I am a big fan of wireless electricity meters – they make electricity consumption visible. And if you are trying to reduce your electricity consumption they are a fantastic tool. I bought one of these units a couple of years ago but it had no memory. So earlier this year I overcame my reluctance to buy another piece of consumer nonsense and bought this Efergy unit. It keeps track of the last 7 days energy consumption in memory and so I can plot a graph showing the household electricity consumption day by day.
The graph is remarkably dull. The 7 day average plummets when we go away (but why only to 5 units a day is a mystery to me) and we have weekly peaks in consumption when we do washing and tumble drying. Surprisingly I haven’t seen much evidence of a dependence on day length yet, but I expect that will begin to show up soon. However despite their usefulness for plotting variations in electricity consumption, the thing I wanted to highlight was the poor accuracy of the units.
As I write my Electrisave meter says I am currently using 0.6 kW while my Efergy meter says I am using 0.55 kW. This 10% discrepancy is typical, and not always in the same direction. In fact this is within specification for the units which only claim 10% accuracy. Because it has memory, I can test the performance of the Efergy unit against the EDF energy meter that determines how much I pay. Since July when I began the comparison, the Efergy unit under-reads by between 15% and 25% depending on exactly which period I compare them. I think this is because the units’ sensors read only the electrical current and the energy consumption depends on the relative phase of teh current and voltage. For households using lots of low energy light bulbs (for instance) the phase can vary significantly. I don’t think this really detracts from the utility of these devices – but it is perhaps worth bearing in mind.