How this is calculated
Two different things are worked out here: a deadline and a duty cycle.
The deadline starts from the published rules: four hours for a refrigerator, forty-eight for a full freezer, twenty-four for a half-full one, doors closed. The tool treats those as ceilings and only ever shortens them.
Two things shorten it. Heat leaks in proportion to the gap between the room and the inside, so a 70 °F room is the reference and a hotter room scales the clock down by that ratio. Door openings dump cold air in seconds: two or three take about 15 percent off, frequent ones about 40 percent.
The duty cycle is separate arithmetic. Running watts times compressor hours gives energy a day, and compressor hours over 24 gives the share of every hour the machine must be powered. A compressor cannot make up lost ground while switched off, so that share does not shrink when you batch the runs.
Worked example: a 15 cubic foot chest freezer in an 80 °F garage, half full. The ambient ratio is 80 over 70, so 1.14, and the 24-hour half-full clock becomes 21 hours.
Everyday compressor time of 8 hours scales the same way, to 9.1 hours. That is 22.9 minutes an hour, 1,371 Wh a day at 150 W, and about 1,520 Wh out of a battery once inverter losses are counted. Two sessions of four and a half hours cover it.
Wattage and duty figures come from the appliance wattage chart, which rests on the Energy Information Administration 2020 Residential Energy Consumption Survey, table CE5.3b, released June 2023 and revised March 2024.
Those figures measure everyday running, doors opening and warm food going in. A box that stays shut works less, so the run time here is an upper bound.
Quick answers
How long does food stay safe in the fridge without power?
Four hours with the door shut, per the FDA and the CDC. After that it goes out unless a thermometer still reads 40 °F or below.
How long does a freezer stay frozen in a power outage?
About 48 hours full, 24 hours half full. The food is the cold store, so an empty freezer fails first.
Can I refreeze food that thawed during an outage?
Yes if it still has ice crystals or reads 40 °F (4 °C) or below. Quality drops but safety holds. Ice cream is the exception and gets thrown out.
How often should a generator run to keep a freezer cold?
Twice a day suits a freezer. A refrigerator needs the gap under four hours, so five or six short runs, or a battery bridging between them.
How much power does a fridge or freezer need during an outage?
Roughly 1,200 to 1,400 Wh a day for a full-size box, plus about 3× running watts of start-up surge for the inverter to clear.
Keep it or throw it out
Once refrigerated food has sat above 40 °F (4 °C) for four hours or more, the call is made item by item and never by tasting. The CDC sets the four-hour line; the item-by-item calls follow Virginia Cooperative Extension FST-518.
| Food | Call | Why |
|---|---|---|
| Raw or cooked meat, poultry, fish, seafood | Discard | Bacteria multiply fast in the danger zone |
| Milk, cream, sour cream, yogurt, custard | Discard | Same |
| Soft cheese: brie, cottage, ricotta, fresh mozzarella | Discard | High moisture, low salt |
| Hard and processed cheese: cheddar, parmesan, Swiss, provolone | Keep | Low moisture holds bacteria back |
| Eggs in shell, egg dishes, egg substitutes | Discard | Salmonella risk once warmed |
| Butter and margarine | Keep | Fat, very little free water |
| Leftovers: casseroles, soup, stew, pasta, rice, cooked vegetables | Discard | Cooked and cooled food is the classic vehicle |
| Cut fruit, cut melon, cut or cooked vegetables, packaged salad | Discard | Cutting exposes the flesh |
| Whole raw fruit and vegetables | Keep | Skin is intact |
| Opened mayonnaise, tartar sauce, horseradish, creamy dressing | Discard | Egg and oil emulsion, only mildly acidic |
| Ketchup, mustard, jam, jelly, relish, olives, peanut butter, soy sauce, vinegar dressing | Keep | Acid, sugar or salt does the preserving |
| Cream-filled pastry, cheesecake | Discard | Dairy filling |
| Bread, rolls, tortillas, muffins, plain cake | Keep | Shelf stable anyway |
| Tofu, opened infant formula | Discard | Low acid, high protein |
| Food | Still icy | Fully thawed and warm |
|---|---|---|
| Meat, poultry, fish, seafood | Refreeze | Discard |
| Casseroles, stews, soups, ready meals | Refreeze | Discard |
| Ice cream, frozen yogurt | Discard | Discard |
| Milk, soft cheese, cheesecake | Refreeze | Discard |
| Hard cheese | Refreeze | Refreeze |
| Juice and juice concentrate | Refreeze | Refreeze |
| Fruit | Refreeze | Refreeze |
| Vegetables | Refreeze | Discard |
| Bread, rolls, muffins without custard | Refreeze | Refreeze |
| Flour, cornmeal, nuts | Refreeze | Refreeze |
Fruit carries enough acid and sugar to survive a warm spell; vegetables carry neither, so a bag that thawed and warmed goes out. Ice cream is the odd one out again: the texture wrecks on refreezing and the sugar-fat emulsion hides spoilage.
Why a full freezer buys twice as long
The cold in a freezer is stored in the food itself rather than in the air around it. Frozen food absorbs heat to warm up and then far more to melt, so the temperature stalls near freezing for hours while the ice turns quietly to water.
Melting a kilogram of ice takes about 334 kilojoules. The same energy poured into a kilogram of liquid water would lift it about 80 degrees Celsius, four fifths of the way from melting to boiling. That latent heat is the flywheel, and half a freezer of food is half a flywheel, which is why the published number halves from 48 hours to 24.
Sealed jugs of water buy the rest of it for nothing, and they become drinking water when they melt.
The agencies publish two points only, full and half full. The three-quarter and quarter settings are this tool running the same straight line between and below them; the quarter-full case prints about twelve hours, which is an estimate to check with a thermometer rather than a figure to trust.
The run schedule that holds the line
Once a generator or a battery is in play the question changes: how little can you run and still keep the food out of trouble. The two boxes answer differently.
A freezer is generous. Its coast time is long, so energy alone sets the schedule and two sessions a day is normal practice.
A refrigerator is mean. Its coast time is the same four hours the safety rule uses, so the gap between runs binds and it wants five or six short sessions rather than two long ones. The usual plan of running morning and evening quietly leaves the refrigerator above 40 °F for most of the day.
- Move refrigerator food into the freezer at hour one. That removes the four-hour constraint for nothing.
- Put the freezer on the generator and the refrigerator on a small battery covering the gaps.
- Load the compressor start-up surge, about 3× running watts, into the inverter sizing before anything else.
Feed those watt-hours and that surge into the backup power calculator with the rest of the critical circuits, then size the bank with the battery runtime calculator.
United States targets: refrigerator at 40 °F or below, freezer at 0 °F, measured with an appliance thermometer left inside each box. Perishable food left at room temperature for two hours is discarded, or one hour when it is above 90 °F.
European targets: refrigerator at 4 °C or below, freezer at −18 °C, measured with a thermometer left inside each box. Perishable food left at room temperature for two hours is discarded, or one hour when it is above 32 °C.
Ice, dry ice and the cooler move
Ice is the cheapest storage here. A cooler packed with block ice or frozen gel packs holds refrigerator food at 40 °F or below for as long as the ice lasts, so the four-hour deadline becomes a question of how often you can restock the ice.
Dry ice does the same for a freezer, colder, and without wetting the food.
Buy 2.5 to 3 pounds of dry ice per cubic foot of freezer space. Fifty pounds holds a fully stocked 18 cubic foot freezer for two days. Wrap it in paper, keep it off the food with cardboard, wear gloves, and ventilate the room.
Buy 40 to 48 grams of dry ice per liter of freezer space, so about 22 kg for a 500 liter freezer, which holds it for about two days. Wrap it in paper, keep it off the food with cardboard, wear gloves, and ventilate the room.
Below freezing outside, the outdoors is a working freezer, in a shaded animal-proof box on the north side of the house. Direct sun ruins the trick even in cold weather.
Where this clock stops
It is a planning tool, and a thermometer beats it every time. A probe reading 38 °F after five hours means the clock was pessimistic. A probe reading 46 °F after three means the food is gone.
Appearance and smell prove nothing, and the FDA says so outright: you cannot rely on appearance or odor. The CDC ends on the same line. If you are unsure whether a food is safe to eat after an emergency, throw it out.
Sources
FDA, Food and Water Safety During Power Outages and Floods, for the 4/48/24-hour hold times, the ice-crystal refreezing rule, the two-hour room-temperature rule and the 50 pounds of dry ice per fully stocked 18 cubic foot freezer. CDC, Keep Food Safe After a Disaster or Emergency, for the 40 °F and 0 °F targets and the four-hour discard line. Virginia Cooperative Extension FST-518 for the item-by-item keep-or-discard calls. North Dakota State University Extension, Keep Foods Safe During a Power Outage, for 2½ to 3 pounds of dry ice per cubic foot. Watts and compressor hours from this site's appliance wattage chart, which rests on the Energy Information Administration 2020 Residential Energy Consumption Survey, table CE5.3b (released June 2023, revised March 2024). All read September 2026.
Go deeper
Size the machine that runs these boxes with the backup power calculator, then price the fuel on the generator fuel consumption chart. A week without the grid covers what fails and in what order, and food preservation cost comparison covers life after refrigeration.