1. The four categories
Backup power comes in four shapes, and they answer different problems rather than competing for the same job.
- Standby generators are permanently installed, start themselves, run on natural gas or propane and can carry a whole house. Highest cost, zero effort during an outage.
- Portable generators are wheeled, gasoline or dual fuel, and cost a fraction as much. Somebody has to go outside, start them and refuel them every 8 to 12 hours.
- Inverter generators are portables that produce clean power electronically, so they are far quieter and gentler on electronics. Roughly twice the price per watt.
- Battery power stations store energy rather than making it. Silent, no fuel, no exhaust, and the only category that is safe to run indoors.
The full comparison, including a table of what each can and cannot do, is in Standby, portable, inverter or battery.
2. Sizing, in one formula
Motors draw three to seven times their running current for a fraction of a second at startup. Only the biggest one counts, because motors rarely start at the same instant. Resistive loads such as heaters and kettles have no surge at all but draw their full rating continuously.
Our sizing calculator does this arithmetic for you, and the sizing guide works through a full example.
3. Getting the power into the house
A generator is half a system. The other half is the device that guarantees your house is connected to the utility or to the generator and never to both. There are three legitimate options: a manual transfer switch, an interlock kit on the existing panel, or an automatic transfer switch.
The illegitimate option, a cord with male plugs on both ends run into a dryer outlet, backfeeds the utility lines and can kill a repair crew. It is prohibited essentially everywhere; OSHA's electrical safety standards exist partly because of it.
Read transfer switches and interlock kits, or plan yours with the transfer switch planner.
4. The safety rules that are not negotiable
- Outdoors only, at least 20 feet from the house, exhaust pointed away from every door, window and vent.
- Never in a garage, carport, porch, basement or shed. Opening a door does not help.
- Working carbon monoxide alarms on every level and outside every sleeping area.
- Engine off and cool before refuelling. Fuel stored outside living space in approved containers.
- Never backfeed a panel. Never run a cord under a door or through a window seal.
The full reasoning, including why a CO shutoff sensor is a backstop rather than a permission slip, is in carbon monoxide and generators. Federal guidance is at Ready.gov and the U.S. Fire Administration.
5. Fuel
Gasoline is cheap and available but goes stale in weeks. Propane stores indefinitely but loses vapour pressure in extreme cold. Natural gas never runs out but depends on the utility and derates the engine. Diesel suits large and sustained loads but needs treatment against microbial growth. Full comparison in the fuel guide, and the runtime calculator will tell you what a three-day outage costs on each.
6. What it costs
The generator is the smallest line on a standby quote. The pad, the transfer switch, the conductors, the gas line, the permit and the commissioning are the rest, and they are why two quotes for the same house can differ by thousands. The eight questions that make bids comparable are in what a standby generator really costs.
7. Then keep it working
Generators fail from neglect, not wear. Exercise monthly under load, change the oil annually, keep fuel fresh and stabilised, replace the starting battery on age rather than on failure, and run a full drill once a year on a pleasant Saturday. The maintenance schedule lays out the intervals, and the readiness checklist tracks the seasonal version.
Frequently asked questions
What size generator do I need for my house?
Add the running watts of everything that must run, then add the single largest starting surge on that list, then add about 20 percent headroom. A typical essentials list lands between 5,000 and 7,500 watts. Whole-house coverage including central air and an electric range is a code load calculation an electrician performs, and usually lands between 14 kW and 26 kW.
Can I plug a generator into a wall outlet to power my house?
No. That is called backfeeding. It energises the utility side of your service and can electrocute a line crew working on what they believe is a dead circuit. The legal options are a manual transfer switch, an interlock kit or an automatic transfer switch, all of which make it physically impossible for the utility and the generator to be connected at the same time.
How far from the house does a generator have to be?
At least 20 feet, outdoors, with the exhaust pointed away from doors, windows and vents. Never in a garage, carport, porch, basement or shed, and opening a door or window does not make an enclosed space safe.
How long can a generator run continuously?
A portable runs until the tank empties, typically 8 to 12 hours, and needs to cool before refuelling. A standby generator on natural gas can run for days, though manufacturers specify a service interval, often around 200 hours, at which the oil must be changed.
Do I need a permit to install a generator?
For any permanently installed generator or transfer switch, almost always. Most jurisdictions require an electrical permit and an inspection, plus a gas permit where a fuel line is involved. A plug-in battery power station needs no permit.
Can a battery power station replace a generator?
For short outages and for essential circuits, often yes, and it is the only option that is safe indoors. For multi-day outages, electric heat, an electric range or central air, a battery large enough gets expensive and a fuel generator is usually the better tool. Many households end up with both.
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