What’s with those watts?

When you talk about renewable energy (or any kind of energy, really) you end up talking about a lot of watts. But what do all those watts mean (and how can you sound like you’re in the know)?

Watts represent a rate (sort of like miles per hour, or cubic feet per second — but in this case, it’s joules per second), so you can’t ever say that a solar system, over a year, produced, say, 5 kilowatts. That’s like saying that a reservoir can hold 10 gallons per second. It makes no sense.

So, when you’re talking quantity (how much electricity did your solar panels produce last month? For example, or how much electricity did you use last month?) you’re talking about watt-hours, which turns the number into a quantity instead of a rate.

Normally, when it comes to home electricity bills or residential-sized solar arrays, you see this written in kilowatt-hours. 1,000 watts (or watt-hours) = 1 kilowatt (or 1 kilowatt-hour) and 1,000 kilowatts = 1 megawatt. You may also see kilowatt-hour or megawatt hour written as kWh.

A measure of capacity:
Watts (and kilowatts and  megawatts) are properly used (without “hour” tacked on the end) when talking about capacity. So, for example, it’s correct to say that a homeowner is installing a 4-kilowatt solar array on his house.

What that means is that under perfect conditions (cloudless skies with the sun exactly overhead and probably on the equator) that solar array can produce electricity at a rate of 4 kilowatts. So, for example, if those perfect conditions existed for an hour, the solar array would produce 4 kilowatt-hours during that time. For solar panels and for wind farms, you can express their size by kilowatts or megawatts.

For example, you could say, “The utility company is building a 40-megawatt wind farm.” But the same holds true for wind-farm capacities — that means 40 megawatts under perfect conditions, which never really exist.

The differences in solar:
Not all solar systems are the same. A residential “solar system” could mean both that the system is photovoltaic (meaning that it produces electricity) or solar thermal (meaning that the sun heats the house’s water. This is usually just a pre-heating, however. Most of these places will have an on-demand hot water heater that finishes the job.)

On a commercial scale, you could have a photovoltaic system like the array set up at the Denver airport, but more often, large solar projects being discussed by utilities are concentrated solar power. This means that mirrors reflect he sun’s rays onto some kind of oil (or oil-like substance) in a tube, heating it. The oil then heats water to create steam. The steam turns a regular turbine, just like the one found in a standard coal plant. This is allows utility companies to store the heat, something that can’t be done with photovoltaic system. (A concentrated solar system will store the heated water in big thermos, essentially.)

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