications and prices change by model and year. Re-read the manual of the specific unit you buy.
Sources
- EcoFlow DELTA 2 user manual (EcoFlow)_1670906417941.pdf)
- AirSense 11 user guide (ResMed)
- UL 2743 Portable Power Packs standards update notice (Intertek)
- [PackSafeLithium Batteries (FAA)](https://www.faa.gov/hazmat/packsafe/lithium-batteries)
