Backup Power for Vital Loads: Freezer, Well Pump, Medical Devices

By GoldBlock WorkshopPublished Updated

The short answer

This page covers backup power for the vital loads in an outage: the refrigerator and freezer, a well pump, a furnace blower, medical devices and a few lights and chargers. The extension method adds up running watts and allows for the starting surge of motors: “usually at least twice” running watts (University of Florida IFAS), sized at four times by Virginia Cooperative Extension. CPSC: generators run outside only, at least 20 feet from the house with the exhaust directed away, never plugged into a wall outlet; any connection to house wiring goes through a transfer switch installed by a qualified electrician (OSHA). The full generator rule is in the notice below.

Key numbers

  • 2 to 4×starting watts for a motor, compared with its running watts; University of Florida IFAS: usually at least twice; Virginia Cooperative Extension sizes at four timesSource: UF IFAS; Virginia Cooperative Extension
  • 20 feetCPSC: outside only, at least 20 feet from the house, exhaust directed away; CDC: more than 20 feet from any windows, doors and ventsSource: CPSC; CDC
  • 400 to 800 Wtypical refrigerator running watts (freezer 600 to 1,000 W)Source: Virginia Cooperative Extension
  • 4 / 48 hourshow long a closed refrigerator / full freezer keeps food safe without powerSource: CDC

Which loads are vital?

The loads covered here:

  • Food: refrigerator and freezer. A closed refrigerator keeps food safe for about 4 hours and a full freezer about 48 (CDC), but those hold times are for a closed appliance during an outage, not a generator schedule. If you run them in shifts, check appliance thermometers: keep the refrigerator at 40°F or below and the freezer at 0°F or below, and follow the official discard rules if they rise (see Short-Term Food Supply).
  • Water: a well pump, if your water depends on one.
  • Heat: the blower and controls of a forced-air furnace; Virginia Cooperative Extension lists the furnace blower at 400 to 600 W. Space heaters, heater safety and other ways to stay warm are in Home Temperature During Outages.
  • Medical: oxygen concentrators, CPAP, powered wheelchairs, refrigerated medicines. FDA’s guidance for home medical devices says “it is important that your device works during a power outage and that you have a plan in place,” including how long the device runs on batteries.
  • Communication and light: phone and radio chargers, a few LED lights.

Why does a motor need more power to start than to run?

University of Florida IFAS: “Starting watts are the extra power needed to start the appliance,” over and above the running watts on the nameplate. The extension sources size for it:

  • Virginia Cooperative Extension: “Motors require four times as much power to start as they do to run.” Its example adds a refrigerator and freezer to 1,800 running watts and concludes: “If so, you would need (1,800 X 4) 7,200 watts.”
  • University of Florida IFAS: starting watts “are usually at least twice the amount needed to operate the appliance.”

So the range is two to four times running watts, with the nameplate or manual settling the real figure for a given motor. Typical running watts from Virginia Cooperative Extension: refrigerator 400 to 800 W, freezer 600 to 1,000 W, furnace blower 400 to 600 W, water pump 800 to 2,500 W. The extension example has you decide whether the refrigerator and freezer are to start at the same time, and sizes for the motors that could start together.

Loads go on one at a time. University of Florida IFAS: start the generator “without the appliances plugged in,” then “add other appliances one at time, allowing the generator to stabilize as each one is added.”

Well pumps

Franklin Electric’s AIM manual lists its 4-inch single-phase submersible motors at 115 volts only in the 1/2 horsepower size and at 230 volts from 1/2 to 1.5 horsepower (Franklin Electric AIM Manual, 2026, p. 39), and it sizes generators so they deliver at least 65% of the motor’s nameplate voltage during starting. Connection to house wiring, including a pump circuit, goes through a transfer switch (next section). Wells, pumps and their components are getting their own section of the site; this page covers only the power question.

Generator, batteries or both?

The choice is between four things described below: a portable generator, a battery system, solar to recharge a battery, and a permanently installed standby generator. Which loads each one can carry follows from the surge and connection facts above and from the ratings on the unit itself.

  • Portable generators need fuel, regular running, and the carbon monoxide and electrical rules in the notice above.
  • Battery systems and portable power stations make no exhaust; use one indoors only as its manufacturer permits. Two numbers describe one: watts, the power it can deliver at a moment, and watt-hours, the energy it stores. Its continuous watt rating must cover everything running at once, and its watt-hours decide how long that lasts: usable watt-hours ÷ load watts = hours, less in practice; the manufacturer’s specifications give the unit’s efficiency and usable capacity. Its surge (peak) rating must cover the motor’s starting watts, and a 240-volt load needs a 240-volt output. For medical equipment, FDA’s power-outage booklet has the user gather the answers from the device manufacturer or medical provider: “Can my device operate on another power source? If yes, what type?” and “Does my device have a back-up system? If yes, how long will it operate and where is it located?”
  • Solar recharges batteries during a long outage. A grid-tied rooftop system without a battery normally stops producing when the utility goes down: its utility-interactive inverter is built to detect that condition (an “island”) and, in Sandia National Laboratories’ words, “cease power conversion and delivery of power to the loads” (Sandia, 2002). Whether a given system has battery backup that keeps it running is a question for its installer.
  • Standby generators with an automatic transfer switch are installed permanently by a qualified electrician.

Why a transfer switch, and why backfeeding kills

A transfer switch connects the house circuits to either the utility or the generator, never both: Virginia Cooperative Extension describes a double-pole, double-throw switch whose one position connects the house conductors to the utility and whose other position disconnects them from the utility and connects them to the generator, which “prevents accidentally feeding power back into the utility lines where it can injure neighbors or utility workers servicing the lines.” Without that switch, a generator connected to the house wiring energizes more than the house: OSHA says attaching a generator directly to a building electrical system without a properly installed transfer switch “can energize wiring systems for great distances.” That is the backfeeding CPSC warns about: “NEVER try to power the house wiring by plugging the generator into a wall outlet, a practice known as ‘backfeeding.’” It “presents an electrocution risk to utility workers and neighbors served by the same utility transformer” and “bypasses some of the built-in household circuit protection devices” (CPSC). OSHA: “Never attach a generator directly to the electrical system of a structure (home, office, trailer, etc.) unless a qualified electrician has properly installed the generator with a transfer switch”; University of Florida IFAS: “the only way to hook up a generator to house wiring is with a transfer switch installed by a licensed electrician.” Virginia Cooperative Extension adds that the wiring and equipment are installed “to meet National Electrical Code requirements, local regulations and the requirements of the power supplier.”

The other connection rules, in the agencies’ words:

  • Cords. CPSC: “Connect appliances to the generator using heavy-duty extension cords that are specifically designed for outdoor use,” and “[m]ake sure the wattage rating for each cord exceeds the total wattage of all appliances connected to it.” OSHA advises replacing underrated cords with properly rated cords that use heavier gauge wire.
  • Wet conditions. CPSC: “Generators pose a risk of shock and electrocution, especially if they are operated in wet conditions”; run it on a dry surface under an open, canopy-like structure, protected from rain and from water reaching, pooling or draining under it.
  • Heat and load. OSHA: “Generators become hot while running and remain hot for long periods after they are stopped,” and overloading a generator “can lead to overheating which can create a fire hazard.”
  • Refueling. CPSC: “Before refueling the generator, turn it off and let it cool down.” “Gasoline spilled on hot engine parts could ignite.”
  • Fuel storage. CPSC: “Never store fuel for your generator in the home.” Gasoline, propane, kerosene and other flammable liquids “should be stored outside of living areas.” OSHA: fuels “should be stored and transported in approved containers.”

Refrigerated medicines

The CDC says: “When the power is out for a day or more, throw away any medication that should be refrigerated,” unless a life depends on it, in which case use it “only until a new supply is available.” FDA adds that if a medicine was not kept at its recommended temperature, check its storage instructions or “contact a pharmacist, health care provider, or the manufacturer to find out if it can still be used.” Insulin is one exception: FDA says insulin in the manufacturer’s vials or cartridges, opened or unopened, “may be left unrefrigerated at a temperature between 59°F and 86°F for up to 28 days and continue to work.” FDA also says: “Do not use insulin that has been frozen.” Keep it out of direct heat and sunlight. Pens and other products may have different limits, so check the package insert. FDA’s guidance for home medical devices advises notifying your local power company when power is lost and restored.

Common questions

What size generator do I need to run a refrigerator and freezer?

Add their running watts (Virginia Cooperative Extension lists 400 to 800 W for a refrigerator and 600 to 1,000 W for a freezer), then allow for the starting surge: two to four times running watts (University of Florida IFAS; Virginia Cooperative Extension), and Virginia Cooperative Extension has you decide whether both are to start at the same time. The nameplates give the real figures.

Can I plug my generator into a wall outlet to power the house?

No. CPSC: “NEVER try to power the house wiring by plugging the generator into a wall outlet.” Backfeeding creates an electrocution risk to utility workers and neighbors (CPSC). House circuits connect through a transfer switch installed by a qualified electrician.

Can a portable generator run my well pump?

Sometimes. Franklin Electric’s AIM manual lists its single-phase submersible motors above 1/2 horsepower at 230 volts, and a generator must hold at least 65% of nameplate voltage while the motor starts. Connection to house wiring goes through a transfer switch installed by a qualified electrician (OSHA; University of Florida IFAS).

How far from the house should a generator be?

CPSC: outside only, at least 20 feet from the house, exhaust directed away, and never in any enclosed space. CDC: more than 20 feet from any windows, doors and vents. The full wording is in the notice at the top of this page and on Carbon Monoxide Safety.

Is a battery power station safer than a generator?

It produces no exhaust, so it can run indoors as its manufacturer permits. Its surge (peak) rating has to cover a motor’s starting watts, which run two to four times its running watts by the extension figures above.

Where to go next

Sources