How to size backup power for a fridge, a well pump, and lights

List the loads that matter

Write the jobs you actually need if the grid is down for a night or a few days. For most houses that is a short list.

• The fridge (and the freezer if it is a separate box).
• The well pump, if that is how you get water — or a lighting plan if you are on city water and the pump is not your problem.
• A few lights in the room you will live in.

That is the list this page will size. Not the whole panel. Not the range, the electric dryer, the central air, or a hair dryer. Not a space heater as the "and also" item. Heat is a room problem first, not a watt problem: heat one room.

A well without power is a closed pipe. If you skip the pump, you need jugs. If you skip the fridge, you need food that does not care: 90 days of food still matters. If you skip stored water because "the well will run," you have a story, not a reserve: 30 days of water.

Walk the house with a notepad. Copy what the plate says. If there is no plate you can read, you do not have a number yet. Guessing is how a pump never starts.

How to read a nameplate

The plate is usually on the back, the side, or inside a door frame. You want watts. If you only get volts and amps, watts are volts times amps for a rough running figure. Motors also have a starting (surge) number that is larger than the running number. The small number is what it uses once it is spinning. The large number is what it wants for a moment to get there.

A fridge compressor and a well pump both lie if you only add the small number.

Typical watts below are labeled TYPICAL only. They are a planning range so you know what a plate often looks like. Read their plate. Measure if you can. Do not buy to a typical.

• Fridge, TYPICAL: 100–200 W running / 600–1200 W starting.
• 1/2 HP well pump, TYPICAL: often 800–1500 W running / 2000–4000 W starting.

Lights are easier. Add the bulbs you will actually switch on. A few LED lamps are a small line. A shop full of old bulbs is not. Write the real count.

A plug-in watt meter is how you stop arguing with a faded sticker. It will not catch every start spike, but it will tell you what the fridge really draws when it is running in your kitchen, not in a catalog. If the plate and the meter disagree, believe the meter for running watts and still leave room for start.

If the plate is gone and you cannot measure, you do not size a box yet. You store water and shelf food and you hire someone who can read the motor.

Add them up without lying

Two columns: running and starting.

Running is what you must hold at once: fridge running + pump running (if you will pump while the fridge is on) + lights. Starting is the ugly minute: the largest motor start plus everything else that will still be on.

Work an example with typicals, then throw the typicals out and use your plates.

Say the fridge is 150 W run / 900 W start (typical-looking). The 1/2 HP pump is 1200 W run / 3000 W start (typical-looking). Lights are 60 W. If the pump starts while the fridge is already running and the lights are on, the surge you care about is about 3000 + 150 + 60, not 1200 + 150 + 60. If both motors can start at the same instant, it is worse. You do not plan for the pretty total.

Leave margin. A box that is "exactly" the surge number will sag, trip, or refuse the pump. The pump is the one that surprises people. The fridge cycles all night. The pump is short and brutal. Size for the brutal start or admit you will not have well water and you will live on jugs.

Do not add a space heater "because there is room." That room disappears the first time the pump kicks. Do not add the whole house "because the generator is big." Big on the carton is not the same as honest addition.

Write the number you will not exceed. Then write the loads you will leave off on purpose.

Runtime

The carton watt number is a ceiling, not a promise that you can hold that ceiling all night. Runtime is hours at the load you actually sized.

A fridge does not run every minute. It cycles. A night of fridge plus a few lights is a modest average if you keep the doors shut and you do not add a heater. A well pump run is a few minutes of high draw when someone opens a tap or a tank calls for water. Your fuel or battery budget is average load over time, with enough headroom that a start does not kill the box.

If the box is fuel, you need fuel you can store safely and a place you can run the machine that will not kill you. If the box is batteries, you need a charge plan for the next day. "The box says 3,000 watts" is not "3,000 watts until sunrise."

Do the boring math once. Running watts × hours you care about, with the fridge as a cycling load, not a constant. Then look at the fuel tank or the battery rating in watt-hours and see if a night fits. If it does not, you drop a load or you shorten the night you expect, or you store more water and more shelf food so the pump and the fridge are not the only plan.

What this size will not run

This size will not run the range, the electric dryer, whole-house air, or a bank of space heaters. It will not run every light in the house because you did not walk around and switch them off. It will not make a well into water storage. It will not heat the house.

Leave those off on purpose. Write them down so a tired person at 10 p.m. does not plug one in.

The fridge is only part of 90 days of food. If the box is down, or you sized lights and water and not cold, the pantry still has to work. Stored water covers you if the pump is down or you chose not to size it. Wash from jugs if the tap is dead.

Heat is the next skill, and it is not a heater on this circuit: how to heat one room.

Hookup, one level up from a death cord

Never backfeed a dryer outlet. Never build or buy a "suicide cord" — a cord with two male ends, or any setup that feeds the panel from a receptacle. That can energize the house wiring and the line back to the street. It can kill a lineman or a neighbor. It can kill you. Hire an electrician for a transfer switch or an interlock that is legal in your jurisdiction. This page will not teach you to wire one.

Never run a generator in a home, a garage, a basement, a shed, or near an open window. The U.S. Consumer Product Safety Commission has said carbon monoxide from a generator can kill in minutes. A cracked garage door is not ventilation. A window you opened "a little" is how people die in their sleep. Outdoors, well away from openings, exhaust pointed away from the house. If you cannot do that, you do not run a combustion generator.

Extension cords have limits. A light cord on a well pump is a heat problem and a fire problem. Use a cord rated for the load, as short as you can, and do not run it under a rug.

This is not an electrical-install guide. If the hookup is more than a listed inlet and a switch a licensed electrician installed, stop.

Size the loads. Write the number. Leave the death cord in the century it belongs in. Then deal with heat as a room, not as another plug: heat one room when the grid is down.