Help understanding meters vs electric heaters

[jtomara37]

I've been in the electrical field for about 12 years (low voltage/controls mostly) and finally doing the school required to get my electrical licence.

Here is what I am perplexing over:

If you have a electric heater that requires 10 amps to operate;
120v x 10amps= 1200 Watts
240v x 10amps= 2400 Watts


Electric meters outside the home bill you for kilo-Watt hours so wouldn't the 120volt be cheaper? Maybe I don't understand what the meter reads. Does it measure amps or does it measure Watts. My guess it is has be be reading amperes (the flow of electrons past a certain point in 1/sec) If it is measuring amps then it would make sence the 240volt would be cheaper to run because it would take 20amps @120v to get the same power (watts) as the 240volt.

Here are 3 questions were given to me to answer:
1. A 5-kW electric heating unit is connected to a 240v line. What is the current flow in the circuit?
my answer: 5-kW= 5,000 watts / 240v =20.833 Amps
2. If the voltage in question 1 is reduced to 120v, how much current would be needed to produce the same amount of power?
my answer: 5,000 watts / 120v = 41.666 Amps
3. Is it less expensive to operate the electric heating unit in question 1 on 240v or 120v?
my answer: I'm stumped because no where up to this point have we been taught how were are billed for the consumption of energy other than kilo-watt hours which I understand to be 1000 watts used in 1 hour. My answer to the question #3 is they cost the same because we have the 5,000 watt(5-kW) that does not change, all we are changing is the volts.

Thx in advance for the support.

Jerry

 

[Besoeker]

I've been in the electrical field for about 12 years (low voltage/controls mostly) and finally doing the school required to get my electrical licence.

Here is what I am perplexing over:

If you have a electric heater that requires 10 amps to operate;
120v x 10amps= 1200 Watts
240v x 10amps= 2400 Watts


Electric meters outside the home bill you for kilo-Watt hours so wouldn't the 120volt be cheaper? Maybe I don't understand what the meter reads. Does it measure amps or does it measure Watts. My guess it is has be be reading amperes (the flow of electrons past a certain point in 1/sec)

The meter records energy. That is, power multiplied by time. It is usually expressed as kWh and that's what homeowners get billed in.

In the example you gave above, the 120V, 10A heater would give you less heat output than the 240V unit. To get the same output, you would need 120V and 20A. If you run either the 240V, 10A unit or the 120V 20A unit for an hour and you'd get billed for the same energy use, 2.4kWh in this case.

 

[jrannis]

Remember that the resistance of the water heater stays the same. Plug in your numbers and check it out.
A resistive load like a 240 volt water heater will operate on a lower voltage like the 120 volts in your example.

Its an easy way to get a warm shower out of a small generator when you have an extended power outage

 

[Dennis Alwon]

Also remember that in order to heat the tank the 120V element at 1/2 1/2 the wattage will take a lot longer to heat the water. This would mean that you are using less wattage at 120 v but it will probably have to stay on twice as long to heat the same amount of water as the 240 v element.

 

[iwire]

Here is what I am perplexing over:

If you have a electric heater that requires 10 amps to operate;
120v x 10amps= 1200 Watts
240v x 10amps= 2400 Watts


Electric meters outside the home bill you for kilo-Watt hours so wouldn't the 120volt be cheaper?

In your example above if both heaters ran the same length of time then the 120 volt heater would only use 1/2 the KWH of the of the 240 volt unit.

But that leaves out other important facts, you would also get 1/2 the amount of heat from the 120 volt heater.

For the heat output to remain the same the resistance of the element would have to change.

So for the same heat output the figures would look like this.

120v x 20amps= 2400 Watts
240v x 10amps= 2400 Watts

Here are 3 questions were given to me to answer:
1. A 5-kW electric heating unit is connected to a 240v line. What is the current flow in the circuit?
my answer: 5-kW= 5,000 watts / 240v =20.833 Amps

I agree

2. If the voltage in question 1 is reduced to 120v, how much current would be needed to produce the same amount of power?
my answer: 5,000 watts / 120v = 41.666 Amps

Correct

3. Is it less expensive to operate the electric heating unit in question 1 on 240v or 120v?
my answer: I'm stumped because no where up to this point have we been taught how were are billed for the consumption of energy other than kilo-watt hours which I understand to be 1000 watts used in 1 hour. My answer to the question #3 is they cost the same because we have the 5,000 watt(5-kW) that does not change, all we are changing is the volts.

The operating costs will be the same. The installations costs will be different.

Higher voltage = smaller conductors = less cost.

BUT

Higher voltage requires breakers and switches that have higher voltage ratings which in some cases can exceed the savings on the conductors.

A 3 pole 15 amp 480 volt I-Line breaker is a lot of money compared to using a 3 pole 30 amp 208 volt NQOB breaker.

 

[ItsHot]

power

power

power- the ability to produce an effect (work). I need more power!:grin:

 

[Steve Nolte]

I've been in the electrical field for about 12 years (low voltage/controls mostly) and finally doing the school required to get my electrical licence.

Here is what I am perplexing over:

If you have a electric heater that requires 10 amps to operate;
120v x 10amps= 1200 Watts
240v x 10amps= 2400 Watts


Electric meters outside the home bill you for kilo-Watt hours so wouldn't the 120volt be cheaper? Maybe I don't understand what the meter reads. Does it measure amps or does it measure Watts. My guess it is has be be reading amperes (the flow of electrons past a certain point in 1/sec) If it is measuring amps then it would make sence the 240volt would be cheaper to run because it would take 20amps @120v to get the same power (watts) as the 240volt.

Here are 3 questions were given to me to answer:
1. A 5-kW electric heating unit is connected to a 240v line. What is the current flow in the circuit?
my answer: 5-kW= 5,000 watts / 240v =20.833 Amps
2. If the voltage in question 1 is reduced to 120v, how much current would be needed to produce the same amount of power?
my answer: 5,000 watts / 120v = 41.666 Amps
3. Is it less expensive to operate the electric heating unit in question 1 on 240v or 120v?
my answer: I'm stumped because no where up to this point have we been taught how were are billed for the consumption of energy other than kilo-watt hours which I understand to be 1000 watts used in 1 hour. My answer to the question #3 is they cost the same because we have the 5,000 watt(5-kW) that does not change, all we are changing is the volts.

Thx in advance for the support.

Jerry

I agree with your answers on 1 & 2.

To solve for 3 you will need to figure the resistance of this heater element.

240v x 240v/5000w=11.52r

120v x 120v/11.52r=1250w

So a 240 volt heater supplied with 120 volt would produce 1250 watts and therefore be less expensive to operate at 120 volt.

I believe this to be the answer for the question being asked.

 

[iwire]

I believe this to be the answer for the question being asked.

It certainly could be, but it is a poorly worded question.

Normally you would not choose to supply a 240 unit with 120 volts.

 

[rattus]

Less expensive?

Less expensive?

Assuming constant resistance, applying 120V to the heater would indeed drop the power to 25% of the power at 240V and would cost less to operate if we consider only the cost/hour.

However, you are getting only 25% of the heat, therefore the cost per BTU would be the same and that is what it is all about.

 

[Besoeker]

Remember that the resistance of the water heater stays the same. Plug in your numbers and check it out.
A resistive load like a 240 volt water heater will operate on a lower voltage like the 120 volts in your example.

But you'd get just a quarter of the power.
If you are heating a tank of water, it would take four times as long to get up to temperature.
It it is an electric shower (one that heats the water as it is used) it might be a shade on the cool side.

 

[LarryFine]

I know some of this will repeat parts of some of the other responses, but . . . too bad. :grin:

If you have a electric heater that requires 10 amps to operate;

10a at what voltage? A heater that will flow 10a at 120v will not be the same heater as one that will flow10a at 240v.

These two heaters:

120v x 10amps= 1200 Watts
240v x 10amps= 2400 Watts

. . . will have different resistances. The first will be (120/10) 12 ohms, and the second will be 240/10) 24 ohms. If you applied 240v to the 120v heater, the current would be (240/12) 20a, and the power would be (20*240) 4800 watts.

If you want to talk about which voltage is more economical or efficient to use to produve a given amount of heat (or work, etc.), you have to treat the power as a constant, and vary the load resistance to suit each supply voltage, giving you two different voltage constants.

Electric meters outside the home bill you for kilo-Watt hours so wouldn't the 120volt be cheaper?

If you mean cheaper when using one heater for both voltages, yes. If you mean cheaper when using two same-power heaters at either voltage, no. Regardless of the voltage, it takes a certain amount of power to provide a certain amount of heat. The advantage of a higher power level is to be able to heat space faster.

Your heater has to be able to keep up with the heat loss of the space (or water, if it's a water heater.) As long as it can keep up with the demand, a small heater and a large one will heat the same volume for the same cost. This takes time into consideration.

While energized, the larger heater will consume more power than the smaller one, but the smaller one will have a greater duty cycle (on time.) If neither one is aksed to heat to the point it never cycles off, they will use the same amount of power over time.

It also doesn't matter what the voltage-to-current ratio is as long as the same power level is acheived. At any voltage, if the appropriate current is delivered, the same power can be produced. Practicality factors into voltage choices. Why isn't 120v used everywhere?

Maybe I don't understand what the meter reads. Does it measure amps or does it measure Watts.

It calculates watts by measuring current and voltage, just like we do.

My guess it is has be be reading amperes (the flow of electrons past a certain point in 1/sec) If it is measuring amps then it would make sence the 240volt would be cheaper to run because it would take 20amps @120v to get the same power (watts) as the 240volt.

Covered this above.

Here are 3 questions were given to me to answer:
1. A 5-kW electric heating unit is connected to a 240v line. What is the current flow in the circuit?
my answer: 5-kW= 5,000 watts / 240v =20.833 Amps

No problem there.

2. If the voltage in question 1 is reduced to 120v, how much current would be needed to produce the same amount of power?
my answer: 5,000 watts / 120v = 41.666 Amps

Right, but what has been left out is that the heater must be altered for this to occur; an element with 1/4th the resistance must be installed.

3. Is it less expensive to operate the electric heating unit in question 1 on 240v or 120v?
my answer: I'm stumped because no where up to this point have we been taught how were are billed for the consumption of energy other than kilo-watt hours which I understand to be 1000 watts used in 1 hour. My answer to the question #3 is they cost the same because we have the 5,000 watt(5-kW) that does not change, all we are changing is the volts.

That depends on how you define "operate," instantly or over time, and if the former, are we replacing the element?

 

[LarryFine]

Normally you would not choose to supply a 240 unit with 120 volts.

Nor, hopefully, the other way 'round.

 

[Besoeker]

It calculates watts by measuring current and voltage, just like we do.

The meter displays energy.
So you need Volts, Amps, power factor, and time.

 

[gar]

080927-1437 EST

What a conventional rotating disk watt-hour meter does is multiply the instantaneous voltage with the corresponding instantaneous in-phase component of the current to produce a torque on the disk. Conteracting this torque is a permanent magnet that restricts the rotation of the disk. This is designed and compensated so that rotor velocity is proportional to torque. The instantaneous velocity is proportional to power. By connecting a gear driven counter to the disk an integrator is created that displays the cumulative energy used.

This type of meter does not measure power factor and then multiply by voltage and current. It does not know or care about the power factor.

A two coil wattmeter also does not measure power factor.

.

 

[dmanda24]

when you say that it multiplies it by the instantaneous in-phase current component that takes into account the power factor, so it may not use the pf directly, but it is involved, if not we would be billed for KVARH (KILO VOLT AMPERE REACTIVE HOUR)

 

[gar]

080927-2052 EST

dmanda24:

Power factor is simply a mathematical tool to relate average power, the rate of doing work, to the product of typically rms voltage magnitude and rms current magnitude. Power factor is only indirectly determined. If you use a device for measuring voltage and/or current that is not an rms device then you may get a different value of power factor. Instantaneous power and average power are real world measurable quantities that can be directly measured.

To repeat the watt-hour meter does not measure power factor and does not use power factor to measure the energy used.

How would you measure power factor for an arbitrary waveform. You would use a wattmeter to measure power, and separately an rms voltmeter and rms current meter to determine VA. Next you divide the measured power by the measured and calculated VA.

There is no way to build a device to directly measure power factor for an arbitrary waveform other than using a wattmeter and the rms voltage and current meters, and combining the outputs to produce a power factor value.

.

 

[dmanda24]

I understand what you are saying but how does the meter measure only the component of the current that is in phase with the voltage

 

[gar]

080927-2152 EST

dmanda24:

See "Electrical Circuits and Machinery", by Hehre and Harness, Vol II, John Wiley and Sons, 1942, page 586, The Watthour Meter.

.

 

[jtomara37]

so how exactly does the meter on the outside of the building record the energy consumption being used, cause I'm unclear. Few people are saying it measures power. Power I understand to be watts (P). Is that correct? My limited knowledge thus far has yet to see a device that records watts. Altho I have seen and used Current Transmitters for opening and closing switches and devices and also seen ones that wrap around wires to measure the current flow (amperes) used in some electronic meters in commercial buildings. If the meter on the outside of a residential home does read watts (power p) could someone explain how it does this. Thx.

 

[iwire]

so how exactly does the meter on the outside of the building record the energy consumption being used, cause I'm unclear. Few people are saying it measures power. Power I understand to be watts (P). Is that correct? My limited knowledge thus far has yet to see a device that records watts.

A Kilowatt hour meter does in fact measure watts (power).