How to find your overnight base load

Your base load is what the house draws when nobody is using anything. It is the cheapest useful measurement in this whole subject, and it often finds money nothing else would.

How to do it

  1. Last thing at night, with everything off that you would expect to be off, read your electricity meter and note the time.
  2. First thing in the morning, before anything is switched on, read it again.
  3. Subtract the first reading from the second to get kWh used.
  4. Divide by the number of hours between the two readings.

The result is your base load in kilowatts — the power your home draws continuously. Leave the fridge and freezer running; they are part of the answer.

What the number means

  • Under 0.1 kW. Unremarkable. A fridge, a freezer, a router and some standby.
  • 0.1 to 0.2 kW. On the high side but not alarming, particularly with two fridge-freezers or a lot of equipment.
  • Above 0.2 kW. Worth investigating. At 0.25 kW you are spending roughly £535 a year on whatever is running while you sleep.
  • Above 0.5 kW. Something substantial is heating. That is over £1,000 a year.

To convert: multiply kilowatts by 8,766 for annual kWh, then by your unit rate. A base load of 0.25 kW is about 2,190 kWh a year — which for many households is most of their electricity, used while nobody is awake.

What to look for if it is high

Roughly in the order they turn out to be the answer:

  • An immersion heater on a timer, or left on after a boiler problem.
  • A heated towel rail with no timer.
  • Underfloor heating on a thermostat.
  • A frost heater in a garage, outbuilding or conservatory.
  • An old chest freezer with failing door seals.
  • A dehumidifier or air purifier running continuously.
  • Desktop computers, servers or consoles in rest mode rather than off.
  • Pond pumps, aquarium heaters, hot tubs.

Narrowing it down

Repeat the measurement having switched off one suspect at the wall. The change tells you what that item was drawing. It is slow but it is definitive, and it needs nothing but a pen.

If you can borrow a plug-in energy monitor, it settles anything on a 13 A plug in minutes. For things that are wired in rather than plugged in — immersion heaters, towel rails, underfloor heating — the meter method above is the one that works.

Why this beats a list of typical appliance costs

Published figures are averages applied to your house. This is a measurement of your house. It captures the things nobody thinks to list, and it is the only method that will find an appliance nobody remembered was there.

Once you have a suspect, the appliance running cost calculator will tell you what it costs over a year. For the wider picture, see why your bill might be high.