Measuring Power Draw

[Mark Bat]

Hi all,

I live in a block of flats with communal garages and have been tasked with resolving a minor electrical issue we may have. (Having no electrical background, thought i would gain some expert advice. Any guidance/help would be very much appreciated).

I have been checking the power supplied to one of my neighbour's garages whilst they are away with a view to determining whether we have a problem with earth leakage or not. Everything is turned off in the garage with the exception of a car battery trickle charger. Over seven days the average daily power consumption was 0.373 kWh and did not hardly vary from one day to another. The main supply meter is analogue and presumably calibrated for 230-240 V. With the power off at the meter the consumption is zero as would be expected.

The question is; to determine what the calculated current draw is do we use 230V (I think that is what the supply is these days?) or 12V which would be the trickle charger voltage (I think). And then; do we appear to have a problem?

Using my schoolboy knowledge of physics:
230V example using W = V x A for direct current

Watts over one hour = 373/24 = 15.542

Current inferred = 15.542 / 230 = 0.0676 Amp or 67.6 mA.

12V example

Current inferred = 15.542 / 12 = 1.295 Amp or 1295 mA.

I do not have any information on the trickle charger in use to get an idea whether 1295 mA would be normal or 68mA. Our neighbour called it a "battery checker", but I am fairly sure this amounts to the same thing.

I do have a trickle charger myself and when I ran it for 24 hours on my car the consumption was 0.764 kWh over 24 hours by way of a general comparison. They are, no doubt, different chargers, different batteries and at a different level of charge. I know that my own charger will supply about 1.2 Amp until the battery voltage reaches about 14V whereupon the current will drop to around 200 mA or less to maintain charge.

If the answer is 68mA I assume all OK, but if it is 1.3 Amp might we have a problem bearing in mind that the power draw was practically the same every day? What is your opinion on this? Unfortunately the neighbour in question has not left a key to his garage so that I could turn the charger off and make some comparison with it disconnected.

 

[Pete999]

The 230Volts you talk of is the harmonised EU voltage, and any calculations would be inaccurate, to gain a true answer you need to actually measure the voltage in the Garage. Of course over the period of time the voltage could alter, not a great deal but it could drop during peak hours, cooking lunch, peak viewing time on TV, if this problem is really bugging then you may want to get a data logger installed, could cost a bit though,just depends on how far you want to take the issue, hope this helps.

 

[The Ghost]

If you think you have an earth leakage problem somewhere just measure the current going through the earth with a clamp meter no?

 

[Pat H]

And to be sure. turn off the battery charger and check no current is flowing. If any is its leakage (or an unknown load)

But based on your results that's 373Watts per day. So divide by 24 and that's 15 watts per hr.
I'd guess that's about right for a battery charger.

 

[PEG]

Hi,are the garages equipped with socket outlets,and are the owners billed individually or collectively,for their usage?

Basically,are your "investigations",safety,or cost driven?

 

[HandySparks]

But based on your results that's 373Wh per day. So divide by 24 and that's an average of 15 watts per hour.
I'd guess that's about right for a battery charger.

Corrected that for you.

 

[HandySparks]

Your calcs are generally right, but simplified. The battery charger is taking about 67mA at 230V from the mains supply, and feeding about 1.2A at 12V into the battery. Various issues complicate the matter, like the mains probably being more like 240V, the battery at about 14V when fully charged, the losses in the battery charger and the power factor (phase angle) the battery charger presents as a load on the supply.

A true earth leakage of more than about 1mA at 230V for the fixed wiring of a garage would be worth investigating as a safety issue.

However, as PEG says, what is the problem you're trying to solve?

 

[Lucien Nunes]

Charger output current at 12V does affect the input power consumption of the charger, but the two relate in a complex way that makes it irrelevant to the present situation. Also watts per hour' or 'watts per day' do not mean anything useful in the context. Talk of watts (power) or watt-hours (energy) but not watts per hour.
Your average consumption appears to be 373/24=15W which sounds about right for a charger, anything from 5 to 25W would be reasonable if it's not slogging away filling up a flat battery. While it is switched on, its consumption will mask any due to leakage, so you cannot investigate any further with the charger on. I do not actually see a problem here from your figures.

 

[Mark Bat]

Thanks to all who replied. Some great advice and further thoughts. Appreciated all.