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The Difference Between DC and AC Watts (and PTC/STC)

Avatar for Dave Llorens
Published on 11/19/2008 in
Updated 07/24/2018

plasma ball 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).

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 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.

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.

Check out here if you want to get some solar quotes for your home or business.

Last modified: July 24, 2018

17 thoughts on “The Difference Between DC and AC Watts (and PTC/STC)

  1. Avatar for mus mus says:

    Thank you for clarifying the difference. One side issue, if i have a 1 MW power plant does this mean it has an out put of 3600,000 kWh (1000kW x 3600). Apologise for asking since I am not an electrical engineer

  2. Avatar for Molly McBeath Molly McBeath says:

    Thanks for this article. You’ve cleared up my confusion on DC vs AC watts.

  3. Avatar for Richard Richard says:

    Actually the AC/DC/STC/PTC/CEC/…whatever rating is not really anything that I care about. I care about how many kWh the system is going to generate in a year. If you have a reputable dealer, they will state that amount and back it up with a written guarantee. If not, be very cautious.

    1. Avatar for Ben Zientara Ben Zientara says:

      Hi, Richard-

      The amount of kWh the system is going to generate in a year is variable, based on how much sun the location gets. The same panels will generate a different amount of kWh on the roof of a Seattle home than they would on the roof of a San Diego home. That’s why PTC/STC are valuable measures of performance.

      Oftentimes, installers who offer PPAs will guarantee a minimum number of kWh produced by a system in a year, and then likely build the system to exceed that number.

  4. Avatar for Anonymous Anonymous says:

    dc or ac watts same or not

  5. Avatar for Bill Bill says:

    I am trying to find out the efficiency of a home made solar panel I made.When I put it in direct sunlight, it reads 10 ac volts or about 70 dc volts on a volt meter. How many watts does this convert to? What size inverter is best to use?

  6. Avatar for MySchizoBuddy MySchizoBuddy says:

    What about power factor?

  7. Avatar for charlieb000 charlieb000 says:

    isnt AC Watts in RMS (root mean square)? its totally different from your converter’s inefficiencies.

  8. Avatar for Dane Ericksen Dane Ericksen says:

    In the second paragraph under “DC Watts,” “on” should be “only.”

    1. Avatar for Tor a.k.a. "Solar Fred" Tor a.k.a. "Solar Fred" says:

      Thanks! Fixed.

  9. Avatar for cmk cmk says:

    i know how many watts ac i need, how do i find out how may dc watts i need?

  10. Avatar for shaheer shaheer says:

    how can i convert dc watts to ac watts??

  11. Avatar for tahir iqbal tahir iqbal says:

    By the way can I use these transformers with dimmer control unit since LED strips are dimmable… Thanks

  12. Avatar for tahir iqbal tahir iqbal says:

    Hi there,
    I really could do with some help here. I have 8 LED strips lights which output 72 watts per strip. I would like to link all 8 strips to one transformer. Since strips are rated at 12Volt, I need to know which transformer do I need to make this work? Please help me as I am not very good at all with AC and DC conversion. Would you recommend any product and what type transformer do I need i.e AC to DC or DC to DC or something else. I been advised to get 12 Volt 600 watts transformer but dont know if it is AC to DC or what?

    I would highly regard your kind help.

    Thanks

  13. Avatar for David Llorens David Llorens says:

    Fixed! Thanks mom.

  14. Avatar for CelticSolar CelticSolar says:

    This explains it.

    We have a 4.0kW (STC) system. The highest output we’ve seen is 3.5kW. The PTC and orientation likely account for not seeing a 4.0kW result.

  15. Avatar for Dave Dave's mom says:

    This post is very easy to read. Maybe one correction –
    In the 2nd paragraph under DC Watts, shouldn’t the 3rd sentence be “What PTC means ….”

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