Most people shopping for backup power start with the wrong question. They ask which product to buy. The question that actually matters comes first: how many hours of backup does your house need? That's not a guess. The U.S. Energy Information Administration publishes the answer every year — and the 2024 numbers make the math unusually clear.
The 2024 outage picture
Here's the headline: U.S. electricity customers averaged 11 hours of power interruptions in 2024, per the EIA's Electric Power Annual 2024 (summarized in a December 2025 analysis). That's nearly double the annual average of the previous decade. Major events, including Hurricanes Beryl, Helene and Milton, accounted for about 80% of outage hours. Interruptions tied to major events averaged nearly nine hours per customer in 2024. Compare that with nearly four hours a year from 2014 through 2023. The routine stuff, outages not tied to major events? About two hours a year, like clockwork.
The industry tracks this with two metrics, and they're worth knowing. SAIDI — System Average Interruption Duration Index — is the total outage time the average customer eats in a year. SAIFI — System Average Interruption Frequency Index — counts how often the lights go out. For sizing backup, SAIDI is the one. Capacity is a function of duration and load, not duration alone.
National averages lie, though. They smooth over the places that got hammered. Helene knocked out about 5.9 million customers across 10 states. Beryl took 2.6 million offline in Texas. South Carolina customers averaged 52.3 hours without power in 2024 — against 2.0 to 5.4 hours in each of the four years before that. A separate five-year cut of federal reliability data found the typical U.S. state lost 4.9 hours a year from 2020 through 2024. Louisiana averaged 33.5. So if you're in a storm-prone region, forget the national mean. The number that matters is the major-event figure for your state. How long it takes to restore power after an outage where you live — that depends on storm severity, crew access, grid damage — and that's what sets your duration target.
From duration to capacity: the sizing equation
Three variables. That's it. Required capacity in watt-hours = planned outage hours × combined wattage of what you'll run × a safety margin of 1.2 to 1.5 for inverter losses and your own estimation error.
Run the numbers once and it clicks. Fridge: about 150 watts running. Add LED lights, the Wi-Fi router, phone charging — call it 100 watts combined. Essential load: 250 watts. Against the 2024 national average of 11 hours, that's 2,750 watt-hours. Apply a 1.25 safety margin: roughly 3,400 watt-hours target. Now stretch the duration to 24 hours for a storm-prone household — same 250-watt load needs 7,500 watt-hours before margin.
What belongs in the load list
The device list matters more than anything else you decide. Start with what protects health and property. Refrigeration. Medical devices — a CPAP runs about 40 watts. A sump pump, roughly 800 watts, where flooding's a risk. Then communications and light: router (~10W), LED bulbs (~10W each), phone chargers (~10W each), a laptop (~65W).
Leave the big heaters out unless your budget has room. A portable space heater at 1,500 watts burns in one hour what a fridge uses in ten. Size for essentials first and the capacity — and the price — stays tied to the outage data instead of worst-case comfort fantasies.
Why the average is the wrong target
SAIDI is an average, and averages get dragged around by extremes. In 2024, 80 percent of all outage hours came from major events. Size for the two-hour routine outage and you're covered for the ordinary days — and exposed the day a hurricane parks over your county. The EIA data points one way: size to the major-event duration for your state, then add margin. Once the device list and duration are locked, a portable power station calculator does the arithmetic and tells you whether a given unit's watt-hour rating actually covers the plan.
Simple version: take the EIA duration for your region, multiply by your measured essential load, add margin, buy once. Outage trends keep tilting toward longer storm-driven blackouts. Capacity planned against that trend is capacity that works when you need it.Anchor table (for client verification)