Sizing guide
What size portable power station do I need?
Sizing a power station is one multiplication problem: watts × hours = watt-hours. A 40-watt CPAP running 8 hours needs 320 watt-hours. Everything else on this page is refining that math.
On this page
Runtime calculator
Pick a device (or enter watts manually), enter how many hours you need it to run, and the calculator estimates the battery capacity required — including ~15% headroom for inverter losses, since no station converts power at 100% efficiency.
You need about
–
Estimate only. Real runtime varies with temperature, battery age, and the device's actual draw. Always confirm a station's continuous and surge wattage against your device.
Common device wattages
Typical running wattages. Your exact device may differ — check its label or manual.
| Device | Typical running watts | Notes |
|---|---|---|
| Phone charger | ~10W | Any station handles this for days |
| Wi-Fi router | ~10W | Keep internet up during short outages |
| LED lamp / bulb | ~9W | |
| Box fan | ~50–120W | Varies widely by size and speed setting |
| Laptop (charging) | ~65W | Varies by model and battery level |
| CPAP, no heated humidifier | ~40W | See our CPAP guide |
| CPAP, heated humidifier on | ~90W | Humidifier roughly doubles draw |
| Chest freezer | ~100W running | Cycles on/off; surge at compressor start |
| Full-size fridge | ~150W running | Startup surge is 2–3× higher for a few seconds |
| Microwave | ~1,100–1,500W wall draw | A "1,000W" microwave draws more than its cooking power; short bursts only on most stations |
| Small space heater | ~1,500W | Drains batteries fast — poor fit for most stations |
Three worked examples
1. CPAP for one night (8 hours, no humidifier)
40W × 8h = 320Wh. With headroom, a station around 400Wh+ covers a full night. (Our CPAP guide does the full math, including humidifier scenarios.)
2. Fridge through a 12-hour outage
150W × 12h = 1,800Wh — but fridges cycle on and off, typically running about a third of the time. Realistic need: roughly 500–700Wh, plus enough surge capacity for the compressor kick (often 600–900W for a few seconds). A ~1,000Wh-class station with 2,000W+ surge is the comfortable fit — see our under-$1,000 picks.
3. Space heater for 1 hour
1,500W × 1h = 1,500Wh, and the heater needs a station with at least 1,500W continuous output. This is why heat is the hardest outage load: you'd need a very large (and expensive) station for even modest heating. Blankets and a small heated throw use a fraction of the power.
Don't forget startup surge
Motors and compressors (fridges, freezers, some fans, power tools) draw 2–3× their running wattage for a few seconds when they start. A fridge that runs at 150W might briefly pull 600W. Your station's surge rating must clear that spike, even if the continuous rating easily covers the running watts. This is the single most common sizing mistake — people size for running watts and the station shuts off every time the compressor kicks in.
Next steps
Best under $1,000 →
The ~1,000Wh sweet spot for home backup, compared spec-by-spec.
Jackery 1000 vs EcoFlow DELTA 2 →
The two most popular mid-size stations, head to head.
Best for CPAP →
Overnight runtime math for sleep apnea machines.
Solar generators & blackouts →
What "solar generator" really means and realistic recharge times.
Must-have accessories →
Cables, covers, carts — and the Kill A Watt that ends load-guessing.
Related guides: Best power stations to run a refrigerator (what the fridge surge means for sizing) and Best budget power stations under $500 (what $500 actually buys).
Related guide: Best Expandable Portable Power Stations: Start Small, Grow Later — start small and grow later — base vs max capacity and cost per added kWh.