Drawing No. EH–HH–006 // Household Engineering Tools
Fridge and Freezer Power-Outage Planner
Reviewed August 2026
Two modes: a food safety timer using official USDA/FDA guidance to track how long refrigerated and frozen food stays safe during an outage, and a backup power sizing tool for the generator or battery needed to keep them running.
What problem does this solve?
A power outage raises two separate questions: how long is the food in your fridge and freezer actually still safe to eat, and if you have a generator or battery backup, is it sized correctly to keep them running? This tool answers both, using official USDA/FDA food safety guidance for the first question and typical appliance wattage data plus real compressor duty cycle behavior for the second.
Inputs
Results
Background
A closed, unopened refrigerator keeps food safe for about 4 hours during a power outage. A full freezer holds a safe temperature for about 48 hours if the door stays closed; a half-full freezer for about 24 hours. These figures come from the USDA Food Safety and Inspection Service and are echoed by FDA and FoodSafety.gov guidance — they assume the door is opened as little as possible, since each opening lets warm air in and speeds warming.
A full freezer has far more frozen mass acting as thermal inertia — all that frozen food has to absorb a large amount of heat before it rises above freezing, which is exactly why a full freezer holds safe temperature twice as long as a half-full one. A refrigerator's published 4-hour figure doesn't carry the same full/half-full distinction in official guidance, since refrigerated (not frozen) food has much less thermal mass advantage and crosses the food-safety temperature threshold (40°F/4°C) relatively quickly regardless of how full the fridge is.
Separately from the appliance-level 4-hour/24-hour/48-hour guidance, USDA's general food safety rule is that any perishable food (meat, poultry, fish, eggs, dairy, leftovers) that has been above 40°F (4°C) for more than 2 hours total should be discarded, regardless of how it got there. The appliance-level figures are estimates of how long it typically takes to reach that threshold with the door closed; an appliance thermometer is the only way to know for certain, which is why USDA recommends keeping one in both the fridge and freezer at all times.
A refrigerator or freezer compressor cycles on and off to maintain temperature rather than running continuously — compressor duty cycle varies widely with appliance design, thermostat setting, ambient temperature, food load and door openings. This duty cycle is exactly why sizing backup power by nameplate running watts alone overstates real average energy consumption; the actual average draw is running watts times duty cycle, not running watts continuously.
A compressor's brief startup surge is commonly 2–4 times its steady running wattage, lasting only a second or two but requiring the backup source's surge or peak rating (not its continuous rating) to be adequate — a generator or inverter sized only for running watts can fail to start the compressor at all, or trip on overload, even though it would run the appliance fine once started. Always check actual appliance nameplate starting watts when available; this calculator's estimate is a reasonable planning figure, not a substitute for the real spec.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
They're representative estimates for a typical closed appliance under normal conditions, not a guarantee — actual time to cross the 40°F safety threshold depends on your specific appliance's insulation, how full it is, ambient room temperature, and how many times the door has been opened. Use an appliance thermometer to confirm actual temperature whenever possible; the time-based guidance is what to rely on before you can check the thermometer, not instead of it.
Every door opening lets a burst of warm room air in and cold air out, and since a freezer's safe-time estimate assumes the door stays closed as much as possible, frequent or prolonged openings shorten the actual safe time below the published 24/48-hour figures — which is exactly why USDA recommends deciding what you need in advance and opening the door as briefly and infrequently as possible during an outage.
The estimate (roughly 3× running watts, within the commonly cited 2–4× range from appliance and generator sizing guides) is a reasonable planning figure for typical residential compressors, but real starting surge varies by compressor type and size. If you're sizing a generator or inverter close to its limit, it's worth checking with the manufacturer or using a clamp meter to measure actual starting current if precision matters.
For a conservative screening case, this calculator now adds both appliances’ starting-watt requirements together so it does not assume the compressors can never start simultaneously. If a controller deliberately staggers starts, a smaller surge requirement may be justified, but that needs to be assured by the actual system rather than assumed.