An outage is several clocks, not one
People plan for outages by asking how long the power will be off, which is the one number nobody can supply. The tractable question is the other way round: given that it is off, what in this house is on a timer, and in what order do those timers expire. That reframing turns an unanswerable forecast into a list you can act on, and the list is usually shorter and more surprising than expected.
On the defaults here, the refrigerator compartment is done inside four hours while the freezer holds for nearly two days, phones outlast the freezer only if there are power banks in the house, and a sump pump on a modest battery is finished in half a day. The gap between the first failure and the last is an order of magnitude. Anything you do in the first hour of an outage should be aimed at whichever clock is shortest, and on most defaults that is the fridge, which is why the standard advice is to move what matters into the freezer or a cooler early rather than opening the fridge repeatedly to check on it.
The freezer is the one you can actually change
Of all the numbers on this page, the freezer hold time is the one under household control, and the lever is how full it is. Frozen food is thermal mass. Every pound of it absorbs a large amount of heat before it rises above freezing, and it takes that heat from the air in the cabinet, which is what keeps everything else cold. Air has almost no heat capacity by volume, so an empty freezer has nothing to buffer with.
The figures in general circulation are roughly 48 hours for a full freezer with the door kept shut and around 24 for a half-full one, which is why this calculator scales linearly with the fill percentage. The practical consequence is that filling the gaps with sealed jugs of water, frozen in advance, buys a day of hold time for the price of the containers. Leave headspace because water expands as it freezes, and keep them out of the air path from the evaporator. If you are sizing the appliance rather than planning around one you own, the refrigerator size calculator covers the volume side of the same question.
Door openings, and why they cost more than they look
| Behaviour | Effect on a 48 hour freezer |
|---|---|
| Nobody opens it | The full 48 hours |
| Four openings a day, a quarter hour of hold time each | About 46 hours |
| Twelve openings a day, same penalty | About 40 hours |
| Someone checking whether it is still cold every hour | Most of the reserve gone in a day |
The penalty per opening is a rough figure and it is an input for that reason, but the shape is right: cold air is dense, it falls out of an upright cabinet the moment the door opens, and it is replaced with warm room air that then has to be cooled by the contents. The behavioural fix costs nothing. Decide what comes out before opening the door, take it all in one movement, and put a thermometer inside so the question of whether it is still cold can be answered without opening anything.
Phones, banks and the number people get wrong
Phone endurance in an outage is consistently overestimated, for two reasons. The first is that outage usage is not normal usage: people check for updates constantly, and a phone that has lost its usual cell site burns battery searching for another one. The default here is 1.2 full charges a day per phone, which is heavier than an ordinary day and about right for a bad one.
The second is that power bank ratings are cell capacity, not delivered capacity. A bank sold as 10,000 mAh is quoting cells at around 3.7 volts, which is about 37 watt-hours, and conversion losses mean you get meaningfully less than that into a phone. The efficiency field handles it. Do the sum honestly and the usual conclusion is that a household with two phones and one small bank has a day and a half, not a week, and that the car is the largest battery on the property. If you want to size a battery properly rather than estimating, the battery runtime calculator does the full version including depth of discharge.
The two things this page will not tell you
Refrigerated medication comes up in every outage discussion and it is not a question a calculator should answer. The storage requirements are specific to the product, they are on the label, and your pharmacist is the person who knows what to do about an interruption for the particular thing you are holding. Ask before you need to know. The same applies to any powered medical equipment in the house, which needs a written contingency of its own and a conversation with the clinician and with the utility, many of which maintain a register of households that depend on power for medical reasons.
The other is generators. Sizing one is a separate exercise covered by the generator sizing calculator and its fuel by the fuel runtime calculator. What belongs here is the single safety fact that outage deaths turn on: generators and fuel-burning heaters produce carbon monoxide, and they must never run indoors, in a basement, or in an attached garage, open door or not. That includes a unit sitting close to an open window or an air intake. Everything else about running one is in the manual, and the household side of the plan is in the family emergency plan.
Questions people ask
How long does a freezer stay cold without power?
The figures in general circulation are about 48 hours for a freezer that is completely full with the door kept shut, and about 24 hours for one that is half full. That is why this calculator scales the hold time with the fill percentage rather than treating it as a constant. Real appliances vary with insulation, age, room temperature and how much cold air escapes each time somebody looks inside, so treat the output as a planning figure. The refrigerator compartment is a different and much shorter story, commonly cited at around four hours.
Why does keeping the freezer full make such a difference?
Because frozen food is thermal ballast and air is not. Every pound of frozen mass has to absorb a substantial amount of heat before it warms above freezing, and it draws that heat from the cabinet air, which keeps everything around it cold. Air has almost no heat capacity by volume, so an empty freezer has nothing to buffer with and warms quickly. Jugs of water frozen in advance do the job exactly as well as food does, which makes this the cheapest single improvement to an outage plan. Leave headspace for expansion and keep them clear of the air path.
How long will my phone last in an outage?
Less than you think, for two reasons. Outage usage runs heavy because people check constantly, and a phone that has lost its usual cell site drains faster hunting for another. And power banks are rated in cell capacity rather than delivered energy: a 10,000 mAh bank is about 37 watt-hours of cells and delivers noticeably less than that after conversion losses. Put your real figures in and the answer is usually a day or two rather than a week. Text rather than call, since texts get through congestion that defeats voice calls, and turn the screen brightness down before the battery is the problem.
What about medication that has to stay refrigerated?
That question belongs with your pharmacist and with the label on the specific product, not with a hold-time calculation, and this page deliberately does not give a number for it. Ask in advance what to do about an interruption for the particular thing you are holding, so the answer is already known rather than being worked out during the outage. The cooler field here is only there to place that clock on the timeline alongside the others. Powered medical equipment needs its own written contingency, and many utilities maintain a register of households that depend on power for medical reasons.
Is a generator the answer to all of this?
It answers some of it and introduces its own requirements. Sizing, transfer arrangements and fuel are covered by the generator sizing calculator and the fuel runtime calculator, and the electrical connection is work for a qualified electrician rather than an improvisation. The safety fact that matters more than any of the numbers is that generators and fuel-burning heaters produce carbon monoxide and must never be run indoors, in a basement or in an attached garage, including with the door open or near an open window. Working carbon monoxide alarms belong in the house before a generator does.