Plug-in energy monitors (Kill-A-Watt, Belkin Energy Use Monitor, similar) sit between a device and the wall socket, measuring how much electricity that device uses. They cost £15-40 and promise to reveal hidden energy drains.
After a year of using one in a one-bedroom flat, here’s what they actually deliver, what they don’t, and whether the £25 was money well spent.
What they measure
A plug-in energy monitor measures the wattage of whatever you plug into it, in real time. Most also track cumulative use over time (hours, days, weeks). The good ones also calculate cost based on your electricity tariff and show projections (annual cost if this pattern continues).
The measurement is genuinely accurate for plug-loads, TVs, computers, kitchen appliances, lights, anything that plugs into a standard socket. They can’t measure devices that are hardwired (heating, water heating, cooker, oven) and can’t measure your whole-house consumption.
What you find when you measure
The first month of measuring is genuinely revealing. The patterns I found in my one-bedroom flat:
- The fridge: 0.5-1 kWh/day, running constantly. Total annual cost: roughly £60.
- The Wi-Fi router: 6-8W constant, 60-70 kWh/year, £15.
- The TV in standby: 4W, totaling 35 kWh/year and £9.
- The set-top box: 15W in standby (!), 130 kWh/year, £33.
- Phone charger (left plugged in): 0.5W, 4 kWh/year, £1.
- Microwave clock display: 3W constant, 26 kWh/year, £6.
- Computer monitor in sleep: 5W, 44 kWh/year, £11.
- Electric kettle: 0.3 kWh per boil (much less than expected), but 200+ boils per year still totaling £20.
The biggest surprises: the set-top box (which I thought was off) drawing more electricity in standby than the fridge during active operation. The microwave clock costing £6 a year. The Wi-Fi router being the constant background drain.
What you do with the data
The value of the monitor isn’t the measurement; it’s the actions the measurement enables:
- Eliminate standby waste. Once you know which devices have significant standby loads, you can put them on switched plug strips and eliminate the waste.
- Identify failing appliances. An old fridge using 1.5 kWh/day instead of 0.7 is failing or due for replacement.
- Compare options. Measure two different appliances (a kettle vs. an electric heater for boiling a small amount of water) to choose the more efficient.
- Validate behaviour changes. See actual results from running the dishwasher on eco mode vs. standard.
- Educate household members. Showing actual numbers is more persuasive than abstract advice.
The “aha moments”
The specific moments where the monitor changed behaviour:
- The set-top box revelation. Discovering it used 15W in standby led to immediately replacing it with a more efficient model (3W standby), saving £25/year.
- The fridge thermostat check. Measuring showed my fridge running 30 percent of the time. After adjusting the thermostat to a slightly warmer setting (still safe), it dropped to 20 percent, saving roughly £15/year.
- The kettle math. Realising that filling the kettle with twice as much water as needed roughly doubles the energy cost, leading to more careful filling and ~£10/year saved.
- The standby total. Adding up all standby loads showed roughly £80/year just for devices that were nominally off. Switching to plug strips cut this to roughly £20/year.
The cost-benefit
The energy monitor cost £25. The behavioural changes it triggered have saved roughly £100-150 per year in electricity costs. The break-even was about three months.
The monitor continues to be useful for occasional spot-checks (when buying a new appliance, when investigating an unexpected bill increase, when comparing two options). But the bulk of the value was extracted in the first three months.
The genuinely useful uses
Beyond the initial audit, energy monitors stay useful for:
- New appliance comparison. Before buying a new kettle, dishwasher, etc., measure your current one and the new one to validate efficiency claims.
- Diagnosing high bills. When the bill spikes unexpectedly, work through the major plug-loads to identify the cause.
- Verifying eco-mode claims. “Eco mode” on dishwashers and washing machines is sometimes meaningfully different from standard and sometimes mostly marketing. Measurement settles the question.
- Computer setup optimisation. Different monitor brightness, screen-off timers, and sleep settings can have measurable effects.
- Heating fact-check. Some electric heaters claim to be more efficient than others. Measurement reveals the truth (usually they’re all similar; what varies is the wattage).
The limitations
Energy monitors don’t help with:
- Heating and hot water (the biggest energy uses, but typically hardwired).
- Whole-house consumption (smart meters do this, but with less detail).
- Hardwired appliances (cookers, hobs, hardwired lights).
- Identifying issues you didn’t think to test.
For hardwired appliances and whole-house tracking, a smart meter with in-home display is more useful.
The smart-meter alternative
If you have a smart meter (most UK flats now do), the in-home display shows whole-house consumption in real time. This is complementary to plug-in monitors:
- Smart meter: total household consumption, including hardwired appliances.
- Plug-in monitor: per-device detail for plug-load appliances.
For most households, a smart meter alone catches roughly 60 percent of the insight value. A plug-in monitor adds the remaining 40 percent by identifying which devices are responsible for the consumption patterns.
Related: Should Renters Install a Smart Thermostat?
The recommended use pattern
For someone buying their first energy monitor:
- Week 1: measure every plug-load device in the flat, one at a time. Note daily consumption.
- Week 2: identify the highest-waste devices and the highest standby drains.
- Week 3-4: implement changes (switched plug strips, replacement of inefficient devices, behaviour adjustments).
- Month 2 onwards: spot-check occasionally; use for new purchases.
- Long term: lend to friends/family who haven’t done their own audit.
Related: Hot Water Savings in a Shared Flat: Negotiating With Housemates
The honest verdict
A £25 energy monitor delivers excellent value for someone willing to spend a few hours auditing their household and then act on the findings. The savings typically pay back the cost in 2-4 months and continue indefinitely.
It’s less valuable for someone who already has a smart meter, who has already done a basic energy audit, or who won’t act on the findings. For most households, particularly those starting to take energy seriously, it’s among the highest return-on-effort sustainability purchases available.
Related: Summer Cooling Without Air Conditioning
The energy math in one table
| Key point | What it means in practice |
|---|---|
| The fridge | 0.5-1 kWh/day, running constantly. Total annual cost: roughly £60. |
| The Wi-Fi router | 6-8W constant, 60-70 kWh/year, £15. |
| The TV in standby | 4W, totaling 35 kWh/year and £9. |
| The set-top box | 15W in standby (!), 130 kWh/year, £33. |
| Phone charger (left plugged in) | 0.5W, 4 kWh/year, £1. |
How to put this into practice this week
The most reliable way to translate a guide like this into something that actually changes your week is to pick the single highest-use change in it and ignore everything else for the first fortnight. The rule is the same one that makes physical training work: load is fine, variety is fine, but the consistent application of a small number of well-chosen movements is what produces results. Sustainability follows the same logic. The dozen interesting interventions you have just read about will all matter at some point. The two you build into your routine first will matter the most.
If you genuinely cannot pick, default to the intervention with the strongest financial signal. Where money and impact line up, the habit forms easily because the reinforcement happens monthly on a bill rather than abstractly in the climate news. Once that habit is invisible, the next one is easier to add. Six months of one small habit at a time produces a meaningfully different household, and you will barely notice the transition while you live through it.
Sources and further reading
Frequently asked questions
For plug-load devices, very accurate (within 2-5 percent for most consumer models). Look for models with energy regulation certification.
Smart meter shows total household consumption; energy monitor shows per-device. They're complementary. The plug-in monitor adds per-device detail the smart meter doesn't provide.
For most users, a basic model (£15-25) is sufficient, Belkin, Kill-A-Watt, OWL are reliable brands. Smart versions with app connectivity cost more and add limited value for one-off audits.
Most value is extracted in the first 1-2 months of audit. After that, occasional spot-checks (monthly to quarterly) are sufficient. Don't need to use daily.
