Fill spare freezer space with bottles of water in case the power goes off

A tip says to fill the empty space in your freezer with bottles of water, so the freezer works less and your bill drops. Some people who worked through the numbers say it will not lower your bill. US food-safety agencies recommend the same bottles for a different reason: they keep food cold longer when the power fails.

If your freezer has spare space, fill it with sealed plastic bottles or jugs of water, leaving a gap at the top for the ice to expand, to keep your food cold longer in a power outage (a power cut in the UK), not to lower your bill.

If your freezer is packed with food already, you do not need this: the food does the same job. If you have no spare space, skip it.

Why it may not lower the bill

Freezing water takes electricity. You pay for that once, and the cold goes into the water. Some people who ran the numbers say that this cost cancels out most or all of what you save later by having less air to cool each time the door opens, and some say the saving is too small to notice on a bill.

What it is good for: a power outage

The US Department of Agriculture says to keep the freezer full and to fill empty spaces with frozen plastic jugs of water, bags of ice or gel packs. The FDA says to freeze containers of water to help keep food cold in the freezer, the fridge or a cooler in case the power goes out. Many people say frozen water keeps a freezer cold for much longer than air does.

Both agencies give the same numbers, for doors that stay shut. A fridge keeps food safe for about 4 hours. A full freezer holds its temperature for about 48 hours, or about 24 hours if it is half full. The UK Food Standards Agency gives the same figures and calls them estimates. Keep the doors closed. Some people say not to open them even to check on the ice, and both agencies say to keep them closed as much as you can.

The pages read for this page do not say how many extra hours jugs add. The 48 and 24 hours are for a full and a half-full freezer, and the Department of Agriculture lists jugs as a way to keep the freezer full. If the power is off longer than a fridge can hold, the Department says you can move food into a cooler with ice or frozen gel packs. Some people say a frozen bottle beats ice cubes in a cooler: it does not melt into water that waters down drinks or makes a mess, and you can drink the water at the end.

How to do it

Use sturdy plastic bottles or jugs with lids. Leave a gap at the top: water expands when it freezes, and some people warn that bottles filled to the brim split or bulge. Some people warn that aluminium bottles can split and steel ones can go out of shape. One person says the plastic bottles that fizzy drinks come in cope with freezing better than still-water bottles. One person warns that a jug can leak if it is moved or if the container fails, which can damage the food beside it.

Add the bottles a few at a time over several days. One person warns that adding lots of warm bottles at one time can thaw food that is already frozen. One person says they broke a freezer by cramming too much into it. Leave space for air to move, especially around any fan, and check your freezer’s manual. One person says a chest freezer suits the idea better, because the cold stays in when you lift the lid.

Water jugs are not the only way to fill the space. Some people say sealed freezer bags of sauce, or simply more food, do the same job. The Department of Agriculture also says to group food together in the freezer, and to freeze leftovers, milk and fresh meat that you will not need soon, so they stay at a safe temperature longer.

If your winters freeze, some people say you can freeze the bottles outside overnight instead of using the freezer to do it. The Department of Agriculture gives the same method for a snowstorm: fill buckets, empty milk containers or cans with water, leave them outside to freeze, and use the ice in the fridge, freezer or a cooler. It does not work where it does not freeze.

After a power outage: check the food, not the jug

Some people lay a half-filled bottle on its side, so they can see afterwards whether the ice melted and froze again. One person puts a coin on top of a frozen bottle for the same reason, and another says an air pocket at the bottom of a bottle gives it away. This page could not find these tricks in the official pages it read. A jug that melted and froze again tells you the freezer thawed at some point. It cannot tell you how warm the food got. Use it as a warning, and use the agencies’ tests.

If you kept a thermometer in the freezer, the FDA says to check it when the power comes back: at 40 °F or below, the food is safe and can be refrozen. If you did not, check each package: if the food still has ice crystals or is at 40 °F or below, it is safe to refreeze or cook, and you cannot rely on how it looks or smells. In the fridge, throw away meat, poultry, fish, eggs, milk and leftovers that were above 40 °F for 4 hours or more. In England, Wales and Northern Ireland, the Food Standards Agency says a fridge under 8 °C is fine in a power cut; if it reaches 8 °C or more, eat chilled food within 4 hours and throw it away after that.

Never taste food to decide. When in doubt, throw it out. For medicines kept in the fridge, the CDC says to throw them away after a day or more without power, unless the label says otherwise. If a life depends on one, it says to keep using it only until a new supply arrives. Ask a pharmacist. The sickness signs and what to do about them are in the safety box above.

The jugs are also drinking water, if the water was safe

The FDA says that when your normal water supply is contaminated or unavailable, melting ice will also supply drinking water. So fill the bottles with water you know is safe. The CDC says not to use water you know or suspect is unsafe to make ice.

If your tap water may not be safe, the CDC gives two ways to treat it. If the water is cloudy, first filter it through a clean cloth, paper towel or coffee filter, or let it settle and pour off the clear water.

Boiling is the best way to kill germs. Bring clear water to a rolling boil for 1 minute (3 minutes above 6,500 feet, about 2,000 metres), let it cool, and keep it in clean, covered containers.

Bleach is the second way. Use plain, unscented household bleach with 5 to 9 percent sodium hypochlorite, and follow the label if it has directions for drinking water. Otherwise the CDC says to add 2 drops to a quart or a litre of clear water, or 8 drops to a gallon, which is a little less than an eighth of a teaspoon. If the water is cloudy, murky, coloured or very cold, add double. Stir it well and let it stand for at least 30 minutes before you drink it. Bleach does not kill some germs, such as the parasites Cryptosporidium and Giardia, as well as boiling does. One person warns that stronger bleach water over a long time can be bad for teeth, so use the amounts above, no more.

Three limits matter. Boiling and bleach cannot make water safe if it holds fuel or toxic chemicals. The CDC says that if you know or suspect it does, you should use bottled water or another source and contact your health department. Also, the CDC says never to use water from radiators or boilers in a home heating system for drinking, cooking or washing. And boiling water can scald you.

In the UK the Drinking Water Inspectorate says that if you get a notice to boil your water, you bring it to a rolling boil and let it cool naturally. Store it in a clean container in the fridge and throw it away after 24 hours. Your water company’s notice is the instruction, and this page gives no UK bleach dose. A gas cooker may still work in a power cut; the Food Standards Agency says most camping stoves and barbecues are for outdoor use only, and the guide here on floods, fires and storms covers carbon monoxide from stoves and generators.

Who this page is not for

If your freezer is packed with food already, you do not need jugs. If you have no spare space, or do not have a freezer, skip it: a cooler and bags of ice from a shop do the same job for a short outage. If you live somewhere warm, you cannot freeze the bottles outside; you would use your own freezer, which costs the electricity described above.

The 4, 48 and 24 hour numbers are estimates for a closed fridge and freezer. If you are pregnant, an older person, looking after a baby or young child, or your immune system is weakened, be stricter than the numbers: the Food Standards Agency says to take extra care with high-risk foods. For a baby’s formula, it says you need to be able to boil the water, and if you cannot, ready-to-use formula is an option.