What size portable power station do I need? Step-by-step sizing math
By Power Portable Editorial Staff · Updated · Picks come from manufacturer specs and current prices. We do not run hands-on tests.
Worked sizing profiles from our runtime engine
The table below computes required battery watt-hours (at 85% usable AC efficiency), simultaneous continuous watts, and peak motor surge watts for two common backup scenarios. Open the interactive runtime and sizing calculator to build your own custom load list.
| Scenario | Included appliances | Outage duration | Load energy (Wh) | Required station (Wh) | Continuous AC (W) | Peak surge (W) |
|---|---|---|---|---|---|---|
| Overnight CPAP comparison (8h) | CPAP without humidifier (11 W) + CPAP with heated humidifier (53 W, 104 W surge) | 8 hours | 512 Wh | 602 Wh | 64 W | 115 W |
| Home storm essentials (12h) | Full-size fridge (1.5 kWh/day) + Wi-Fi router (10 W for 12h) + 1/3 HP sump pump (800 W for 1h, 1,300 W surge) | 12 hours | 1,670 Wh | 1,965 Wh | 810 W | 1,310 W |
Step 1: Add simultaneous running watts and the largest motor startup surge
Every portable power station has an internal AC inverter with two watt ratings: continuous watts (what the inverter can supply indefinitely) and surge or peak watts (what the inverter can supply for 1 to 3 seconds while an electric motor starts). Your station's continuous watt rating must exceed the sum of every appliance you plan to run at the same time.
Induction motors inside refrigerators, chest freezers, air conditioners, and basement sump pumps draw two to three times their running wattage for a fraction of a second at startup. For example, a 1/3 HP sump pump draws 800 W while pumping water but spikes to 1,300 W at startup. When sizing for multiple appliances, add all simultaneous running watts together and add only the single largest surge delta (surge watts minus running watts) on top.
Resistive heating appliances such as coffee makers (900 W), microwave ovens (1,100 W), hair dryers (1,200 W), and portable space heaters (1,500 W) have zero motor surge, but draw high continuous power that quickly drains small batteries.
Source:U.S. DOE and Penn State EGEE 102 Appliance Power Ratings (data/appliances.json)(checked 2026-11-06)Step 2: Calculate required battery watt-hours (Wh) using the 85% usable rule
A power station's sticker capacity in watt-hours (Wh) is measured at the internal DC battery cells. When you plug 120 V AC household appliances into the inverter, DC-to-AC inversion heat, cooling fans, and the battery management system's low-voltage safety reserve consume roughly 15% of the stored energy.
To find the rated battery capacity you need to buy, divide your total appliance watt-hours by 0.85 (our default usable AC fraction). If your appliances consume 850 Wh of energy during a 12-hour outage, dividing 850 Wh by 0.85 tells you to buy at least a 1,000 Wh power station.
For cycling appliances like a full-size refrigerator, never multiply nameplate watts by 24 hours. Because the compressor cycles on and off with the thermostat, use U.S. Department of Energy daily consumption figures (1.5 kWh/day, or 62.5 W average draw) scaled to your outage length.
Source:U.S. DOE Building Technologies Office Refrigerator Baseline & Power Portable Methodology(checked 2026-11-06)Step 3: Match your calculated Wh and W to the four standard power station tiers
Once you know your required watt-hours and continuous watts, match them to one of four standard hardware classes. Stations in the 300 Wh class (200 to 499 W output) weigh 7 to 12 lb and cover CPAP machines without heated humidifiers (11 W), laptops (50 W), and Wi-Fi routers (10 W).
Stations in the 1,000 Wh class (1,500 to 1,800 W output) weigh 24 to 32 lb and bridge an overnight outage for a full-size refrigerator plus electronics. Stations in the 2,000 Wh class (2,000 to 2,400 W output) cover 24-hour refrigerator and sump pump outages, while 3,000 Wh to 5,000 Wh+ expandable cabinets feed multi-circuit home transfer switches.
Source:Power Portable Runtime and Sizing Calculator Engine (lib/calc/runtime.ts)(checked 2026-11-06)Frequently asked questions
Why does my 1,000 Wh power station only deliver about 850 Wh through the AC outlets?
Converting low-voltage DC battery power into 120 V pure-sine-wave AC power generates heat in the inverter MOSFETs and transformers, and the BMS reserves 5% to 10% state of charge to prevent cell damage. Together, those factors yield roughly 85% usable AC energy.
How do I size a power station for a refrigerator without overestimating?
Use the refrigerator's daily kilowatt-hour rating (1.5 kWh/day for a typical full-size fridge, or 62.5 W average) rather than multiplying compressor nameplate watts by 24 hours, then divide by 0.85 for inverter efficiency.
Does running appliances on 12V DC or USB-C give longer runtime than AC outlets?
Yes. Using regulated 12 V DC or USB-C Power Delivery skips the 120 V AC inverter and eliminates its 15 W to 35 W idle tare loss, increasing usable capacity from ~85% to ~92%.