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steel 02-26-2007 09:12 PM

Load management
I posted a question about a method of controling my load meter, but the equipment I was told about by the electric company seems to be obsolete. Maybe I need to try a different approach. Can someone explain to me how the load meter works?

From what I have gathered, it is supposed to measure the amount of watts being used at the same time. The meter is supposed to be set to take a reading over a half hour period. This to me means that if I were to use 10kW for 15 minutes and 5kW for the other 15 minutes, I should get a reading of 7.5 kW for that interval. The highest amount registered for the month is then what I am charged for. Am I understanding this correctly?

I have also been told on numerous occasions that the key is to control the hot water heater. I went outside and took a reading under normal conditions (1.5kW), then I turned the hot water heater on and took another reading (2 kW). I was expecting a much larger difference. Again, I may very well be confused about how it is calculated. My tank is listed at 3380 watts for one of the elements and 4500 for the other. Assuming both were on, shouldn't I get a reading of 7.9 kW just from the tank?

Is there any way to monitor the meter? I would love to be able to get a printout of the usage on an incremental basis. Is there such a piece of equipment that could be wired up to do this for me? It would definately help to be able to figure out this mystery. I can never seem to catch the meter anywhere over 5 kW. I even had my old meter tested, so I know it was good and I was getting the same sort of readings out of it as I am now with my new meter.

elementx440 02-26-2007 10:09 PM

wattage is your voltage times your current. Resistance (load) of anything conductor changes with heat, so the current passing through the heater elements will actually change as it heats up... that will change the load dynamically. I have no idea how they actually calculate the ratings on the sticker, maybe like a RMS value or something....

isn't the load measured over time? the kW/hour?

the SI unit equivalent of kw/hour is Joules (1watt/second)

1kW/hour = 3600000 joules or 3.6 mega-joules
perhaps you'll find more info by googling how to measures joules?

this was interesting from wikipedia:
1 joule in the real world is approximately:
the energy required to lift a small apple (102 g) one meter against Earth's gravity.
the amount of energy, as heat, that a quiet person produces every hundredth of a second.
the energy required to heat one gram of dry, cool air by 1 degree Celsius.
one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer.

jwhite 02-27-2007 05:41 AM

I do not know what load metering is. Demand metering is when you are charged a higher rate depending on time of day, and day of year. High demand days, like in the summer have a higher rate. You can also be charged a higher rate if you exceed a max demand level for a certain period of time.

In your hot water tank experiment, how can you be sure that the hot water tank came on. The temperature could have been satisified for one or both of the elements. Do you have a hot water pump? That may have been the only thing that came on.

In you example 10 kw for 15 min plus 5 kw for 15 min is 7.5 Kwh (kilo watt hour). it is 15 kw for 1/2 hour

there are lots of load management meters out there. I have installed a viriety in different settings. I have yet to find one priced for residential applications. The payback time for the equipment would be too many years in a house, to justify the initial cost.

Check out the sq d web site. or rockwell automation. Most of these products are designed for places that already have a communications back bone in the facility. this makes the product easy to hook up to a computer system. In your case you are likley to need to buy the comm back bone as well.

There are other vendors and you could even look on ebay. Good luck with your project.

steel 02-27-2007 09:05 PM


Originally Posted by elementx440 (Post 34917)
isn't the load measured over time? the kW/hour?

With the metering system I am on, it takes into account the usage over time (kWh) and my maximum load (kW). The kW are then used as a multiplier to factor my bill. The numbers side of it I have all figured out. The electric company sent me the information and I was able to set it up in an Excel spreadsheet to calculate it. If anyone is interested in getting a crash course in how the bill is calculated, then I have a thing or two I can teach you. It apparently takes quite a bit more knowledge to be able to outsmart the meter.


Originally Posted by elementx440 (Post 34917)
one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer.

I am going to go refresh my system with a few joules of energy.

steel 02-27-2007 09:12 PM


Originally Posted by jwhite (Post 34930)
In you example 10 kw for 15 min plus 5 kw for 15 min is 7.5 Kwh (kilo watt hour). it is 15 kw for 1/2 hour

Is this how they factor it? If this is true, then it would certainly explain why I can't figure out how to beat this thing.

So it does not necessarily measure the load at one time, but it takes the total for the half hour?

Let's just say that it is as much fun to turn a drier on and off as it is for my kids to turn lights on and off. If the dryer uses 5,000 watts and it is turned on three times in the half hour interval, does my meter then read 15kW?

jwhite 02-28-2007 05:57 AM

No it reads only the time it was running.

kwh = T x KW

Say the dryer ran six times for one miniute each.

kwh = .1 hr x 5 or 0.5 kwh

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