Since MW is actually the rate of energy is being used, why would we use MWh/h as in Fingrid's electricity market Load and Generation forecast page?
Wouldn't MWh/h be the same as MW???
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3\$\begingroup\$ must be some power-company way of looking at things. i'd agree that MW-H per hour would be MW. maybe utilities look at it as buying and selling units of energy measured in MH-H, and then measure the rate of that, rather than just looking at it as straight power. \$\endgroup\$– JustJeffOct 11, 2010 at 23:48
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4\$\begingroup\$ it could represent the rate of change of energy I suppose, but I agree it looks like they're just misusing \$\endgroup\$– Toby JaffeyOct 11, 2010 at 23:50
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2\$\begingroup\$ @endolith: It's their measure of power production. They produce/sell in MWh. The total they produce/sell in a period is measured in MWh/h or /day, /month or /year. This is standard practice internationally. \$\endgroup\$– TransistorJan 2, 2016 at 19:38
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1\$\begingroup\$ There are lots of interesting units out there that do not make sense if you look at them in isolation. My favourite so far is "Hz/s" (speed of an RF spectrum sweep). \$\endgroup\$– Simon RichterJan 3, 2016 at 21:54
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1\$\begingroup\$ I think it is about the software's programming which creates that graph. Software might decide to keep the h/h to help you to back track the steps of the calculation. \$\endgroup\$– AyhanSep 9, 2016 at 18:56
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5 Answers
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MW is a unit of power. MWh is a unit of energy.
MWh/h is a unit of AVERAGE power during a certain period, so it is used to show ENERGY consumption.
For example, I as a consumer can be buying power from the utility with a 1 MW power (that is, I pay to have a MAXIMUM DEMAND of 1 MW) but I only consume 12 MWh of energy per day, so I have an ENERGY consumption equivalent to 0.5 MW - flat.
To distinguish between one and the other, the first is referred to with MW, while the second one might be referred to as MWh/h.
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4\$\begingroup\$ "MWh/h is a unit of AVERAGE power during a certain period" So is MW. "so it is used to show ENERGY consumption" No. MWh/h is a measure of power, not energy. \$\endgroup\$– endolithJan 4, 2016 at 17:14
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1\$\begingroup\$ It is strange that this answer was accepted. MWh/h is silly, as the ‘h’ in the denominator is arbitrary. Why not take minutes, weeks, or decades? A bit of looking around would have shown that this is not some kind of industry custom. For example, Elia just uses MW. \$\endgroup\$– equaegheSep 9, 2021 at 19:19
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No, it doesn't. h/h = 1, so it's MW. But power companies never made sense anyway :)
Maybe they think the target audience doesn't understand watts but have a vagues idea about what MWh is and know how to think of magnitudes per hour.
(And fingrid? What shame. ;)
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1\$\begingroup\$ I don't think that this makes it easier for the target audience. Here, we have a sometimes very trashy daily "science" program on tv, and the often also claim a power consumption of 60 watts per hour... (but they also say the want to entertain, not to teach. but people believe in their show...) Another fact: Power is P=UI, so the unit is VA=W. But VA is also used to denote it's apparent power. \$\endgroup\$– sweberApr 14, 2015 at 8:01
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\$\begingroup\$ @XTL: It does make sense. It's a bit more obvious if we consider production in MWh/day. Here you can see that just giving MW would require averaging, etc. They're selling in MWh so they record production in MWh/h. If they said they produced 5 mega-units / hour you probably wouldn't have an issue. \$\endgroup\$ Jan 2, 2016 at 19:44
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\$\begingroup\$ Seconded. Many people, even among those that trade, consume "kWh" and "MWh" as atomic notion, thus the need to weird kWh/h etc. \$\endgroup\$ Feb 6, 2018 at 14:33
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agreed, it is power companies trying to keep their end simple consistent.
As people are generally billed in kWh or (MWh for big business?) it is simpler to indicate the a unit of power per time period.
* kWh and MWh are alternatives to joules for representing an amount of energy,
* kWh/h and MWh/h are alternatives to kW and MW for representing power (or energy production/consumption).
As appliances as generally rated in W (or kW) telling people they have been charged for 100MJ of electricity might be confusing to the end user, so instead they have opted for kWh as the unit for energy. The kWh/h and MWh/h is just an extension of that, to provide end users with units that have meaning to them.
For example Fingrid is now generating enough power to supply a 1,000,000 fridges (assuming 1kW fridge). So it does make sense, in weird non scientific kind of way
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3\$\begingroup\$ I just drove for an hour at 60 km/h. So I must have driven a distance of 60 \$km\cdot h/h\$. \$\endgroup\$– KazMar 3, 2013 at 0:09
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It clearly makes sense. Consider it as Joules rather than kWh:
kW = J/s Instantaneous energy consumtion. A graph would often look like white noise.
kWh/h = J/h Energy consuption per hour, you will see the morning, dinner and night in a graph.
kWh/month = J/month Energy used per month. In a graph you see summer and winter.
kWh/y = Joule per year Energy per year. In a graph you will see "hot" and "cold" years and climate change.
For the total world consumption:
16 kWy/y ca 400 EJ/y = ExaJoules per year. To write 16 kW here would express worlds instant energy consumption or Joules per second, not very informativ.
All are energy per unit time, but by changing the actual time periode one can address different aspects of energy consumtion.
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1\$\begingroup\$ I really do not think it makes any sense to relate unit to "kind" of quantities. E.g. volts is volts there's no different unit for DC, peak instantaneous, average or RMS voltage. They all are simply volts. \$\endgroup\$– carlocMay 24, 2017 at 11:45
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\$\begingroup\$ A second is not Instantaneous. Actually, any period is just as good as any other for the purposes of dimensional, and it has nothing to do with averaging period. \$\endgroup\$ Feb 6, 2018 at 14:30
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\$\begingroup\$ Small correction: 1 W = 1 J/s, so 1 kW = 1000 W = 1000 J/s = 1 kJ/s = 1 J/ms, not 1 kW = 1 J/s. \$\endgroup\$ Dec 5, 2021 at 20:31
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\$\begingroup\$ @carloc I agree, but that’s even used for instantaneous power, active power, apparent power and reactive power, where we use different “units” (W, W, VA, VAr). \$\endgroup\$ Dec 5, 2021 at 20:33
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Most people are using this kind of unit all the time. How much electric energy does your house consume per year? Let’s say 20000 kWh. How would you write this in short hand? 20000 kWh/yr.
Makes sense, doesn’t it? Would it be as clear if I wrote: 20000/(355*24) = 2.28 kW? How much electric energy do you pay for with a larger house, 3.1 kW? But energy is measured and priced in kWh, not in kW which is energy per second, i.e. power, not energy.
The unit kWh/h expresses that you use x kWh energy each hour while kW tells what you are using per second.
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3\$\begingroup\$ Welcome to EE.SE. You were doing so well until your last sentence of paragraph 2 and 3! "kW tells what you are using per second." No, kW tells you what you are using at this instant (or an average over some period of time). The amount of energy used in 1 s would be given in kWs (or kWh/3600). Remember, power is instantaneous and energy = power x time. Fix that and you'll get an upvote from me. \$\endgroup\$ Dec 5, 2021 at 20:22