SIZING GUIDE

What size portable power station do I need for a power outage?

Updated October 2026 · Educational guide, not electrical advice

Portable power stations are sold by battery capacity in watt-hours (Wh) and by output in watts (W). You need enough of both: enough energy to last the outage, and enough power to run everything that is on at the same time, including motor startup surges. This guide walks through the math the HEA calculator uses so you can check any product listing yourself.

Step 1: List what must stay on

Write down only the essentials for an outage: refrigerator, internet router, a few lights, phone chargers, a CPAP machine, a fan. For each one, find the running watts on its label, nameplate or manual. If a label shows amps and volts instead, multiply them (amps × volts ≈ watts) as a rough estimate.

Step 2: Estimate energy (Wh) over the outage

Energy depends on how long each appliance actually runs, not just how long the outage lasts. A refrigerator compressor cycles on and off; lights may only be on in the evening.

These values are an illustrative example, not measurements of a specific product. Use your own appliance labels; a refrigerator's EnergyGuide label (kWh per year) is a better source for its daily energy than its running watts.

Step 3: Check power (W) and startup surge

Add the running watts of everything that can be on at the same time. In the example that is 10 + 120 + 30 = 160 W. A safety margin of about 20% gives roughly 190 W of continuous output.

Motors and compressors (refrigerators, freezers, well pumps, sump pumps, air conditioners) briefly draw a startup surge that can be several times their running power. Look for the surge or "peak" rating of the power station and how long it can sustain it. A peak number without a duration does not show that a compressor will start.

Step 4: Convert to battery capacity

Not every watt-hour on the box reaches your appliances. Three things reduce it:

• Inverter efficiency. Converting battery power to 120 V AC loses energy; 85–90% is a common assumption.
• Reserve. Many people keep 10% or more in reserve rather than draining the battery completely.
• Self-consumption. The station itself draws power while its inverter is on, often 10–30 W. Over 24 hours that alone can be hundreds of watt-hours.

With the example above, 90% inverter efficiency, 15 W of self-consumption for 24 hours and a 10% reserve (90% usable):

So a station of roughly 2,000 Wh would be a reasonable starting point for that one-day scenario, and a ~1,000 Wh unit would fall short even though the appliances alone use 1,110 Wh. Battery health also declines with age, which is another reason to leave headroom.

Step 5: Decide whether solar matters

For outages longer than a day, solar panels can replace part of what you use. Production depends on location, season, weather, panel angle and shading, and on the station's solar input limits (voltage, current and maximum charging watts). Typical-weather estimates are useful for planning but are not a forecast for a specific storm.

Common mistakes

• Multiplying every appliance by the full outage length, which badly oversizes the battery for cycling loads like refrigerators.
• Ignoring the station's own idle consumption.
• Assuming 220 V or 240 V appliances will run from a 120 V outlet, or that a 220 V rating automatically covers 240 V.
• Plugging a power station into a wall outlet to "back-feed" the house. This is dangerous to you and to utility workers; panel connections need a proper transfer switch or interlock installed by a licensed electrician. See backing up a home panel.

Home Energy Advisor is an independent pilot project. Some store links on this site are affiliate links: HEA may earn a commission on eligible purchases, and commissions do not change how equipment is compared. A favorable estimate does not certify an installation; connecting to a home electrical panel requires a qualified professional.