(From the standby power thread) How do energy companies calculate KWh? - Page 1
Forum section: electronics / general

author: icon
author_rank: Regular Contributor
timestamp: July 02, 2012, 09:49:57 pm
content: HiSpurred on by another thread about standby power, I was intrigued to know how much various items around the home were costing me. So, I can measure the RMS current being drawn by a device and calculate the VA at any particular moment. But voltage can change over time, and going from VA to watts requires a power factor connection. Does anyone know - a) what nominal (UK) voltage power companies assume and b) what power factor correction they use to calculate watts?(First thing I looked at was a transformer for a halogen lamp - uses 8VA switched off! What the hell!? That's maybe 10 beer tokens per year right there! Sadly the next thing I looked at blew the 400mA fuse in my multimeter, which rather offsets any savings I might make.)CheersJohn

author: madires
author_rank: Super Contributor
timestamp: July 02, 2012, 10:25:29 pm
content: Hi!Don't know the details for the UK , but the old fashioned power meters for private households over here measure just the active power (Ferraris type). Companies with heavy machinery got power meters measuring apparent power. The electricity suppliers want to keep their power grid clean and money is always a great incentive :-)Best regards,madires

author: Rufus
author_rank: Super Contributor
timestamp: July 02, 2012, 11:01:36 pm
content: Old electromechanical meters use a complicated induction motor with voltage and current coils, phase shift, and permanent magnet braking mechanism. The end result is an aluminium disc which turns at a speed proportional to true power.Modern meters measure voltage and current separately at high sampling rates and multiply the two digitally to obtain instantaneous power and integrate that to accumulate energy.Kill-a-Watt type devices operate the same way and are a cheap and moderately accurate way to measure mains power consumption.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 03, 2012, 01:37:15 am
content: Surely the logical thing is to institute a regime where you do the "turn all the standby stuff off at the mains switch" thing for a month,then do another month with your normal set up,reading the power meter at the beginning & end of each,& comparing the results of both periods.Both months would obviously need to have similar weather,as otherwise, if you use Electrical heating & clothes drying equipment,that could distort things.

author: madires
author_rank: Super Contributor
timestamp: July 03, 2012, 09:28:25 am
content: Hi!Quote from: vk6zgo on July 03, 2012, 01:37:15 amToo much trouble for a low accuracy! i would ask my electricity supplier what type of power the power meter measures, and get one those power meter plugs (a better one with readings of active/apparent/reactive power and cos phi). And then check the suspicious devices one by one. Most "green" electricity suppliers are even lending out power meter plugs to their customers for free.Best regards,madires

author: Rufus
author_rank: Super Contributor
timestamp: July 03, 2012, 08:17:56 pm
content: Quote from: madires on July 03, 2012, 09:28:25 amThey are not free. Expenditure on them probably qualifies as carbon credits or some other political bollocks and you and everyone else pays for them with increased electricity prices.

author: tom66
author_rank: Super Contributor
timestamp: July 03, 2012, 11:23:09 pm
content: It can be in the energy supplier's interest to reduce demand, or to keep you as a valued customer - those meters cost £10 in retail shops so they're probably only a couple of quid bulk from China.