BACK PAGE BASICS: Comparing Ratings of Energy Sources

One hundred feet of head and 120 gpm of flow could produce 1 kW continuously with the right hydro turbine. With only 10 feet of head, it could take 1,200 gpm of flow (and a different turbine design) to generate the same 1 kW.

How much does a 1-kilowatt (kW) PV array produce compared to a 1 kW wind turbine? And how does that compare to a microhydro turbine? These are common questions that make poor assumptions about how electricity generating devices are rated. Let’s sort out the tech—and the misinformation.

Starting with nonrenewables, if you buy a fossil-fueled generator that is accurately rated at 1,000 watts, it will be capable of that level of production continuously—as long as you keep supplying it with fuel and until it breaks down. An honest rating for a device defines its continuous operating output. Over a 24-hour day, that fueled generator will produce 24 kilowatt-hours (kWh)—its actual power output times the number of hours of operation.

A 1 kW microhydro turbine is quite similar. If supplied with sufficient water power (head and flow), it can kick out 1 kW continuously, and therefore produce 24 kWh per day. A well-designed system will endure many years of continuous operation.

How about a 1 kW PV array? Unlike a fossil-fueled or microhydro generator, it does not receive a continuous supply of power (sunshine). It gets no sun at night, and optimum sun for only a small part of each sunny day. The solar resource in a particular area is measured in “peak sun-hours.” The San Juan Islands in Washington (where I live) average 4 sun-hours per day, or the equivalent of one-sixth of a day. If the rating was accurate and there were no other losses, a 1 kW PV array here should produce an average of 4 kWh per day. Not accounting for losses, a 6 kW PV array would be needed, then, to produce 24 kWh per day at my location. 

Wind-electric generator ratings are even less predictable and calculable. To get to kWh, we need to either make a prediction based on the turbine’s swept area and average wind speed (the wind equivalent of sun-hours), or use test or predicted numbers from real-world sites or manufacturers’ calculators. We cannot predict wind energy output in any simple and accurate way from a wind turbine’s peak rated output.

Several factors contribute to this problem. A 1 kW wind turbine refers to the turbine’s production at the peak wind speed. But wind turbines only witness that speed—and therefore rated production—a very small portion of the time. Manufacturers may also choose different wind speeds to rate their machines. Because wind is a cubic resource, a small drop in wind speed means a dramatic drop in output. So, in the wind world, “1 kW” is just a shorthand for a turbine’s general size—it’s not a number that’s used to predict energy (kWh) output.

In the end, since you want to generate and use energy from the generating source, getting hard production numbers for each technology is the goal. Comparing microhydro, PV, and wind by rated power (kW) is not sensible. It’s all about energy, so cut to the chase and get accurate energy numbers (kWh)—and then compare.

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