[EvaH (OP)]

Derek says:

My home is all electric and when I get my bill, it shows how many units of electricity I have used. It is currently around 17,000 units a year. I believe that a unit is one kilowatt hour. My question is, is a unit of energy (one kilowatt hour) the same, whether it is produced by gas or by electricity? In other words, if my heating was by gas and I kept my house temperatures about the same, would I still use around 17,000 units regardless of whether it was supplied by gas or electricity (this might, of course depend on the efficiency of the heating system)? Perhaps another way of looking at it would be to ask whether boiling a kettle of water by gas or electric would use the same number of units.

Can you help?

[Halc]

My home is all electric and when I get my bill, it shows how many units of electricity I have used. It is currently around 17,000 units a year. I believe that a unit is one kilowatt hour. My question is, is a unit of energy (one kilowatt hour) the same, whether it is produced by gas or by electricity?

A kilowatt hour is a unit of electrical energy. If you're heating your house by gas, you're not getting energy in the form of kilowatts.  Essentially, what you want to compare is BTU (a unit of thermal energy) per unit of cost.

Electric is often significantly more expensive per BTU than is a gas/oil burner, but this assumes 100% efficiency for the electric heat. A good heat pump can deliver something more like 300% efficiency depending on the ambient outside temperature during the heating season.

(this might, of course depend on the efficiency of the heating system)?

Exactly. Straight electric is 100% efficient (no waste going up the chimney), but is very expensive compared to gas which might be 80% efficient at a far lower cost. As I said, electric might be better only if you can get the efficiency far above 100%.

Perhaps another way of looking at it would be to ask whether boiling a kettle of water by gas or electric would use the same number of units.

Never seen a heat pump used to heat water, and a gas stove is typically not vented (and thus not a good choice for heating your home), so a kettle on the stove is far more efficient to heat with gas than electric.

[alancalverd]

Energy is energy. 1 kWh = 3412.14163312794 BTU but for historic reasons UK and US gas suppliers use BTU and guarantee their product in BTUs per cubic foot.

An immersed element electric kettle converts 100% of the electrical energy into heating the water and the element, with whatever losses occur from the surface of the kettle.

The efficiency of a kettle on a gas ring depends principally on the amount of heat  lost by radiation and convection from the flame to the surrounding air. The smaller the flame area with respect to the kettle, the less inefficient. 

[Bored chemist]

If you're heating your house by gas, you're not getting energy in the form of kilowatts. 

My gas bill says otherwise.
It tells me my energy consumption in KW Hr.
There's a very simple reason why they took to doing this.
A KW Hr of gas heating is a lot cheaper than the same energy delivered as electricity.

A kilowatt hour is a unit of electrical energy.

No, it isn't.
It's a bit more than a horsepower hour.
It is enough energy to move a force of 3600 Newtons through a distance of 1000 metres.

[charles1948]

You can measure some forms of energy easily enough.  The "heat" energy produced by a 1-kilowatt  electric fire can be measured by putting a thermometer in front of it.

But what about other forms of energy.  Such as "potential" energy.

How do you measure the potential energy of say, the Moon.  Or the M.31 Andromeda galaxy?  What units would you you use to express the potential energy of these bodies?

[Halc]

A kilowatt hour is a unit of electrical energy.

No, it isn't.
It's a bit more than a horsepower hour.
It is enough energy to move a force of 3600 Newtons through a distance of 1000 metres.

OK, I cannot argue with that. Energy is energy after all. It's just that where I live, only the electric company bills me by that unit.  My gas bill *used to be) in terms of volume delivered at some standard pressure.

My gas bill says otherwise.
It tells me my energy consumption in KW Hr.
There's a very simple reason why they took to doing this.
A KW Hr of gas heating is a lot cheaper than the same energy delivered as electricity.

Perhaps this is a more recent trend. Been a while since I've had gas service, despite the fact that the guy in the next lot over has it. They won't run a line over to us on the other side.

I see MMbtu now, which can be translated directly to KWh, but it assumes some sort of efficiency rating of both your electric or gas system that is probably not representative of your exact situation.

I maintain that usable heat per price is the only measure worth comparing.

[Halc]

You can measure some forms of energy easily enough.  The "heat" energy produced by a 1-kilowatt  electric fire can be measured by putting a thermometer in front of it.

Temperature and energy output are two different things. A magnesium candle burns at a far higher temperature than my heater, but puts out far less heat.

But what about other forms of energy.  Such as "potential" energy.

As BC points out, all energy can be expressed in any valid unit of energy such as KWh. Potential energy is negative and relative, so a rock on the ground (of Earth) might have -X KWh of potential energy on the ground and -Y KWh of PE up on the hill, where X > Y.  Zero PE is attained by the object lifted to infinite altitude from the object (Earth) relative to which the PE is measured.

How do you measure the potential energy of say, the Moon.

Depends on that which you choose for the relation.  The moon has so much PE relative to the Earth, but a much larger magnitude figure relative to the sun.

So as for Andromeda, relative to what?  Itself? That's a valid thing. An object's PE relative to itself is the energy needed to disperse the mass of the object to infinity.

What units would you you use to express the potential energy of these bodies?

KHw will do, or Joules (exajoules?), or any unit you like.

[Bored chemist]

units would you you use to express the potential energy of these bodies?

Joules.
That's the point.

[Bored chemist]

OK, I cannot argue with that. Energy is energy after all. It's just that where I live, only the electric company bills me by that unit.  My gas bill *used to be) in terms of volume delivered at some standard pressure.

Realistically, the gas and electricity companies bill me in the same unit. Pounds Sterling.

[evan_au]

Is a unit of energy the same, no matter how it is produced?

One measure of energy these days is how carbon-intensive it is.
- Around here, it's been banned in city residences now (because of pollution), but burning wood or coal is extremely carbon intensive
- Burning black coal is quite carbon intensive; a lot of the energy goes up the chimney. Turning it into electricity in a big plant is only 30-40% efficient; lose 10% of the remainder in transmission. A heat pump can regain most of those losses; an electric radiator does not.
- Burning gas (often methane, today) is less carbon-intensive, as some of the energy comes from burning hydrogen atoms. It is also able to be burned safely in residences - but some goes up the chimney.
- Solar and wind are carbon-intensive during construction, but not so much when running.
- Nuclear is very carbon-intensive during construction, but not much when running

All of these technologies also have carbon costs at end-of life.

[charles1948]

units would you you use to express the potential energy of these bodies?

Joules.
That's the point.

So, how many Joules has the Moon got, as it orbits the Earth.  Could we use these Joules as an energy source.
To generate electricity on Earth.  If not, why not?

[Colin2B]

So, how many Joules has the Moon got, as it orbits the Earth.  Could we use these Joules as an energy source.
To generate electricity on Earth.  If not, why not?

The first tidal energy scheme was in La Rance, France. Others include Strangford Lough in Eire.
I think there is a big one in S Korea.

[alancalverd]

Quote from: Halc on Today at 15:01:43
A kilowatt hour is a unit of electrical energy.
No, it isn't.

Oh yes it is. Energy is power integrated over time, so 1 kWh (= 3.6MJ) is a perfectly respectable unit of energy that by convention happens to be used by the electricity supply industry but almost nobody else (apart, it seems, from some gas companies). And that is why your electricity bill mentions "units". Not an SI unit, admittedly, but neither is the nautical mile or the pound sterling, both of which are very useful.

[alancalverd]

Others include Strangford Lough in Eire.

One day, perhaps.

[Colin2B]

Others include Strangford Lough in Eire.

One day, perhaps.

I thought the turbine was working, must ask folks who sail there.

[Bored chemist]

Quote from: Halc on Today at 15:01:43
A kilowatt hour is a unit of electrical energy.
No, it isn't.

Oh yes it is. Energy is power integrated over time, so 1 kWh (= 3.6MJ) is a perfectly respectable unit of energy that by convention happens to be used by the electricity supply industry but almost nobody else (apart, it seems, from some gas companies). And that is why your electricity bill mentions "units".

Thanks for the elementary physics lecture.
Now read the post you commented on.
A kilowatt hour is not a unit of ELECTRICAL energy; it is a unit of energy.
That's the point I was making.

[alancalverd]

Please name a unit of electrical energy that is not a unit of any other form of energy.

[charles1948]

Please name a unit of electrical energy that is not a unit of any other form of energy.

How about "volt"?

[alancalverd]

 Not a unit of energy.

[Janus]

Please name a unit of electrical energy that is not a unit of any other form of energy.

How about "volt"?

A volt is a measure of electrical potential.
It, pushing current through a 1 ohm resistance  results in 1 watt (Volts x Amps*), a measure of power, which over a duration of 1 sec, gives a watt-sec or 1 joule of energy.

* If you are dealing strictly with DC current or AC current with a purely resistance load. Otherwise, you have to consider the Power factor caused by either inductance or capacitance in the circuit.