When you receive a solar quote from an installer it will be broken down to a dollar per watt ($/watt) price.
The question is, is the quoted price in Direct Current (DC) or Alternating Current (AC) watts?
First of all let’s talk about the difference between the two. DC electricity flows in one direction and is the type of electricity generated by ordinary batteries and solar panels. AC power is electricity that constantly changes direction between positive and negative sides and is used in your home to power most appliances. As an aside, the inverter that is installed as part of a home solar power system transforms the DC current to an AC current so that it can be used in your house and sent to the grid. Now that you understand the fundamental difference we can talk about why it makes a difference when reviewing and comparing solar quotes.
What happens if my solar quote is in DC Watts?
When looking at a solar quote in DC Watts you could see a Stand Test Conditions (STC) rating or nameplate rating. This rating system makes it easy to understand the quote. If you are quoted a system that uses 10 SunPower SPR-315E-WHT-D panels (315-watt panels) you would have a 3,150 watt (3kW) DC STC sized system.
The other rating system that you might come across when you get a quote for solar is Performance Test Conditions (PTC). As the name suggests this rating is determined by operating the panels in outside test conditions and seeing how much they actually generate. In the field, a 315-watt STC rated panel may only produce 300-watts because under the PTC test there is line loss from the connecting wires.
So how do I compare DC to AC watts?
To calculate the AC wattage if you are given DC numbers simply multiply the PTC DC wattage by the inverter efficiency. The inverter efficiency should be around 94 to 95 percent.
Now that you know the difference between the rating systems it doesn’t really matter how your system is quoted although in California the standard is CEC (California Energy Commission) AC watts. However, if you are dealing with installers who are based outside of California you might find that the quote is is DC watts. Also note that is some locales like in Pasadena (which we just covered as part of our “Solar Cities” blog) the rebate you will receive is based on CEC AC watts and not the higher DC number.