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Converting "1000 imp/kWh" to watts
Modern domestic electricity consumption and microgeneration meters commonly have lights that flash 1000 times per kWh
/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:
= 3.6 / seconds per flash
(or, if you have a metronome, kW = 0.06*BPM)
|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:
- the idea that "imp" somehow stands for impedance on these meters (or "impressions", but that misunderstanding is not as bad as "impedance"; meter data sheets say "impulses"),
- or that one flash of a 1000 imp/kWh meter means you've "used up one watt" (please stop guessing)---a flash of a 1000 imp/kWh meter does correspond to a watt-hour (3600J), but as a unit that's not generally as useful as the kWh (3.6MJ), and the watt itself is a measure of rate (it's a joule per second)---you can't "use up one watt" unless you're a circuit designer worrying about a power budget---kWh is to kW as miles is to miles-per-hour (one is something you can "clock up", the other is the speed at which you're doing so; I suppose the placement of the word "hour" might look inconsistent to people unfamiliar with SI units, since on their cars the unit for speed is the one that includes "hour", whereas when measuring energy flow the unit for rate is the one that does not include "hour"---sorry if this is confusing).
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 sanity checker)
All material © Silas S. Brown unless otherwise stated.