The difference between DC and AC watts (and PTC/STC)

Solar panel power output is rated as the number of watts of direct current (DC) power a solar panel can produce under full sun at 25 degrees celsius. These measurement parameters are also called “standard test conditions,” or STC for short.

But real-world operating conditions are not like the laboratory, so the panels rarely ever produce exactly that much power. A better measure of potential total output is PTC, or PVUSA test conditions. Tests for PTC are done under conditions much more like the real-world.

Furthermore, our homes and appliances use AC, not DC power, so the output of the solar panels must be converted to AC watts, and that conversion can cause some power loss. That’s why your 6-kW solar system will probably never produce 6-kW of AC power if you look in your solar monitoring app.

The key thing to know here is to make sure that you’re looking for the same power output numbers (DC vs AC, and STC vs PTC) when you’re comparing quotes for solar panels.

DC Watts

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 PVUSA 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 DC. PTC ratings take into account everything, including loss from wires, etc.

AC Watts

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.