What Size Generator Do I Need? Free Outage Calculator
Tick what you need to keep running when the power cuts out. Get your watts, your startup surge, and the generator or power station size that covers it.
Answer in 30 seconds: tick your devices below. The calculator adds up running watts and startup surge, then suggests a generator size and a power station size. It runs in your browser, free, no sign-up.
Generator size calculator
Tick each item. Set how many and how many hours a day it actually runs (a fridge compressor runs about 8–10 hours a day, not 24). Values are typical published figures; your nameplate label wins.
Estimates only. Check the label on each appliance (or its manual) for its real watts, and have an electrician install any transfer switch. Never run a fuel generator indoors or in a garage.
Planning for the outage itself? Blackout Protocol covers heat, water, food and light. See the guide →
How to read your result
Running watts is what everything you ticked draws if it all runs at the same time. Peak surge adds the biggest startup spike on top, because motors in fridges, freezers, pumps and furnace fans pull two to three times their running watts for a second or two when they kick on. Your generator or power station has to handle both numbers, or it trips the moment the well pump starts.
Energy per day matters for a power station, because a battery holds a fixed amount of energy, measured in watt-hours (Wh). A generator just needs fuel, so for a generator the running and surge watts are what count.
The calculator adds headroom on purpose. A generator running flat out all night burns more fuel and wears faster, and a power station loses about 10–15% of its stored energy turning battery power into wall-socket power. Sizing to about 80% of the rated output is the common rule of thumb.
What size generator do I need for a whole house?
Most people asking "how big a generator do I need" mean: what keeps my house livable through a winter outage? For essentials only (fridge, freezer, furnace fan, sump or well pump, lights, phones, router), most homes land around 5,000 to 7,500 running watts on a portable generator. Adding central air, an electric water heater, an electric dryer or an electric range quickly pushes you past 10,000 watts and into standby-generator territory.
You rarely need everything at once. Stagger the big loads: run the well pump to fill the tank, then the microwave, not both together. That alone can drop you a whole size class. Tick only what you would really run at the same time, and the calculator shows the difference.
Generator or power station?
A portable power station is a big rechargeable battery with outlets. It's silent, makes no fumes and is safe indoors, so it's ideal for a CPAP at the bedside, phones, a router, LED lights and a fridge for a short outage. Recharge it from the wall, a car, or solar panels.
A fuel generator (gas, propane or dual-fuel) gives more watts for as long as you have fuel. It handles well pumps, furnaces and multi-day outages better, but it is loud and must run outside, well away from doors and windows, because of carbon monoxide.
Plenty of households use both: a generator for the heavy loads during the day, and a power station for quiet overnight essentials.
Power stations sized for your result
Match the size tier from your calculator result. These are broad tiers, not specific models; check each listing's capacity (Wh), continuous output (W) and surge rating against your numbers.
Small (about 300–700 Wh): CPAP for a night or two, phones, router, lights.
Mid (about 1,000–2,000 Wh): a fridge for around a day plus the small stuff.
Large (about 3,000 Wh and up, often expandable): fridge, freezer and furnace fan, or several days with solar.
What size power station for a CPAP?
A CPAP is often the most important thing on the list, and luckily one of the smallest. Without the heated humidifier, many machines draw around 30–60 watts, so an 8-hour night uses roughly 250–500 Wh. A 500–1,000 Wh power station covers one or two nights. Turn the humidifier and heated tube on and the draw can double, so check your machine's power-supply label, and ask your supplier about a DC adapter, which skips the inverter loss and stretches the battery further.
What size generator or power station runs a fridge?
A modern full-size fridge runs at around 100–250 watts while the compressor is on, but needs a startup surge of roughly 1,000–1,500 watts. It cycles on and off, so over a day it typically uses about 1–2 kWh. That means a power station needs a surge rating that copes with the compressor start, and around 1,000–2,000 Wh to carry it for about a day. On a generator, a fridge is easy: even a small 2,000-watt inverter generator starts one. Keep the door shut and a full fridge holds its cold for hours on its own.
What size generator to run a furnace?
A gas or oil furnace won't run without power for the blower fan and control board. A typical blower draws a few hundred to around 800 watts with a startup surge on top, so a 3,000–5,000 watt generator usually covers the furnace with room for a fridge and lights. Larger power stations can run a blower for a few hours, which gets a cold house through the night. Our furnace watts and startup surge guide goes deeper, and you must connect through a transfer switch or interlock, never a back-fed outlet.
Recharge with solar for longer outages
A power station only lasts as long as its battery unless you can refill it. Solar panels plugged into the station's solar input can top it up each day. The calculator estimates how many panel watts would replace your daily use on a typical day, assuming about 4 hours of good sun and some losses. Winter sun is weaker, so treat it as a top-up, not a guarantee.
Keep your backup power plan on paper
The Backup Power Planner kit is a printable workbook and binder for the records side: your appliance watt list, fuel and battery logs, a test-run schedule and an outage checklist, so the numbers you found here are on the fridge, not lost in a browser tab.
Get the Backup Power Planner on EtsyThe gear is half the plan
A generator sized right still runs out of fuel, and a power station still goes flat. What gets a family through a long winter outage is the plan around the gear: one warm room, water you stored on purpose, food that doesn't need the fridge, and light that keeps going. That's what Blackout Protocol walks you through, step by step.
Watch the Blackout Protocol walkthroughFAQ
What size generator do I need to run a house?
For essentials only, most homes land around 5,000 to 7,500 running watts. Central air, an electric water heater or an electric range pushes that to 10,000 watts or more. Use the calculator for your own number.
What is the difference between running watts and starting watts?
Running watts are what a device draws once it's going. Starting (surge) watts are the short spike a motor needs to start, often two to three times higher. Cover your total running watts plus the biggest single surge.
Can a portable power station run a refrigerator?
Usually yes, if its inverter handles the compressor surge and it has enough watt-hours. A typical fridge uses about 1–2 kWh a day, so a 1,000–2,000 Wh station covers about a day.
How big a power station do I need for a CPAP?
About 250–500 Wh per night without the humidifier, so 500–1,000 Wh covers one or two nights. The humidifier can double that.
Can I run my gas furnace on a generator?
Often yes, through a transfer switch or interlock installed by an electrician. The blower typically needs a few hundred to around 800 watts plus startup surge.
Is a generator or a power station better for an outage?
A power station is silent and safe indoors, which suits CPAP, phones, lights and a fridge. A fuel generator gives more power for longer, which suits pumps, furnaces and multi-day outages, but must run outdoors.