kW and kWh Explained – hopefully.

What’s the difference between a Kw and a Kwh?

My solar sales rep is tempting me to buy a complete solar solution, and I’m trying to understand as much as possible before making the plunge. This question is one that puzzled me for a while, but now I get it! – Here’s the scoop:

First, ‘kilo-Watts’ are a power measurement, while ‘kilo-Watt-hours’ are an energy measurement. Huh? Well, ‘energy’ is referring to a measurement of how much fuel something contains or may use or generate over a period of time, just like when people on diets talk about calories (another measurement of energy). ‘Power’ measures the speed at which energy is used or made. I know this can be a bit confusing, but bear with me.

For the purpose of conversations relating to solar energy, that energy is typically measured in kWh (for kilo-Watt-hours) – sometimes mWh’s (for mega-Watt-hours in big installations).

And power is typically measured in kW.

A simple example will help to illustrate the difference: A solar panel may generate 250 Watts (or 0.25kW) of power when the sun shines on it. No matter how long the sun shines, 250 Watts will be the power output from the panel (more or less – this is a complicated area, but for now we’ll just say it’s 250). When the sun goes down, the panel stops functioning and 0 Watts are produced. However, if you want to know how much energy it will produce, we need to consider a time factor – how many kWh’s can the same panel deliver in, say, 1 month? To answer this, we need to know how many hours the panel is generating power for each day during the month. Let’s estimate 5 hours per day, for 30 days. This means the panel will deliver 250 x 5 x 30 = 37,500 Watt-hours in a month – we can divide this big number by 1,000 to convert to kilo-Watt-hours – the panel makes about 37.5 kWh’s of electrical energy per month. Simple, right?