Back to Silas S. Brown's home page

Converting "1000 imp/kWh" to watts

The UK has seen three general kinds of domestic electricity meter: very old mechanical meters (with dials or mechanical counters) that have no lights, moderately-old digital meters that still need to be read, and now "smart" meters that transmit their readings automatically. The more recent two of these three (the moderately-old digital meters and the "smart" meters) both tend to have flashing lights, but the meaning of the lights is different.

On a "smart" meter, the meaning of the lights may vary with manufacturer, and in some cases indicates signal transmission status rather than energy use. But on a moderately-old digital meter (typically installed for consumption or microgeneration before some time in the 2010s), the light usually flashes a set number of times per kWh, typically 1000 imp/kWh. You can multiply the time between two flashes by 1000 to find the time it will take to "clock up" 1 kWh at the current usage rate, but if you'd rather have everything in watts (and you do not possess a wireless energy monitor, or there is insufficient wiring-board room to install one), then you can use:

Average kW = 3.6 / seconds per flash

(or, if you have a metronome, kW = 0.06*BPM)

For example:
1s (60 BPM)3.6kW
1.5s (40 BPM)2.4kW
For the rarer 800 imp/kWh meters, use kW = 4.5 / seconds per flash (or 0.075*BPM). In case there are any meters out there with other imp/kWh values, the general formula is kW = 3600/(imp/kWh) / seconds per flash (or 60/(imp/kWh) * BPM), but it seems 1000 imp/kWh is most common (at least in 2014).

I put up this page because there seemed to be some misinformation going around, for example:

If an imp/kWh light is lit continuously then this indicates no current flow (but some meters show this as no light rather than a continuous light). In some cases an extremely small current (such as a Raspberry Pi B+ playing audio to non-amplified speakers) will count as "no current", but the meters are usually quite sensitive.

Some two-rate meters used in Economy 7 (e.g. 5246C) also have a second light, confusingly placed near the maximum-current label (e.g. 100A), which is always lit continuously when the off-peak circuit is energised, regardless of whether or not it's being used. The primary imp/kWh light is the only one whose flashing or continuity indicates use or no use.

(See also current-transformer "energy monitors", how to use Economy 7 effectively and my energy deals coherence checker)

All material © Silas S. Brown unless otherwise stated.