What is the Difference Between Kilowatts DC (kWDC) and Kilowatts AC (kWAC) for Solar Electricity?

Solar Electricity:  Kilowatts DC (kWDC) and Kilowatts AC (kWAC)
If you have been investigating solar you have probably already heard the terms kWDC and kWAC.  But the differences between these two are often explained very poorly or not at all.  

kWDC or Kilowatts DC are the direct current (DC) power produced by the solar electric panels prior to any conversion to household electricity (see also inverters). 

KWAC or Kilowatts AC is the alternating current (AC) power that is used by your home.  This is electricity that has been converted ("inverted) from the DC produced by the panels to the AC used by your home and provided by the electric company (see also Solar Electricity Basics). 


What are Watts and Kilowatts? 
Watts and Kilowatts are a measurement of instantaneous power (1 kilowatt = 1,000 Watts). To help put this in perspective:  You are probably familiar with a 60Watt light bulb.  A 60 Watt light bulb consumes 60 Watts of power when it is on.  Also "kilowatt" or 1,000 Watts is often abbreviated as"kW". 

Electrical Energy Losses
Between the DC from the panels and the AC used in the home there are losses caused by multiple factors.  These losses come from things like the conversion to AC (inverter), wire losses, dirty panels, shading, and other imperfections in the system.  It is not possible to get this to 0% loss.  There will always be some loss going from DC Solar Panels to AC Electricity Used.  So there is a conversion factor that says some percentage of the power from the panels will be available to your home (or electrical grid - see also Solar Electricity Basics).  Often this conversion factor comes out to be between 70% to 85%. 

How Losses Affect Your Electrical Generation
What this means to you is that if your solar panels produce 1kW of power then you will likely get 0.70 to 0.85kW of power to use in your home.  If someone says they can get you a 95% conversion rate, then they are not including everything, lying, or just don't understand how to properly calculate the conversion losses.  I would avoid doing business with this person.  Anything over an 85% conversion is pretty unlikely. 

kWDC vs kWAC Explained
Now kWDC is the power produced by the solar electric panels; and kWAC is the usable power to your home after the losses described above.  

Using the same example as above:  What this means is if your solar panels produce 1kW of power this is 1 "kWDC".  And since your conversion factor for losses are likely to be in the range of 70% to 85% then you will get 0.70 to 0.85 "kWAC" of power to use in your home. 

The actual conversion factor will be determined by many design details like the specifications of the inverter, wire diameter and length, operating temperatures, and many other details.  This is where choosing a good design and installer is crucial. 



Comparing Competing Bids for Solar Electric Systems - kWDC vs kWAC
You should be careful when comparing kW numbers.  First be sure to compare kWDC to kWDC.  And kWAC to kWAC.  Never compare kWDC to kWAC numbers because that's like comparing "apples to oranges".  Also, it's probably best to look at both the DC and AC numbers when making comparisons between competing systems or bids. 

kWDC Comparisons
The kWDC is a rating on the solar panels based on standardized test conditions.  This is a pretty good comparison to make between different systems.  Especially, when comparing prices on competing bids.  And when you consider that the solar panels are typically the most expensive part of the entire system. 

kWAC Comparisons
The kWAC is what you will be actually using or deducting off your electric bill (see Solar Electricity Basics) so this is what you are probably most interested in. 

HOWEVER, remember that the conversion factor for losses is an ESTIMATE only.  The only way to know what this will actually be is to install a system and take measurements over time. 

Conclusions
I would recommend looking at both the kWDC and kWAC when comparing competing systems.  Make sure the conversion factor looks reasonable (probably in the 70% to 85% range).  There are some unscrupulous individuals that will try to mislead you on comparing kWDC and kWAC or unreasonably inflate the conversion factor.  Be informed, keep reading, and compare both the kWDC and kWAC numbers. 




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Robert Morse
BSEE, MSEE
SunCal Solar, Incorporated
www.SunCalSolar.com


 

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