Power vs. Energy

Solar Energy Meter

Power or Energy?

A previous poston derates resulted in a few follow up emails where these 2 terms—power and energy—were used interchangeably by the writers, which is a common occurrence despite significant differences in meaning. Understanding this terminology makes understanding various solar energy concepts easier to grasp, especially when talking about derates and how they are calculated.

Power and Energy are two distinctly different but interrelated electrical principles:

  • ENERGY is the AMOUNT of power produced or used and is denoted in Watt-hours (Wh) or Kilowatt-hours (kWh)
  • POWER is the RATE that energy is produced or used and is denoted in Watts (W) or Kilowatts (kW)
First Solar Module Label Detail

A 60W Power Rated PV Module

For example,  solar energy module output is denoted in Watts – the rate of POWER they will produce under Standard Test Conditions (i.e. a 220W rated module). Installed PV systems have a POWER production output rating in Watts, but they are also typically discussed in kWh’s – the amount of ENERGY the system will produce over a period of time.  Here is an example from a SunEdison media article describing the completion of a 2.2MW system at the University of Maryland: “. . . . the 2.2MW (MW = Megawatt) rated farm will generate more than 3.3-million kWh of energy in the first year and over 61-million kWh over the next two decades.”

Rather than energy production, a simpler way to look at this terminology is from an energy use standpoint.  Utilities and their customers are all looking for ways to reduce utility bills. Emphasis is put on lower POWER appliances and the amount of time we use them.  ENERGY (the kWh charge on your bill) is calculated as follows:

Energy  =  Time X Power

An uncomplicated example is a 100 watt light bulb. One hundred watts is the POWER (rate) the bulb uses. If you leave that bulb on for 24 hours it consumes 2,400 watt-hours of ENERGY or 2.4 kilowatts.

Lowering of either or both the POWER and time, will lead to reductions in ENERGY costs. The converse is true of solar systems – increase the POWER rating of the system and multiply by a given time frame and the amount of ENERGY output will increase.

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