Thousands of American homeowners decide to go solar every year. Residential and commercial solar systems are usually sized by customers, measured in watts or kilowatts (instead of number of panels).
A watt is a unit of power, measured by taking energy over time. When you receive your solar system quote, you are usually seeing all sizes in Direct Current (DC) watts. However when the contract is signed and the utility applications are being submitted, the same customer might see Alternating Current (AC) watts instead on the utility form. On the other hand solar inverters are sized in AC Watt terms. So needless to say, this can be very confusing!
The difference between DC and AC watts seems technical, but is important. DC watts and AC watts are different sizes, which can mean different sized solar systems. A rule of thumb — DC system size is related to the power rating of the solar panels, while AC system size is related to the power rating of the inverter. Different jurisdictions and utilities require standard measurements. This article is meant to help homeowners understand this important difference.
DC Watts
The terms AC and DC refer to the current flow of an electric charge. Direct current, or DC, is an electric charge that flows in one direction. Solar panels produce electricity in the form of direct current. The storage batteries that store unused solar energy also charge and discharge with direct current.
Products marketed as AC solar panels or AC batteries simply have inverters that convert to AC. Solar bids can be quoted in DC can take two forms — Standard Test Conditions (STC) and Performance Test Conditions (PTC) — though most quotes use STC system sizes in the US market.
Standard Test Conditions (STC)
Standard Test Conditions is the first method of quoting DC watts. STC is measured under laboratory conditions. Almost all quotes are given using STC watts because it’s the simplest to understand.
When solar professionals talk about solar panel sizes, it’s about the STC size. If your friend tells you they bought a 5.5 kilowatt system for their home, or your neighbor receives a quote for $3 per watt — it’s STC.
You can take the wattage of the panel and multiply it by the number of panels that will be installed. If you have ten 525 watt Jinko Solar Eagle G5 panels, you would multiply 525 watts by ten — which means you would be quoted for a 5250 watt system. It’s simple multiplication.
PV USA Test Conditions (PTC)
The second method of quoting DC watts is the PV USA Test Condition method. This method is typically used for engineering calculations and for utility companies. PTC is not often used These quotes will be lower than an STC quote.
PTC numbers are found by testing solar panels outside a laboratory and determining the actual output. This takes into account power loss due to wires or climate conditions. Some people find PTC to be more “practical” than STC. The PTC rating is often around 10-15% lower than the STC rating.
Some states and areas, like California, require rebates to be calculated in PTC.
AC Watts
Alternating current refers to an electric charge that flows in both directions. The power grid and virtually all of your appliances use AC. Your solar panels generate electricity in DC form, but inverters convert the electricity to AC form to power your home. This is why Solar inverters are always sized in AC terms. So a 7.4 kilowatt inverter is a 7.4 kilowatt AC size and not DC size.
To convert DC wattage to AC is by multiplying the PTC wattage by the inverter efficiency. Most inverters have an efficiency around 95% — which means you can multiply the PTC rating by 95% (0.95). The AC wattage is typically the lowest number of the three options when describing the system size. This lower number addresses power loss due to real world efficiency.
Which is right — DC or AC?
During the sales process, everything is quoted in DC Watts. A customer buys a 10 panel system, using say QCells 400W panels — as sold by the manufacturer, which is an STC rating — so it’s a 4 kilowatt system in DC rating. The difference between STC or PTC versus AC quotes doesn’t really matter so long as you’re comparing using the same method. Though this is almost always in STC (DC) watts or kilowatts.
However, the customer should know that while solar generates DC current our home appliances only consume electricity in AC current. The Solar inverter converts the DC to AC and a 4 kilowatt DC solar system might only generate 3.5 kilowatt AC Power on a given day. Also utility interconnection applications accept AC power ratings, often a source of confusion for a new solar buyer.
When looking to go solar, make sure you know which numbers you’re looking at so you can make an informed comparison. If two installers quote the same solar panels using two different methods, you might think one produces more power than the other.
Our mission is to empower clean energy consumers through transparent pricing and the ability to monitor your solar system’s performance.