An electricity puzzle

One of our venues has very high electricity use at night, and we’re wondering why.

The plot shows the electricity use for a community building. It appears to consume around 3 kW even when the building is unoccupied. The puzzle for us is to figure out what’s using all the electricity and whether anything can be turned off.

The venue comments that they have two freezers and a commercial refrigerator in a big kitchen, and asks whether the 3 kW is from them. Our gut instinct was that they wouldn’t use nearly this much power. To back that up, consider that the government has rules about how to get a commercial refrigerator on their list of recommended, energy efficient products. Here’s an example of a top-sized commercial freezer from their list:

It takes 7 kWh/day to run when carrying out a standard test. A less efficient one might waste 25%, so 3 very large pieces of refrigeration kit might take 27 kWh/day. It seems sensible to take this as a maximum. We don’t know what size their units are, but they are unlikely to be the biggest. On the other hand, they’re unlikely to have cleaned the compressor coils recently, and that can make refrigeration draw a lot more power.

27 kWh/day averages as 1 kW. That’s significant, but nothing like the 3 kW we’re observing at night, and presumably most of it would be during the day just after the door gets opened.

Our first plot is for 30 minute updates like a smart meter would give, but the venue doesn’t actually have a smart meter. They’re using a three phase clamp on meter of the sort some utility companies used to give out to domestic customers. That gives a more detailed view that looks surprising. We’ve checked that the clamp readings are sensible by taking some timed meter readings by hand to make sure that the kilowatt hours match with the two approaches. The readings are every 6 seconds and appear to give the root mean square of the raw data. Here’s a random midnight hour:

What’s really going on appears to be a bunch of roughly 2.5 kW appliances coming on and going off regularly, day and night. A really big refrigerator seems to have a rating of something like 4 amps. What we’re seeing is more like 10 amps. We suspect some thermostatically regulated electric heating of some kind is being left on – perhaps on purpose, but more likely not. They have a gas boiler, but they have sometimes seen electric space heaters on site.

For comparison, here’s a Tuesday morning. That’s one of their busier days. For them, 20 kW is pretty standard. Those blue spikes that start around 12:00 are about 1.5 kW and cycling repetitively roughly every 30 seconds. That’s pretty distinctive but we aren’t sure what causes that kind of pattern.

We can’t go further than this without detective work on site. Here’s an interactive view of the data:

If you have alternative theories about what’s going on, we’d be happy to hear them. We’d like to tell them what to look for.

2 replies on “An electricity puzzle”

Is there data for outside temperature to see if a correlation implies frost protection ?
Frost protection would involve a number of low output heaters and not result in the sharp square wave that phase 3 shows although they may all be controlled by one thermostat.

A water heater such as a tea urn permanently left on is a possibility. A 2.5kW element with thermostat and heating every half hour with 15- 30 litres gives about the right numbers. Taking half an hour to cool back down implies little insulation on it though.

It wasn’t cold enough for frost protection, and I think pipework warmers would be a lot lower output and controlled on a local thermostat, not coming on together. There are seven electric hot water heaters and they’d be rated for around 2.5 kW, so after the break we’ll find out if they have tanks and what kind of control they’re under, and perhaps attempt a walk-round inspection for space heating at the same time that the clamps are measuring. We’re also interested in the steady 2 kW load on the blue phase, since that seems excessive.

Here’s the building manager’s list of larger electrical items:

1 x 3 door commercial fridge

2 x commercial freezers

2 x domestic fridges

Emergency lighting throughout

The lift


The Fire alarm

The intruder alarm 

7 x Electric water heaters for sinks

2 lights that don’t have a functioning switch (snagging item)

The phones and 8 extender hubs run through the data cabinet

5 x Desktop computers that run to allow remote access 

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