How can I measure kWh used over time for a chest freezer?

March 23, 2025 6:29 AM

Is there a tool or device I can buy that will let me accurately measure kWh used over time for a chest freezer (UK electrics), without requiring bare wire work?

Something in my house (perhaps more than one something) is using approx 10 kWh/day more than this time last year. I suspect two chest freezers. I bought an Energenie meter but it is giving absurd results for the chest freezers (45kWh overnight when the main house meter shows 8kWh used) and further research suggests that this type of meter doesn't really work on appliances that cycle their power draw. Is there something that does?

Willing to invest some ££ for a good quality useful tool if one exists, strongly prefer not to get anything that wants a cloud connection, ideally not WiFi either. Bluetooth would be okay. Needs to be available in the UK.

To prevent any confusion: I graph my daily avg kWh/day by month, year on year. I am not confusing cost with kWh. My heat and hot water is not electric and is not the source of the increase. Thanks!

posted by Rhedyn to Home & Garden (13 answers total)

Kill A Watt is the popular tool on this side of the Atlantic - and I did a brief search and they do have an "international" version but the price I saw was unreasonable. This type of thing should be about 20 pounds.
posted by zenon at 6:43 AM on March 23 [3 favorites]

Response by poster: Thanks zenon, I did see a lot of mention of Kill A Watt and have found it for a reasonable price in the UK, but I haven't been able to find a straight up confirmation that it will work for a chest freezer or similar intermittently running compressor sort of appliance, unlike the Energenie. Do you know?
posted by Rhedyn at 6:49 AM on March 23

Best answer: Kill A Watt will absolutely track consumption of a chest freezer or equivalent. The display shows instantaneous voltage, current draw, and wattage, and it also maintains a running count of watt-hours consumed since the measurement was started. The latter is what you're interested in for your situation. It's nothing like a fancy graph of usage per hour of the day, but that total number will tell you if one freezer or the other is running its compressor far too much.

Also note that at least in our area of the US, many public libraries lend Kill A Watt meters for a week or two at a time. Might be worth an ask.
posted by ReferenceDesk at 7:03 AM on March 23 [5 favorites]

I run Home Assistant for home automation and monitoring, which is a whole big thing way beyond what you're trying to do, but I mention that because my search for power-monitoring devices has been motivated by trying to bring in as much sensing/monitoring equipment as possible to that framework. For plug-in power monitoring, there are power-monitoring smartplugs, which you can either hook up to a larger ecosystem (as I do), or just use their own cloud-based service to monitor. My plug of choice is the Sonoff S31, but I'm in the US and that's a 120V US device. One well-regarded option in the UK is the LocalBytes smart plug, but that one has no cloud service at all, so you'd have to set up an (at least minimal) local server to record and report data.

If you want something cloud-based and cheap, there are plugs in the Tuya ecosystem (e.g. this one: link to the US store, but it's a UK device) which can do this, but Tuya devices are kind of the epitome of cheap Chinese IoT crap, and I wouldn't use a Tuya device where I would be very unhappy if it suddenly stopped delivering power (specifically, a freezer which I wasn't regularly monitoring to make sure it hadn't abruptly failed), or in a high-power-draw situation where I would be distressed if the Tuya device caught fire. So, depending how risk-averse you are, a Tuya smartplug might not be your thing.
posted by jackbishop at 7:05 AM on March 23 [1 favorite]

The model demonstrated in this YouTube video seems to show that the model monitors instantaneous power draw, and infers the energy consumption from that. (He runs a 2.8 kW electric kettle for about 1 minute and it shows the total power consumed as 0.044 kWh, which is pretty close to correct.) Might be worth tracking that model down and giving it a shot.
posted by Johnny Assay at 7:11 AM on March 23

Check your local library. Here in the US, my (cool, old) public library lends out these devices for free!
posted by SaltySalticid at 7:15 AM on March 23

The device you have might be giving a "correct" reading, but it's not measuring the same thing your electric meter is measuring. A freezer which is pretty much just a big motor running a compressor can have a very low "power factor", which could easily make your device read too high by 2x or more, depending on how it makes its measurement.

You might find your device is accurate for resistive loads (traditional light bulbs, space heaters, electric kettle, etc) but reads too high (compared to the electric company's meter) for things that are motors.

I believe the Kill-A-Watt corrects for PF and can even display the PF in real time. If one of your chest freezers shows a PF near 1 and the other shows it very low (0.3-0.5) then perhaps you have a failed run capacitor?
posted by fritley at 10:02 AM on March 23 [1 favorite]

Adding to fritley's point, fridges and freezers don't have a flat load, they chill to a cutoff temperature then turn off, then cut back in when the temperature goes up to a certain point. So the cycle tends to look flat with occasional blocks when they are drawing power. You can see more on this in this article.
posted by biffa at 10:32 AM on March 23

The simplest (but still sufficiently capable) device would be one like this. No cloud, no installing apps, just occasionally use your Mk.1 Eyeball to read it and transfer that info onto something that supports note-taking.
posted by Stoneshop at 11:09 AM on March 23

Both the EnerGenie, the Kill-A-Watt and the device I linked simply calculate energy draw from voltage times current times length of time it draws current, with a power factor correction. With the freezer not running (but plugged in), the display for energy used should be stationary or at most ticking up very slowly.

Also, check what the power draw the Energenie indicates when the freezer is running, and compare that to the manufacturer specs for that model.

What model is your main house energy meter? If it's still a rotating disc it can read low on loads with a power factor away from 1 so the EnerGenie et.al could be more correct, but 45kWh overnight (say, 10 hours) is absurdly wrong. That's 4.5kW load during the entire night, about two cooker hobs on full. Even when it's 45kWh over 24h it would be roughly equal to a cooker hob or electric kettle drawing power the entire time.
posted by Stoneshop at 12:28 PM on March 23

Response by poster: @Stoneshop, my main house meter is an old traditional (rotating disc).

I reset the Energenie for a fresh read and let it run for an hour on the freezer. The freezer light is green and the motor isn't audibly running. At an hour the Energenie says 2.328 kWh used, 0.9W, 241.3v, 0.135A, and 0.00 power factor (!!!). Wtf.
posted by Rhedyn at 2:43 PM on March 23

At an hour the Energenie says 2.328 kWh used, 0.9W, 241.3v, 0.135A, and 0.00 power factor (!!!). Wtf.

Watt the Flux indeed. That 0.00 PF is messing with the Energenie calculations, as 0.9W looks plausible for the power draw with the compressor off, 240V*0.135A would be 32.5W at PF=1 (high-ish for a freezer with only a bit of electronics running if it's not just a good old thermal switch controlling things), which, over an hour, would have added 32.5Wh to your leccy bill. With a PF of 0.5 (pretty bad already) it might register as double that on the Energenie if it's not correcting for PF, and actually less than 32.5Wh on the house meter. So my conclusion is the EG is off its rocker and should be returned.
posted by Stoneshop at 4:03 PM on March 23 [2 favorites]

I was just reading that a full unopened chest freezer will keep food frozen for 48 hours (24 if half full), maybe you can just unplug one of them for a day and see if you’re no longer seeing that mysterious power draw?
posted by cali59 at 7:35 PM on March 23