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With us having to research so much information about solar and other renewable energy we came across an enormous number of articles in which no respect is given to the units of measure in which electric power and energy are being measured. In actual fact when the correct units of measure are not used the statements do not make any sense.

What do we mean? Much too often watt hour (Wh) and the derivatives kilowatts hour (kWh) and megawatts hour (MWh) – as a measure of produced energy – are used wrongly instead of watt (W) or it’s derivatives kilowatt (kW) and megawatt (MW) and the other way around.

Let’s deal first with the well known kilo- and mega- prefixes. These are used to make a 1,000 and 1,000,000 of any unit. So 1kW equals 1,000W and 1,000,000W equals 1,000kW which also equals 1MW. Same goes for Wh – 1MWh = 1,000 kWh = 1,000,000 Wh. This was the easy part – kilo = thousands, mega = millions.

Now let’s look at the difference between W (watt) and Wh (watt hour). This is what confuses people more.

The watt (W) is a unit of measure for the power output a machine can produce. Strictly speaking from science point of view this is the amount of work (as in the scientific term) a machine can do (the amount of energy it can output) in a unit of time. Since the more standard scientific units for work (energy) and time are joule (J) and second (s) the standard unit of measure for power is actually a joule per second  (J/s). That is what is used more often in reference to mechanical devices and to chemical phenomena. In electricity the equivalent  is Watt (W) and mathematically 1W is exactly equal 1J/s. When a machine outputs 1 joule of energy every second it is said that the machine is working at 1W power.

So, a watt (W) is a measure of power – the ability for a machine to do work or produce energy in an amount of time. Another way of looking at it is as the speed at which a machine can output energy. If it can output energy at a rate of 1 joule per second that is power of 1 watt.

This is why power and watts (W), correspondingly kilowatts (kW) and megawatts (MW) should be seen as the potential of a machine to output certain amount of energy per unit of time. In energy production industries, like the solar energy industry, machines/generators (solar power stations, any other solar system) are given a number called watts peak (Wp) which is the nominal power they can output. So if a system is rated at 500 Wp that means the system can produce 500 joules of energy per second. If it is easier for you to think of energy produced per hour you have to multiply by the number of seconds in an hour – 36,600.

For those who are good at volts (V) and amperes (A) – if a generators outputs 1A current at 1V voltage the machine is working at 1W. Yes, that simple – power in watts is equal voltage in volts by current in amps.

The other unit – watt hour (Wh) is the unit of measure for amount of energy. If a machine with a power of 1W works for an hour it will produce 1Wh of energy. Now, wait a second, why did we suddenly switch from seconds to hours. Well if in the previous statement we replaced the word hour with second everything would have been correct but the unit of measure for amount of energy would also have to be watt second Wc – this is not used practically. The unit in use in the energy industry and electricity is watt hour (Wh). This is also how usually electricity is sold and priced. If you look at your utility bill you will see you are paying certain cents per Wh, well actually to confuse you now more you are paying cents per kWh but you already know why the k- is in front – the price is for 1,000 watt hours not for one small watt hour.

If we have to look again at the example with volts and amperes – if an electrical system is outputting 1A of electric current at 1V of voltage it outputs 1W of power. If that same system  works for 1 hour it would produce 1Wh energy, for 1,000 hours – 1kWh. At the regular household voltage of 110V a light bulb that uses 0.5A current consumes 55W (110V x 0.5A), in a thousand hours it will burn 55kWh and if you are paying $0.09 for kWh that will cost you $4.95.

Also in respect of generating electricity by solar panels, module – if a solar panel’s rating is at 100W and 25V that means at peak conditions the panel will be outputting 4A current (25Vx4A=100W) and every hour of peak performance will be producing 100W x 1h = 100Wh energy.

OK,  so let’s summarize and see how we should correctly use the units of measure.

Power is measured in watts (W) and systems usually have a rated peak power in watts (W). This of course could be in kilowatts (kW) and megawatts (MW) if it is a large number. Power should be perceived as the rate at which it produces energy (joules per second).

So correct expressions with it will be: “… the station has peak power of 300kW … ” or “.. the solar park’s capacity is 2MW … ” etc.

Energy, or amount of energy, is measured in watt hours (Wh) or the corresponding kilowatt hour (kWh) and megawatt hour (MWh) and it is the useful amount of work output by the system. This is why consumers or the grid pays in cents per watt hour (Wh). If a system works at 100 kW power for an hour it will produce 100 kWh, if this is paid at $0.09 per kWh that would earn $9.00.

Correct expressions using energy and Wh will be: “… the system will produce 500 MWh per year …” or “… the system will generate 700kWh per day …”

What are examples of wrongly put statements. “The power station will produce 15MW per year”. MW is power it is the rate at which energy is produced so it can not be per year, or per day, or per hour. If we know it will work at a capacity of 15MW we should simply say “… will produce 15MW …” may be add “…of electric power…”.

If we want to say “per year” (or per other period) we should calculate what the expected nominal insulation is (in the case of solar power stations and working, up-hours hours for other systems) for that period (that will usually be hours for that period, like in 1,400 hours insolation per year), multiply by the nominal power in watts and we will get watt hours. So then we can say “… the system will generate 700 MWh of electric energy per year…”.

Also often in news reports journalists will write that certain system will be able to power that many households. And this is the correct expression – “able to power (or feed with electricity) certain number of households”. A household needs certain power in kilowatts (kW) let’s say 4kW. So a 400kW station will be bale to feed 100 households. The incorrect statement is “… the system will power 100 households per year …”. We do not need “per year”. It will power the 100 house holds as long as it is operational – 10 years, 20 years – whatever the plan is.

As with all articles on SolarByTheWatt.com, please, do comment on this one, especially if you have recommendation of what to improve to make the content more useful and readable for you.

Related resources:

Wikipedia – Watt

Wikipedia – Kilowatt Peak

Wikiepedia – Kilowatt Hour

How Stuff Works – Amps, Watts and Volts

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