I have a 3 phase prepaid meter at home (model: ACTARIS ACE 9000 ITP). It has a little red LED that blinks at a rate of 1000 impulse per 1 kWh.
This if I measure the time between pulses I will have instantaneous consumption in Watt where:
W = 3600/t
t in seconds
W in Watt
Below are two screenshots. We can see a couple of things:
- In the first we can see that the pulses are almost 100ms in duration.
- After I had a bit of hot water out of a tap, the geyser switched on and there was a time of about 700ms between pulses, thus 5.14kW (3600/0.7)
- because the pulses are 100ms (and there has to be at least a little time between the pulses), the max power that can be measured is just a tad over 35kW
for this I used a phototransistor (Vishay TEPT4400) that has a good gain in the red part of the light spectrum… with a max spectral sensitivity at around 575nm and matched that with a 100k pull-up!