Why backup at all
Solar and batteries are sized for typical weather. But some weeks are cloudier than any plan. On the grid, outages happen in storms and heat waves.
Backup power fills those gaps. For most homesteads, it means a generator: an engine that burns fuel, such as gasoline, propane or diesel, to make electricity.
Sizing solar and batteries to cover the worst week of the worst year is very costly. Many off-grid homes accept a few generator hours a year instead.
Carbon monoxide comes first
Generators make carbon monoxide (CO): a gas you cannot see or smell. It can kill within minutes at high levels. The CPSC says portable generators are linked to more than 100 CO deaths in the US each year.
The rules from the CPSC and NFPA are plain:
- Never run a generator indoors, in a garage, basement, crawlspace or shed, even with doors or windows open.
- Keep it well away from the house. CPSC advises at least 20 feet, with the exhaust pointing away from doors, windows and vents.
- Install battery-powered or battery-backup CO alarms in the home, and test them.
- If anyone feels dizzy, sick or weak while a generator runs, get to fresh air at once and call 911.
Text description
A plan view, seen from above and not to scale. A house has an attached garage, doors, windows and a vent. A portable generator sits outside, at least 20 feet (6 meters) from the house and garage, as the CPSC advises, with its exhaust pointing away from the building. A generator drawn inside the garage is crossed out, and so is one inside a shed: never run a generator indoors, in a garage, basement, crawlspace or shed, even with doors or windows open. Inside the house, CO alarms with batteries or battery backup are marked. Notes say carbon monoxide cannot be seen or smelled and can kill within minutes, and fans and open doors do not make it safe.
Never backfeed
Backfeeding means sending generator power into the house wiring through a wall outlet or a homemade cord. It is extremely dangerous and illegal in most places.
It can push power back out to utility lines and kill line workers. It can also overload house wiring and start a fire.
Safe ways to connect a generator to house circuits are a transfer switch or an approved interlock, installed by a licensed electrician. These make it impossible to connect the generator and the grid at the same time. Otherwise, plug appliances straight into the generator with heavy-duty outdoor cords.
Text description
A block diagram of the concept, not a wiring diagram. The utility grid and a generator both lead to a transfer switch or approved interlock, installed by a licensed electrician, which connects only one source at a time. From it, power goes to a critical-loads panel that feeds the circuits you most need, such as the fridge, the well pump and a few lights. A crossed-out path shows a generator plugged into a wall outlet with a homemade cord, called backfeeding: never do this. It can send power out to utility lines and kill line workers, and it can overload house wiring and start a fire.
Critical loads and load shedding
A backup source rarely runs everything. Critical loads are the ones you most need in an outage or a cloudy spell. Load shedding means switching off the rest so backup power and the battery last longer.
Many homes put their critical loads on a separate panel or set of circuits. In a long outage, only those stay on.
In a long winter outage, is each load usually critical or one that can wait?
- Clothes dryer
- Fan or pump for a wood stove or furnace
- Medical equipment someone relies on
- Fridge and freezer
- Second TV and game console
- Well pump (for drinking and flushing)
- Electric oven
- A few lights
Generator top-up in an off-grid system
Off the grid, a generator usually charges the battery through an inverter-charger, rather than running the house directly. It runs for a few hours, fills the battery, then shuts off.
Set how much energy you are short each day and the charging power. How long does the generator run, and how much fuel does it burn?
About 1.8 hours of running per day.
Fuel: roughly 0.2 to 0.4 gal of gasoline a day.
Assumptions: Includes about 15% charging loss. Small generators use roughly 0.08 to 0.13 gallons per kWh delivered at part load. Never run one indoors or near windows: carbon monoxide kills.
Small generators use more fuel per kWh at light load. Charging the battery at a steady rate is often more efficient than running the generator for a single lamp.
Choosing a backup source
Store fuel in approved containers, away from the house and any flame, and follow your fire department’s rules on how much you may keep.
Pro and permit
Check
It is snowing hard during an outage. Where is it safe to run a portable generator?
Summary
- Backup power covers what solar and batteries cannot. A few generator hours a year is often cheaper than a much bigger system.
- Carbon monoxide kills. Never run a generator indoors, in a garage or near windows. Keep CO alarms working.
- Never backfeed. Use a transfer switch or interlock installed by an electrician.
- Pick your critical loads and plan load shedding ahead of time.
Go deeper: why light loads waste fuel
A generator engine burns a good share of its fuel just to keep turning, whatever the load. At a light load, most of the fuel goes to that overhead, so each kWh costs more. At a steady, moderate load, more of the fuel becomes useful electricity. That is why off-grid systems often run a generator hard for a few hours to charge the battery, then switch it off, instead of running it lightly all day. Inverter-type generators throttle the engine down at light load, which helps, but the principle holds.