These are terms that fox most non-electrical people when it comes to understanding solar power and a good way to tell that someone really doesn’t understand what they’re talking about is if they use the term “kilowatt/hour”.
All will be revealed, however…
Let’s start at the beginning, with the difference between a kW and a kWh
The k stands for kilo, or one thousand; W stands for Watt, a measure of power and h stands for hour.
So, kW means kilowatt, or 1,000 Watts. When you’re talking about a solar power system, you talk about its peak, optimal, best-ever performance, so a 5kW system can knock out 5kW on very sunny days.
Then, kWh stands for kilowatt-hour, which is a measure of energy (rather than power). If your panels kick out 1kW of power for an entire hour, you’ve produce 1kWh of energy. You define the amount of electricity you generate in terms of kWhs.
The difference is, then, that kW measures power and kWh measures energy.
It’s important to differentiate between power and energy
Most people use the words power and energy interchangeably because they don’t realise that there’s actually a difference. If you want to drive an electrical engineer to distraction, carry on doing this, if not, listen up!
If you’re telling your sparky friend about your electricity usage, then you should say that you used 7kWh the previous day, not 7kW. It’s a good habit to get into because when you come to buy your solar array and you say you want a 7kW system you could end up buying a 7kW set-up, which will cost over $15,000 and produce 28kWh a day.
What you meant to buy, of course, was a system to cover your usual usage of 7kWh each day, which will be a 2kW system. Which won’t cost anywhere near $15,000.
How do kW and kWh work when it comes to batteries?
When you’re looking at batteries, you need to look at both the power and the energy storage capacity they have. The storage capacity is expressed and measured in kWh. Tesla’s Powerwall 2 can store 13.5kWh of energy and it has 5kW of power, so if it’s being run at full speed (as it were…) it can discharge fully in around two hours and 40 minutes.
The more kWh a battery has, the more energy it can store and the longer it’ll last. The more power the battery has, the faster you can put energy in and get it back out.
In the real world, this means that more powerful batteries can run more appliances at once and the more energy it has, the longer it can run them for.