The simple freezer habit that protects your food during power cuts – and why most people do the opposite

Sophie Bennett • December 10, 2025 16:47

The power went off halfway through a Sunday roast. The oven died with a sulk, the radio fell silent, and the freezer in the corner kept humming for a few stubborn seconds before giving up. We lit candles, checked our phones, and did the usual mental maths: how long will the food last, really?

Three hours later the lights blinked back. The freezer started its low growl again. I opened the door for a quick look, felt a vague wave of cold, shut it, and did what almost everyone does in that moment: shrugged and hoped for the best.

A week afterwards, talking to a food safety scientist over coffee, I discovered that was precisely the wrong instinct. My freezer had just lived through a small crisis, and I’d learned absolutely nothing about what had actually happened inside.

The fix, it turns out, takes less than three minutes, costs nothing, and lives quietly on the top shelf next to the frozen peas.

You freeze one small pot of water, drop a coin on top, and then, crucially, you leave it alone. It becomes a tiny time‑stamp for every power cut your freezer ever has.

Most people either don’t do this at all, or they do something that cancels it out without realising. That’s how food that “feels cold enough” ends up back on the plate when it should have gone in the bin.

The day an expert told me my freezer had no memory

The expert was a chilled‑foods microbiologist called Priya, the kind of person who can make you faintly afraid of a prawn sandwich. I told her my storm‑night story, expecting a pat on the back for my restraint in not flinging everything away.

She listened, nodded, and then said something that lodged in my brain.

“Your freezer doesn’t remember a power cut. If you don’t give it a marker, you’re guessing.”

She explained that during a cut, the temperature inside your freezer climbs in slow motion. A well‑packed, unopened freezer can hold safe temperatures for up to 48 hours. A half‑empty one that gets opened “just for a second” can warm dangerously in less than a day. Once food spends too long above about –9°C and then gets refrozen, it can look fine but quietly lose safety and quality.

“The problem,” she said, “is that by the time the power’s back, the ice on the packets has refrozen, the outside feels solid again, and you have no idea how far it warmed up. That’s when people eat food they shouldn’t-or throw out food that was actually fine.”

Her solution sounded like something off the internet, but she swore by it in her own kitchen.

A small tub of ice and a coin.

What the coin‑and‑ice habit really does

The method has a few names online-“freezer coin test”, “blackout marker”-but the principle is simple. You freeze a shallow container of water solid, put a coin on the top, and keep it in your freezer. The coin tells you how much the ice melted during a power cut.

If the freezer warms enough for the ice to melt, the coin sinks. When everything freezes again, the coin is trapped wherever the water level got to.

Coin still sitting on the top: the ice never fully melted, so the freezer stayed cold enough throughout.
Coin buried halfway: things got warm enough for partial melting.
Coin at the bottom: the ice completely melted at some point, meaning the freezer spent serious time above freezing.

It’s not a precise thermometer, but it gives you a story line. Instead of “it feels cold, probably fine”, you get a visual clue about how bad the interruption really was.

You’re not trying to be perfect; you’re trying not to be blind. That’s all.

How to set it up in three minutes

You don’t need special containers, gadgets, or branded “emergency packs”. Just use what you already have.

Find a small, clear container.
A plastic takeaway tub, yoghurt pot or small freezer‑safe glass dish is ideal. Shallow works better than deep.
Fill it with tap water and freeze it flat.
Pop it on a level shelf in your freezer and leave it until the water is solid all the way through. This might take a few hours.
Place a small coin on the frozen surface.
A 1p or 2p piece is perfect-heavy enough to sink when things melt, small enough not to be annoying.
Move the tub to an easy‑to‑see spot.
Front of the top shelf is ideal. You want to spot it the moment you open the door, without moving other food.
Make a simple rule: never reset it casually.
Only empty and refreeze the tub if:
- you’re doing a full defrost, or
- you’ve decided to throw away everything inside and truly start from scratch.

Treat it like a black box flight recorder. If you keep fiddling with it, it stops telling you the truth.

The habit isn’t just making the tub. It’s resisting the urge to top it up, rinse it, or move the coin “back to the top” after every small wobble. You’re banking knowledge for future you.

How it protects you during and after a power cut

The marker itself doesn’t keep food cold. What it does is change your decisions in the murky hours and days after something’s gone wrong.

It shows you if a cut happened while you were away.
Maybe you’ve been on holiday. You come back, the clock on the oven is blinking, the freezer is humming as if nothing’s wrong. The coin at the bottom tells you there was a meaningful warm spell. You know to be cautious.
It helps you decide what to save and what to bin.
If the coin is still on top, most frozen food will have stayed below –9°C, especially if the freezer was full and the door stayed shut. A coin that’s dropped right down says: don’t trust anything that’s meat, fish, dairy or previously cooked.
It cuts down on waste when the cut was short.
Without a marker, people often throw away whole freezers’ worth “just in case”. If your coin hasn’t moved, you have some reassurance that your stash of batch‑cooked meals is still safe.

Here’s the rough rule of thumb Priya uses at home:

Coin position	What likely happened	What she does
On or very near the top	Partial warming, likely stayed frozen	Keeps properly frozen items; uses ice cream and soft foods soon
Stuck halfway down	Significant warm spell but not a full thaw	Keeps only solid blocks that still feel rock‑hard; bins anything soft, meaty or home‑cooked
On the bottom	Fully thawed at least once	Discards high‑risk foods; may keep only plain bread or raw veg if they still look and smell normal

It’s a guide, not gospel. If in doubt, she leans towards caution-especially with foods that can harbour nasty bacteria once they’ve had a chance to wake up and multiply.

Why most people do the opposite when the lights go out

The moment we hear that familiar click and everything stops, most of us react the same way: we rush to the kitchen and fling the freezer door open to “check” on things. It’s understandable-and spectacularly unhelpful.

Every time you open the door during a power cut, you let cold air spill out and warm air rush in. You turn your freezer from an insulated box into a glorified cupboard. The food starts warming faster, which is exactly what you were worried about.

The second mistake usually comes later, when the power returns:

We open the door and squeeze a packet. If it feels cold and has a bit of ice on it, we declare victory.
We refreeze anything slightly soft “for next time”.
We tell ourselves we’d notice if something was truly off.

That’s how we end up eating chicken that’s had a quiet spa day at unsafe temperatures before being frozen again. It smells fine, cooks fine, tastes fine-and sometimes carries a risk we never see.

The simple habit that protects your food isn’t heroic rescuing during the cut. It’s doing less at the right times, and paying attention to one small tub of ice afterwards.

Turn your freezer into a cool box, not a cupboard

The coin habit works even better when the freezer itself is set up for resilience rather than convenience.

Priya’s three boring, powerful rules:

Keep it at least three‑quarters full.
A full freezer stays cold longer because all that frozen mass acts like an ice pack. If you don’t have much food inside, fill gaps with:
- bottles of water (leave a bit of headroom for expansion)
- bags of ice
- even loaves of cheap sliced bread.
Know your time limits.
With the door shut:
- A full freezer can often keep food frozen for around 48 hours.
- A half‑full freezer may only manage 24 hours.
- A fridge is usually at risk after four hours without power.
Label what matters.
A bit of tape and a date on tubs of cooked meals or meat helps you decide what to prioritise if you know a cut is coming (for example, a planned network outage). Eat the oldest or riskiest things first.

Think of your freezer as a giant cool box that sometimes loses power. Your job is to give it as much cold “ballast” and as few door‑open moments as possible.

The coin then becomes the final check, not your only hope.

Small rituals that actually stick

No one is going to turn into a full‑time food safety officer for the sake of a bag of chips. The habits that work are the ones you can fold into a normal, slightly chaotic life.

These are the ones that tend to stick:

Make the coin tub once, then ignore it.
Let it sit there for months. The less often you think about it, the more useful it becomes when you need it.
In a cut, tell yourself: “Fridge first, freezer last.”
Use up what will spoil quickly in the fridge. Don’t so much as peek at the freezer unless you absolutely must.
After a cut, check the coin before you check the chicken.
Open the door, look at the tub, and decide from there. If the coin’s at the bottom, don’t talk yourself into keeping high‑risk food.
Once a year, do a “blackout drill” on paper.
Ask: if the freezer died tonight, what would I be most upset to lose? Use that as a nudge to declutter and eat through the mystery tubs at the back.

There’s a quiet satisfaction in knowing you’ve built a bit of resilience into the most taken‑for‑granted box in the house. You’re not preparing for the end of the world. You’re just making sure a random Tuesday power cut doesn’t turn into food poisoning on Thursday.

FAQ:

Does the coin‑and‑ice method replace a proper thermometer?
No. A freezer thermometer gives you an actual temperature reading, which is better. The coin method is a low‑effort back‑up that tells you whether things melted significantly while you weren’t looking.

Is food always unsafe if the coin is at the bottom?
Not automatically, but it’s a strong warning sign. High‑risk items (meat, fish, dairy, cooked meals) are best discarded. Low‑risk foods like bread or plain vegetables may be fine to cook from frozen again if they still look and smell normal.

Can I use more than one marker tub?
You can, but one is usually enough. If you like, put a second tub in a different part of the freezer to show if some shelves warm faster than others.

What if my freezer is frost‑free-does this still work?
Yes. Frost‑free models cycle air to reduce ice build‑up, but they’re still vulnerable to cuts. The coin will still sink if the ice fully melts.

Is it safe to refreeze food that has thawed but still feels cold?
Official guidance is cautious: once high‑risk food has fully thawed and warmed, it shouldn’t be refrozen raw. If it’s only partially thawed and still icy, you can usually cook it straight away and then refreeze the cooked dish once cooled. When in doubt, err on the side of safety.