What is the difference between a kWh and a kW?

We recently published a post about the average electricity requirements of a typical home.  Thank you to everyone who emailed us, we had some great feedback and some suggestions for new topics. One of the most popular requests was for some information on what a kWh is and how it differs from a kW. So, here goes:

A kWh is a measure of energy, a kW is a measure of power. But what are energy and power?

Physics textbooks will explain that Energy is the “capacity to do work”, but in the case of residential electricity it might be easier to think of Energy as the total of what we need to buy to run our households.

Power is the rate at which energy is generated or used.  For our purposes it might be easier to think of boiling a kettle. It takes a certain amount of energy to boil water (this is actually the definition of a calorie, but we’ll leave that for another day).  If we want to boil the water faster then we need a kettle with more power.

In equation terms, we can express the relationship between Energy, Power and Time as follows:

Energy = Power x Time.   (or kWh = kW x h)

Another example might help with the explanation.  The average solar panel installation in America has a power rating of 6kW.  We know from our previous blogs that the average home has a typical energy consumption of 30kWh a day, so we can work out theoretically how many hours of sunshine we need to provide all the energy the home needs as follows:

Time = Energy / Power

Time = 30kWh / 6kW

Time = 5hrs

This calculation tells us that the average American home equipped with solar panels should have the ability to produce all of the required energy from the sun, provided the sun shines for at least 5 hours a day.

In actual fact, the relationship is more complicated than this.  The power output of solar panels is sensitive to the angle of the sun’s rays and accordingly don’t produce the maximum power output throughout the day even in consistent sunshine.  The power output of solar panels is also sensitive to any barriers between the panel and the sun, for example, cloud, snow etc.  Finally the energy requirement of the home does not typically coincide with the peak power output of the solar panels.  This topic will covered in our next blog.