# Is MW/hr a valid unit?

Hello all,

I'm new to the forums and this seemed like the perfect place to discuss an issue that has been bugging me. I know MW refers to power (the rate of energy production basically in millions of Joules per second) and MWh refers to energy (essentially the total amout (in millions of Joules) of energy produced in an hour). But from time to time I've seen "MW/hr". Sometimes I think it must be a mistake and whoever wrote it must have meant "MWh", however on other forums (e.g. Yahoo! Answers) I remember seeing that MW/hr would indicate the rate of change of power but there didn't seem to be agreement on whether it was actually a real unit. So is this really a valid unit?

I saw a similar physics error on a sign in a local transit train a while back. I don't know what the writers mean, but energy per unit time squared is not a useful unit. If you know what the writer is talking about, and can look up real physics units elsewhere, you might be able to work out what he/she should have written.

i don't see why not, MW/hr is the rate of change in power, or the "acceleration" of energy. while power describes the amount of energy used per unit time, power/s would describe the amount of power used per unit time, or how the energy per unit time is changing. i don't think there's a distinction between a "real unit" and a "not real unit", the ones you usually see are the useful ones. for example the gravitational constant has units N*m^2/kg^2 or m^3/(s^2 * kg), I don't think theres such thing as a unit that is "not valid", maybe just not useful

Pengwuino
Gold Member
There would be very specific cases where such a unit could make any sense. If I asked how is the power generating abilities of a country changing over time, someone could say that country X is increasing its power generating abilities by 35 GW/year.... which you could reduce down to MW/hour, although that would be kinda silly.

Your ideas of what the units mean are correct.

Where I saw the unit was on a sign trumpeting the fabulous amount of power available from the sun, quoting in MW/hr. This does not make sense. Yes, one could imagine a scenario where MW/hr is meaningful, with respect to the ability of a power plant to respond to rapidly changing demand, but in the context of the sign it's incorrect.

Another possibility is that they don't mean to use it as a unit. In one hour the sun emits 1.385×10^30 J of energy coresponding to 3.846×10^20 MW. Perhaps they meant MJ/h but meant to say that in one hour, this is how much power is produced by the sun

MW-Hr is a valid unit as a measure of energy produced or consumed, 1MW-Hr is 3600 MJ (Mega Joules). It's rather easier to picture 24 Kw-Hrs as a fan heater running all day rather then 86,400 KJ.

Redbelly98
Staff Emeritus
Homework Helper
Where I saw the unit was on a sign trumpeting the fabulous amount of power available from the sun, quoting in MW/hr. This does not make sense. Yes, one could imagine a scenario where MW/hr is meaningful, with respect to the ability of a power plant to respond to rapidly changing demand, but in the context of the sign it's incorrect.
There is a lot of misconception, in the general public, on the distinction between energy and power. As you said, it doesn't make sense to say the power is ___ MW/hr. Whoever wrote that does not understand it, and we can only guess as to the actual meaning.

there is nothing invalid about MW/hr, but I don't think that it means much as a unit. I don't think it has many applications. MW-hr is a measure of energy.

QuantumPion
Gold Member
What is the ramp rate of the reactor? 90 MW/hr. Usually it's expressed in percent full power per hour but MW/hr works as well.

MW-Hr is a valid unit as a measure of energy produced or consumed, 1MW-Hr is 3600 MJ (Mega Joules). It's rather easier to picture 24 Kw-Hrs as a fan heater running all day rather then 86,400 KJ.

I know MW-hr is a valid unit. I talked about it in the original post (as MWh). MWh or MW-hr though refers to MW x hr. I was talking about the usage of MW / hr or MW per hour. I've seen it used on occasion but very rarely and often it seems incorrectly. I figured that there must be a correct usage for it otherwise some people on other forums wouldn't say it could mean anything, but in my Physics education I never came across it (I came across all the usual suspects of course like J, MJ, W, MW, MWh or MW-hr, etc).

i don't see why not, MW/hr is the rate of change in power, or the "acceleration" of energy. while power describes the amount of energy used per unit time, power/s would describe the amount of power used per unit time, or how the energy per unit time is changing. i don't think there's a distinction between a "real unit" and a "not real unit", the ones you usually see are the useful ones. for example the gravitational constant has units N*m^2/kg^2 or m^3/(s^2 * kg), I don't think theres such thing as a unit that is "not valid", maybe just not useful

Thanks gordon.

I suppose I should have asked if it was a useful unit but the rarity of it lead me to wonder if it was a proper standard/accepted unit or if it would be the equivalent of using "cubits/hr" as a measure of speed.

There would be very specific cases where such a unit could make any sense. If I asked how is the power generating abilities of a country changing over time, someone could say that country X is increasing its power generating abilities by 35 GW/year.... which you could reduce down to MW/hour, although that would be kinda silly.

Your ideas of what the units mean are correct.

there is nothing invalid about MW/hr, but I don't think that it means much as a unit. I don't think it has many applications. MW-hr is a measure of energy.

What is the ramp rate of the reactor? 90 MW/hr. Usually it's expressed in percent full power per hour but MW/hr works as well.

Thanks guys. I suppose that is why I have only seen it rarely as the ramp rate of a reactor isn't going to be something that is often disseminated to the public. Usually any article about a reactor or power plant would only refer to it's power generating capabilities (which would usually be expressed in some form of watts such as MW or kW).

QuantumPion, would it be possible to determine the power production of the reactor only from the ramp time? Because 90MW/hr could mean the reactor goes from producing 0 to 90 MW over the course of an hour right? Or it could go from producing 0 to 45 MW over the course of a half-hour. So if it had a power generating capability of only 45 MW the ramp rate wouldn't seem particularly helpful in determining that from the ramp rate alone in a case where someone might have mistakenly used the ramp rate figure as the power generating capability.