Power & energy
A pet peeve of mine is the number of people in the solar industry who don’t know the difference between these two (even some PV accredited designer/installers). It’s not that difficult and it makes you look a bit silly (or unprofessional) if you get it wrong in front of somebody who knows.
Power is measured in watts (W) and is instantaneous i.e. for a typical PV system it’s constantly changing. It will start at zero in the morning before the inverter switches on, then on a clear day it will approximately follow a curve which peaks around midday and starts to tail off into the late afternoon.
A lot of inverters will show this curve on an LCD screen and also show
the power production at that particular time as a wattage reading.
Energy is measured in kWh (for electrical applications – also measured in joules J for thermal systems). Energy is measured over time and is equal to power x time. This you can see from the units: kW x h.
1 kWh is the amount of energy a 100 W bulb uses if it’s on for ten hours, or a 1000 W toaster used for 1 hour. It’s what your electricity bills are measured in and how we pay for the power we use.
So a PV system outputs power which goes up and down during the day depending on many factors – the season, the time of day, the weather, shading on panels etc. What you should be interested in is the amount of energy a system produces over a day (kWh/day) or a month or an average year as this is the figure you use to determine how much your electricity bill will be reduced by or how much excess you’ll have to feed into the grid.
The kWh figure is also used when you compare systems. For example if you’re talking with a neighbour who also has a PV system (not necessarily the same size) it’s easy to compare outputs if you divide the kWh you generate over a day and divide it by your system size. If both systems are working properly (and similar orientation & pitch) then the figures should generally be similar.
So no more talk of kW per hour or day – this just doesn’t make sense. If it’s instantaneous use W or kW, if it’s over time use kWh and the solar gods will be appeased.
