Editing Kilowatt-hour (section)

==Distinction between kWh (energy) and kW (power)==
A kilowatt is a unit of power (rate of flow of energy per unit of time).  A kilowatt-hour is a unit of energy. Kilowatt per hour would be a rate of change of power flow with time.

[[work (physics)|Work]] is the amount of energy transferred to a system; [[Power (physics)|power]] is the ''rate of delivery'' of energy.
Energy is measured in ''joules'', or ''watt-seconds''. Power is measured in ''watts'', or ''joules per second''.

For example, a battery stores energy. When the battery delivers its energy, it does so at a certain power, that is, the rate of delivery of the energy. The higher the power, the quicker the battery's stored energy is delivered. A higher power output will cause the battery's stored energy to be depleted in a shorter time period.

===Annualized power===
Electric energy production and consumption are sometimes reported on a yearly basis, in units such as megawatt-hours per year (MWh/yr) gigawatt-hours/year (GWh/yr) or terawatt-hours per year (TWh/yr). These units have dimensions of energy divided by time and thus are units of power. They can be converted to SI power units by dividing by the number of hours in a year, about {{val|8760|u=h|up=yr}}.

Thus, {{nowrap|1 GWh/yr}} = {{nowrap|1 GWh/8760 h}} ≈ {{val|114.12|u=kW}}.

===Misuse of watts per hour{{anchor|Watt per hour}}===
Many compound units for various kinds of rates explicitly mention units of time to indicate a change over time. For example: miles per hour, kilometres per hour, dollars per hour. Power units, such as kW, already measure the rate of energy per unit time (kW=[[kilojoule|kJ]]/[[second|s]]). Kilowatt-hours are a product of power and time, not a rate of change of power with time.

Watts per hour (W/h) is a unit of a ''change'' of power per hour, i.e. an acceleration in the delivery of energy. It is used to measure the daily variation of demand (e.g. the slope of the [[duck curve]]), or ramp-up behavior of [[power plant]]s. For example, a power plant that reaches a power output of {{val|1|u=MW}} from {{val|0|u=MW}} in 15 minutes has a ramp-up rate of {{val|4|u=MW|up=h}}.

Other uses of terms such as ''watts per hour'' are likely to be errors.<ref>{{Cite book |last=Woofenden |first=Ian |url=https://books.google.com/books?id=iYktDwAAQBAJ&dq=%22watts+per+hour%22+%22nonsensical+phrase%22&pg=PA48 |title=Wind Power For Dummies |date=5 October 2009 |publisher=John Wiley & Sons |isbn=978-0-470-49637-4 |pages=48 |language=en |quote=Much too often, I hear people say something like ... 'watts per hour'...}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=NvRIAQAAIAAJ&q=%22watts+per+hour%22+(mistake+OR+error+OR+blunder) |title=Home Power |date=2007 |publisher=Electron Connection |language=en |quote="Watts per hour. This nonsensical phrase tops my electrical terminology pet peeve list."}}</ref>