significance of VA on power monitor

[andyv]

I've been experimenting with a plug in power monitor. It reads out in watts and also VA.

As I understand it VA and watts should read the same when it's a straight resistance circuit, but differ when there's a capacitor or an inductance in the circuit. I'm really only interested in how much power comes in and I'm charged for through my external electricity meter.

Is the VA reading of any practical use to me?

[The Wanderer]

Domestic supplies are charged on kwh's, not kvah.

Haven't you answered your own question? (Rhetorical!)

[John Rumm]

The better ones do...

Only in that it lets you see when you have a reactive load.

(Depending on what other variables the meter can show you it may also be useful to help you estimate the current carrying capacity required for a given load).

[andyv]

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Thanks folks. I've been measuring individual circuits with the power monitor and also the whole supply with a data logger attached to the incoming meter. I've run this in lots of different ways with different circuits on or off in an attempt to get an idea of electricity use. Gradually I've found items of waste which I've been able to eliminate.

There's still 0.5 kwh going astray midnight to 6.00am and maybe all day too, after I've accounted for everything I know about - fridge, security lights, clocks, central heating and various other things like garage door on standby. I wondered if there might be a clue in the VA reading.

[John Rumm]

There may well be - in that it will give you some clues as to type of load you are looking for.

[TheOldFellow]

Most likely: pumps (CH) and motors (fridge/freezer), both inductive. Do come back and tell us when you do find out.

R.

[geoff]

In message , andyv writes

So what's the stated accuracy of your new little toy then?

Are you losing watts or measurement errors ?

[andyv]

I have thought of that but the declared accuracy seems quite good - see

of my readings come from the data logger though. This is separate to the power monitor. I have a device which counts the red flashing light pulses on the main meter. Each flash is 1 watt hour so I can build up a picture of what passes through the meter over different time periods. I'm particularly interested in what happens when everything is idling in the middle of the night.

One potential item which I'm investigating is the frost thermostat on the boiler.

[geoff]

I would expect something like that to be responsible for 3/5th of bugger all (choose your own units)

[andyv]

You are probably right that it doesn't amount to much. My central heating uses about 15 watts of electricity when it's off. I think however that recent cold weather has been causing the frost thermostat to trip and so run the full central heating in the middle of the night. This then steps up to about 125 watts for the control gear and pumps, and of course it uses gas and heats the whole house. The thing is that the thermostat seems to be faulty and must be tripping in at somewhat above the required 5=BA. I don't yet know if this is happening regularly and is the only issue.

When you add up several small items like this it comes to a fair bit of waste. Not massive but worth preventing, if you care about such things.