Actual electrical meter reading.

[gar]

160321-2004 EDT

michalspike:

No way are you going to make a simple current measurement in a residential application and determine energy usage for a month or year time period. An instantaneous current measurement with some additional information can give you an instantaneous power measurement, but that is not energy.

To determine energy used over some time period means you need to integrate the instantaneous power over that time with respect to time. That is what the power company electric meter does.

Instantaneous power is not the same as short time average power, but knowing short time average power and adding contiguous elements of short time average power times the averaging time over some long time period will give you the energy over that long time.

How to estimate the correctness of the power company meter? If the meter is electronic, then it is probably more accurate than instruments that you may have unless there is a gross error.

Find out how to use the power company meter to make short time energy measurements. Get information from the meter face and ask meternerd how to use that information.

You have two good resistive loads (the water heaters) to provide a known load. Turn off everything in the apartment except the water heaters. Drain enough hot water to force the water heaters to be full on for the time of your test. It is possible that the test time does not need to be more than a few to 10 minutes. This will be a function of how easy it is to read short time energy from the power company meter, and use of a stopwatch.

The water heaters are most likely 208 or 240 V units. Assume no major current from motors or controls goes to neutral. This means one or two ammeters and one voltmeter are your test instruments. Measure the current to each water heater, add the two currents, measure the line-to-line voltage, and multiply the summed current by the voltage to get average power. It is also necessary to measure and verify that this short time average power is constant during the test.

Assuming that the water heaters have stablized in power consumption, then multiply this average power by the time duration during which you read the power company meter. This time has to be in hours. So a 6.4 minute time period is 0.1067 hours. Compare this kWH estimate with the measured kWH from the power company meter.

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[Jraef]

Aside form the kVA vs kW issue, you have another problem in attempting to use a clamp-on portable meter; the "time" factor. Let's keep it simple by looking at the water heater, because in THAT case, kW and kVA will be the same.

So let's say you clamp on with your meter and read 18.75A and it's a 240V heater, and assume, for simplicity, that you measured EXACTLY 240V at the line terminals. So that means the heater is drawing 4500W (18.75 x 240). That reading however is only good for that INSTANT in time. 2 minutes later, let's say you clamp on again and read zero. Why? Because the water is now hot and the heater is no longer necessary.

So how long are you going to have that meter clamped on? If you stood there for 24hrs and wrote down the instantaneous readings every minute for 24hrs, you might have a decent estimate. Are you going to do that?

What a Watt-Hour meter does is use a "totalizer" that sends a string of pulses to an accumulator from what are called "KYZ" contacts. This is a way of telling the meter that a particular amount of energy passed through the meter, then the accumulator adds those pulses all up to give you kW-hours. Without that kind of meter, you can't really tell them anything useful.

The TED meter mentioned by gar is one of those types of kWh meters that you can use to verify the PoCo meter. It's not as accurate however. PoCo meters have to be certified by the Dept. of Weights and measures to an accuracy rating referred to as "revenue class" metering. Commercial off-the-shelf meters do not.

The bigger problem, and the reason he might be concerned, is that despite the required accuracy of the meter, a lot of PoCos are allowed to "estimate" your power use without actually reading the meter! In that case, a recording kWh meter like the TED would be a decent investment.

 

[kwired]

The bigger problem, and the reason he might be concerned, is that despite the required accuracy of the meter, a lot of PoCos are allowed to "estimate" your power use without actually reading the meter! In that case, a recording kWh meter like the TED would be a decent investment.

If they do estimate your usage, at some point they do have to periodically take an actual reading and make up for errors in estimating. If they underestimated over several months then take an actual reading, depending on how they handle the makeup, you could have a one time high bill, or one time low bill if they over estimated for several months.

Some companies maybe find ways to make up differences over one billing period so you don't get that one time jump every so often.

 

[jaggedben]

A revenue grade meter is supposed to be accurate to within 2%, I believe. Something like the TED may only be accurate to within 5% (check the specs). So if you install a TED and get a result that is, say, 7% or more different from the utility meter, then you have a decent case to take to them that their meter is wrong.

 

[robbietan]

In our part of the woods, the revenue meter should be at least 99.98% accurate, certified by a government agency. And meter readings should be between 29 to 31 days apart, no estimating is allowed.

 

[kwired]

A revenue grade meter is supposed to be accurate to within 2%, I believe.

Something tells me it is more like .02%. Other measuring devices used for trade that I have encountered is pretty strict. I mostly have experience with weighing scales - I don't know what laws on tolerances are but the goal from customers was always to be about as right on as possible, if weighing something to be sold (by weight) any error better be high instead of low, but still needs to be as close to accurate as possible.

 

[ggunn]

A revenue grade meter is supposed to be accurate to within 2%, I believe.

I don't know it for a fact but that number sounds very high. A 2% error for a big account over several years' time would be a lot of money either lost by the POCO or overcharged to a customer.

 

[kwired]

I don't know it for a fact but that number sounds very high. A 2% error for a big account over several years' time would be a lot of money either lost by the POCO or overcharged to a customer.

Big or small user, it amounts to more then an added local sales tax often around 1% or so.

 

[michalspike]

For now I'm planning to have 2 test readings morning and at night.
See what I will get from it. I don't need to be 100 % accurate on this.
Thanks for your help.
Will keep you posted

 

[gar]

160322-2129 EDT

michalspike:

Your last post indicates that you do not understand how to determine if the utility meter is approximately correct. A short sample of power at two points in a day is highly unlikely to provide you with any reasonable way to estimate energy usage over a day. It is a waste of your time, and your customer's money.

Get some books and study the relationship of power to energy over some time period with the power constantly varying.

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[gar]

160322-2343 EDT

michalspike:

Using some data from my home using the procedure I believe you have proposed I get the following results:

All data is from the same day.

One set of data is:
2 kW at 0700 and 2.5 kW at 1700 (7 AM and 5 PM).
The average is 2.25 kW, and calculated energy is 54 kWH.

Second set of data is:
1 kW at about 0800 and 1.5 kW about 1600 (8 AM and 4 PM).
The average is 1.25 kW, and calculated energy is 30 kWH.

That is a big range that result from a random time of making the measurements.

The actual total energy used was 37.61 kWH.

You have an even bigger problem with your method because of those big water heaters.

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[winnie]

If the goal is a ballpark accuracy test, meaning that you would only discover a broken meter, then you should probably look at this from the other direction.

Rather than try to estimate energy consumption over the course of a day using clamp on meters to compare to the utility meter, you should figure out how to read short duration _power_ consumption from the utility meter.

With old mechanical meters, the speed of the spinning disk indicated power consumption. The meter would essentially count the number of turns to get energy. With newer electronic meters there is usually something that flashes every time a small amount of energy is consumed; say 1000 pulses per kWh. The flash rate is a good indication of short duration average power consumption.

You can figure out the power consumption from the utility meter and compare that to your measurements.

Given all of the approximations involved, if the two numbers agree to within a factor of 2 or 3, I would say they agree.

-Jon