Definition #
As a start it would be wise to explain both Energy & Power such that it helps prevent confusion later on. The two terms are often mixed up and although most people will get the gist of what you mean it does help avoid confusion when speaking to your installer or when trying to decipher your quotes as to what your actually getting.
There are many scientific definitions but for clarity we are just going to talk about these terms in respect to what you will come across as part of your electricity bill or PV installation:
Power: often expressed in Watts or Kilowatts (kW)
This is what you will most likely know from the ratings on your appliances etc… It is the instantaneous power consumed by a device when in use at a snapshot in time.
Example: if you were to measure (say on your in home display) a 3kW kettle, when switched on it would consume 3kW of power at any moment you looked at the display while it was boiling.
This is instantaneous power – the power consumed at a particular point in time.
Energy: expressed as kilowatt-hour (kWh).
In the context here, this is basically referring to Power again, but measured over time. You may recognise this term from your Energy Bill / Tariff / Meter as to what you pay each month or sometimes on appliance efficiency labels.
Example: lets use the same kettle from the above example for clarity.
Say we have a very very big 3kW kettle. From the above we know that if we switch it on it will draw 3kW. But this time we account for how long it’s consuming that 3kW for (just like your Energy Supplier does!)
Lets go big and say it takes an entire hour from start to finish before it turns off. We multiply the Power by the timespan (3kW x 1 Hour) which gives you 3kWh.
Your energy company would then bill you for 3kWh @ £0.40 so £1.20
Obviously this is the most basic of examples but you get the idea. You can have all sorts of devices which create a power demand switching in & out throughout the day. These all add up over the 24hr period to form part of your total house energy consumption.
Confusion #
Now it’s easy to get the two terms muddled up, so in the context of PV systems & batteries it works as follows:
PV Panels: #
These convert the suns energy into electricity (DC) and are measured as power (Watts or kW.)
There are 2 different power expressions that can be used:
Peak Power (kWp) – such as 5kWp. It is the maximum instantaneous power that the panels could provide the house (if the inverter is big enough) in perfect conditions to run appliances.
Live Power (kW) As PV panels can generate anywhere from 0 to 100% of their peak rating (depending on how sunny it is) this is the power output of the panels “now”.
Side note: Systems are made up of multiple panels therefore you may get quoted the individual panel ratings, so you will need to multiply the panel rating by how many you have to get your total peak power)
Inverters: #
These convert the DC electricity from the PV panels into an AC waveform & are too measured as Power (Watts or kW). In the case of a Hybrid Inverter, these can also send some DC to the battery too. See Hybrid Inverter wiki page.
The Power rating on the inverter is a maximum output figure it can provide to the house (obviously factors like the amount of sunlight or battery power available needs to be taken into account. See the Inverter wiki page)
For Givenergy this rating is normally 3kW, 3.6kW or 5kW total power output to the house as AC.
Batteries: #
These store energy and as such are expressed in their capacity in kWh. They are attached to an inverter.
An example is a 6kWh battery charged to 100% contains enough energy to run a 6kW appliance for an entire hour. (From our examples this would be the equivalent of 2x 3kW kettles boiling for an entire hour)
BUT
This is where the confusion normally occurs as people often get Power (kW) & Energy storage capacity (kWh) mixed up.
It is very unlikely that a 6kWh hour battery could also deliver 6kW worth of Power to run appliances.
Therefore as well as capacity (kWh) they also have a Power Rating which is the maximum power output the battery can sustain until it is empty.
Example: a 100% charged 6kWh battery with a 3kW Maximum Power Rating would be able to provide 3kW of power to the home for a duration of 2 hours (6kWh / 3kW = 2).
Essentially this would provide enough power & energy to boil our (probably now exhausted) 3kW kettle for 2 entire hours.
Finally #
This is the very barebones examples of the differentiation between Power & Energy. It’s important to get these bits right to help explain things later on.
There are a lot more “if” & “buts” to consider (voltages, temperatures and so on…) which we’ll explain in more depth on other pages of the wiki for each part of the system.
Time to give the kettle a rest…
