Anybody know how accurate/useful these electricity usage meters are.
There seems to be a rash of them around and I'm having difficulty making a decision on which (if any) to buy. I'd like to be able to configure it to display the current cost (in £) for the units being consumed for ALL usage. I guess the ones that connect to the household supply at the meter stage would be the best (?) but which one?
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Neil
All the ones I've seen (including that one) are Oregon Scientific, or rebadged/OEMed (as that one appears to me). They have no actual connection to the mains, so they can only actually measure current, and have to make assumptions about voltage and phase angle to get the power consumption. As such, the plug-in meters are going to be significantly more accurate, but that's not to say you wouldn't get some useful indications from this type you install at the incoming supply, but beware it won't give accurate readings for IT equipment, compact fluorescents, and microwaves, nor if your mains voltage fluctuates much.
Yes, I've just been looking at the manual for the 'Owl' device [1] and it doesn't mention power factor at all. Phase is only mentioned to distinguish single and 3-phase systems. It has a clip-on current transformer that it says you must attach to the line meter tail (neutral would work equally well in a sound single-phase installation). The CT plugs into a battery-powered transmitter unit - I bet that eats batteries, wiping out some of the energy cost savings - but there's no voltage sensing connection.
There's a brief spec that gives accuracy figures: unspecified below 1 A, ±10% between 1 and 3 A, and ±5% above 3 A. This is for RMS current, but it's not clear whether the reading is true RMS or simply rectified mean. The upper current limit is 71 A, which suggests to me 100 A peak as the scale limit for its DAC. (Potential clipping problem for high-current loads with poor power factor, but not likely to be an issue for domestic use.) For calculating the power does it assume unity PF, or some lower value like 0.9 or 0.95? Who knows?
Agree entirely. And of course it will over-read, giving a somewhat optimistic view of the potential savings.
[1]
Doh, ADC.
When I first saw them, I wondered if they picked up the phase angle capacitively via the CT lead. However, there's no earth clip (or similar) to measure that relative to.
I'm somewhat surprised the CT can be unplugged from the sender unit. I wonder if the CT part includes anything to prevent high voltage on the open circuit secondary? Perhaps the secondary load is inside the CT housing?
I've just installed one of these
The sensor clamp's transmitter is quite heavy and bulky, and I understand it contains replaceable D-cells which are intended to last for 6 years. I've only had it for one day, so I can't comment on actual battery life.
It has a clock, so it knows the time but not the date. It can be programmed with different unit costs, depending of the time of day. It can't be programmed with the "the first 1000 units costs 15p each, after that 10p per unit" tariffs that are common these days. In use, it seems quite sensitive. It makes the running cost difference between a 60W incandescent and a 20W CFL bulb obvious. I was surprised by how much power fridges and freezers use. For techies, it also has an RS-232 interface (haven't tried that yet either).
Overall, it looks like a good buy so far, given the rising cost of electricity.
Although I've only had it for one day, another thing I like about it is that the receiver is mains powered rather than battery powered, which should reduce running costs. The wall-wart is claimed to draw less than 2W, which is plausible because it doesn't get very warm.
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