The Difference Between DC and AC Watts (and PTC/STC)
As you may know, solar is bid for in dollars per watts ($/W). But are those DC watts (direct current, like in a battery) or AC watts (alternating current, like in a light socket) watts? Hopefully this post will get you informed enough to be sure you’re comparing apples to apples when reviewing multiple solar bids.
The size of your solar system will be referred to in watts. For example, a 3000 watt (W) system will produce 3000 watts (or 3 kilowatts) during optimal conditions. We’re talking about watts, here, not watt-hours, which are different. When getting a system quote, whether they are referring to AC or DC watts matters, because those numbers are different sizes.
The idea is, if you got two bids you’re looking at, both of which are $30,000, and one is in AC watts and one is in DC watts, you have an easy decision (assuming both installers are well-vetted, quality installers).
There are two ways to quote DC watts. One is called STC, or Standard Test Conditions, also known as “nameplate rating.” This is the most simple and easy to grasp way to quote, because you just take the wattage of the panel and multiply it times the number of panels. For example, if you had 10 SPR230 (Sunpower 230-watt panels) panels, you would have a 2300 Watt DC STC sized system.
The other way is PTC, or Performance Test Conditions. This number will be slightly less than STC. What PTC means is they put the panels under outside test conditions and see what they actually pump out. A 200-watt Panel may actually produce only 180 Watts. PTC ratings take into account everything, including loss from wires, etc.
You get to AC watts by multiplying the PTC DC wattage by the inverter efficiency. Many inverter efficiencies can run around 95%… so just take the DC rating an multiply by .95. This will be the lowest number of the three.
So what should I be quoted in?
The answer is that it doesn’t really matter, as long as when comparing quotes, you’re looking at the same number. In Northern California, the standard is to quote in CEC (California Energy Commission) AC wattage… but in many other places in the nation, the standard is to quote in DC Watts. Naturally, if you are comparing a quote from a multi-state installer and say, one that does most of their business in northern California, you will be looking at different numbers.
Additionally, some disengenious installers may try to pass off DC wattage as AC because it makes the number bigger, and makes $/W look cheaper. This is rarely the case, but it’s what this post if for.