My wife will tell you that I was screaming at the TV last night while watching an episode of CSI. I’m one of those guys that points out every little factual mistake in TV dramas. The crack team of CSI agents were trying to determine which home in a neighborhood was using inordinate amounts of energy to identify a drug grow house. They had obtained the “energy” usage of a neighborhood of homes, and identified a home that consumed 5,300 “kilowatts” in one month – a sizable amount of energy supposedly. The problem is, a kilowatt is a measure of power, not energy.
It’s been a couple of years since I discussed the difference and relationship between power and energy, two words that many people use interchangeably, including CSI script writers apparently. Solar power and solar energy are often misused phrases as well, and it’s important to understand the two concepts to evaluate solar energy system performance (note that I called it a solar energy system).
To recap my previous post, energy is power over time. Power measures the instantaneous output (or input). Power is the ability to do something (to do work in terms of Physics). Energy is the amount of power consumed, or work performed, over time. We “consume” or “produce” energy in our homes and businesses, not power.1
In theory, the CSI team could identify a potential grow house if they were able to see how much power was being consumed. In fact, it might even be easier to use power as an identifier, but they were clearly using energy figures to make their analysis.
How does this relate to solar energy?
What can be confusing is that solar energy systems are often (usually) rated in terms of watts or kilowatts, which are measures of power. When you go to buy a solar electric system, you may be quoted a “5kW System.” That system rating is a measure of power – the ability to create energy. However, what you really care about is how it will affect your electric bill, or how much usable energy the system will supply to offset your utility bill. So why do solar energy contractors sell solar energy systems in terms of power, not energy?
The first answer is that it provides an apples-to-apples comparison point for you to make a purchasing decision. While all solar equipment is not created equally, at least there are standard test conditions on which we can compare component and system ratings. Using energy production as a comparison point can be dangerous because assumptions made by manufacturers and contractors may be biased or have different degrees of conservatism applied. Since there are various models and sources available for energy production data, identical systems could be quoted as producing dramatically different energy values.
The second answer is that solar energy is variable. We have good models that estimate solar energy production, but we can’t predict the weather precisely. We can’t tell you exactly how much energy your system will produce in a day, week, year, or even a lifetime, but we can come close. Predicting precise solar power at any given instant is even more impractical. Nonetheless, we have standardized ways of determining maximum power output that can be duplicated and tested, but we have no definitive, accepted, or regulated way by which we can provide an energy rating. Furthermore, we would have to wait 25+ years to prove out energy production on a given system.
The final answer is that it’s far too dangerous to “promise” an energy production figure unless the promise is grossly underestimated, which puts dealers at a competitive disadvantage. In some states, power production agreements (PPAs) are popular because the system installer is paid for energy produced rather than charging you up front based on a power rating. PPAs typically skew benefits to the seller, but do provide a way for people to get into solar with low or no up front cost. Note: PPAs are not legally available in Florida due to arcane and outdated laws protected by well funded utility lobbyists. Nonetheless, PPAs often provide the power ratings of systems for comparison purposes.
Would you buy one car over another just because it’s the same price, but has double the horsepower? Of course not – fuel efficiency, size, and many other factors come into play. more importantly, how often do you utilize “peak horsepower” anyway? That brings me to my next point…
The rated power of a solar energy system will almost never be seen. Since the power rating of the system is based on standard test conditions of the solar modules, a lot of things have to come together for peak power to be observed. The sun’s angle, irradiance, temperature, haze, cloud cover, time of year, time of day, module soiling, age degradation, shading and other factors all impact solar module performance. In addition, solar modules are often, if not usually, rated well above the inverter output rating. For example, it’s common practice to pair a 250W solar module with a 215W microinverter.2 As a result, the theoretical power limit of the system can by substantially lower than the rated power that is given for comparison purposes when proposed to a potential buyer. That’s okay – remember that we are ultimately concerned with energy production, not power output. “Oversizing” the solar modules relative to the inverter typically results in overall greater energy production and cost effectiveness.
Misunderstanding about how power and energy differ is prevalent, and it’s frustrating to solar energy professionals, as proven by my recent outburst at an inanimate object. In many countries you can prepay for energy in “units,” and I have found that people there are much more cognizant of energy use. Knowing how much power your devices and appliances require is only half the battle. Understanding energy, power over time, is critical. The same applies to solar energy systems – the focus should be on energy production, not power output alone. Nonetheless, we are stuck with power ratings to provide meaningful comparisons.
1 Energy cannot really be produced or consumed (created or destroyed) – it’s just transformed – see the Physics Law of Conservation of Energy.
2 For a discussion of why, see Enphase’s “module rightsizing” paper here.