Electric bill

Hi!
I have this question in my homework:
A heater has power of 2.5 KW, it has been on for 12 hours.
a) How much will cost... (that is easy)
b) Explain why the electrical bill for such a device is smaller then what was calculated before (in A).

Our teacher said this in class: the consumer unit counts the incoming power and subtracts the outgoing power.
I may have misheard exactly what he said, but I think he said it like this. This really confused me! If the consumer unit really subtracts unused electricity then shouldn't it be too easy to make a "electric bill hack" - uses something like a generator?

I have heard about "hacking" the consumer unit by using magnets to stop the rotor, but I have also heard about some other devices, that plug in to the socket. One was something like a impulse generator, that does damage to the consumer unit, but the other one seems to cost few 1000$ and seems to somehow lower your bills. I don't really know nothing more about them and that is maybe why I don't know this answer.

Btw, output power is the power that will be turned in to practical energy (microwave oven, electricity to microwaves). It's just said here that microwave has output power of 650W and uses 50% only, so input power will be 1300W... isn't that too much for a microwave? And btw, the reading on the back of a device - it is the input power, right?

WG1337 said:

Hi!
I have this question in my homework:
A heater has power of 2.5 KW, it has been on for 12 hours.
a) How much will cost... (that is easy)

Click to expand...

The heater will use 2.5 x 12 = 30kWh of energy, but it is impossible to calculate the cost because the unit cost of the electricity is not stated.

WG1337 said:

b) Explain why the electrical bill for such a device is smaller then what was calculated before (in A).

Click to expand...

This is nonsense.
There is no reason for the bill to be less.
However if the heater is thermostatically controlled and does not need to be on 100% of the time, it will use less than 30kWh of energy. But this is not stated in the question.

WG1337 said:

Our teacher said this in class: the consumer unit counts the incoming power and subtracts the outgoing power.
I may have misheard exactly what he said, but I think he said it like this. This really confused me! If the consumer unit really subtracts unused electricity then shouldn't it be too easy to make a "electric bill hack" - uses something like a generator?

Click to expand...

Yes back-feed ing the grid through the meter using a generator will make the meter go backwards (to the best of my knowledge) but the reduction in the bill will be more than offset by the cost of running the generator.
Maybe you need to pay more attention in class, or, just as likely, you need a better teacher!

WG1337 said:

I have heard about "hacking" the consumer unit by using magnets to stop the rotor, but I have also heard about some other devices, that plug in to the socket. One was something like a impulse generator, that does damage to the consumer unit, but the other one seems to cost few 1000$ and seems to somehow lower your bills. I don't really know nothing more about them and that is maybe why I don't know this answer.

Click to expand...

Mostly old wives tales and ill informed nonsense.

WG1337 said:

Btw, output power is the power that will be turned in to practical energy (microwave oven, electricity to microwaves). It's just said here that microwave has output power of 650W and uses 50% only, so input power will be 1300W... isn't that too much for a microwave? And btw, the reading on the back of a device - it is the input power, right?

Click to expand...

The magnetron will have a conversion efficiency which could be as low as 50%.
Having just disturbed the spiders living around the back of my microwave oven, the rating plate states that the microwave power is 650W and the mains input power is 1300W. The other 650W is dissipated as waste heat out of the back of the microwave oven.

JimB

For got to give the cost, but that wasn't the big problem, just need to multiply kWh with the cost.

The thing is that it stated there that the cost is dramatically smaller. I had a thought also about the power control, like a oven - first heat up the spiral then lower power, because that much power isn't needed. I also thought about resistance (active), resistance for metals grow if they are heated, but that is a small fraction, don't think the current will change a lot. Then I thought about inductive resistance (it is stated here that it is a heating spiral), but isn't it already calculated to the main input power? But on the other hand, the resistance changes with frequency, I live where AC is at 50Hz, in USA it's 60Hz, many devices these days work in 230 and also in 120 volts.

So if the consumer unit does not subtract unused electricity then it is simply wasted.
I have read about devices called ECO-SAVER, from the description they should work (stabilize current, reduces reactance, stabilizes voltage), don't know if they really work, but should reduce the bill if they work as said.
Can't blame the teacher, his first year teaching physics, probably meant that consumer unit only counts electricity one way.

Wait guys. Is this a trick question?

1. Most heaters have thermostats and thus there would be a duty cycle associated with it.
2. Is there a positive temperature coefficient associated with the heating element?

What is this "unused electricity" ??
Never heard of that before!

I think your ECO-SAVER is just a power factor correction capacitor which will do next to nothing in terms of saving any electric bill in a domestic environment.

JimB