Capacitor in grid-tie system?

benjam47 Registered Users Posts: 10

I recently received a quote for a grid-tie system using SunPower panels and 2 Sunny Boy inverters. The quote also includes a 1kw capacitor, and the explanation I was given is that it would reduce my energy usage by 5% due to the startup power used by my AC system and pool pump, and provide surge protection to the rest of my equipment.

Is this legitimate? Even if it works, I can't imagine that the few seconds of power used when my AC or other motors turn on is using that much power.

It kinda sounds like the "hard start kits" that AC contractors always try to get you to install on your compressor.

I have a separate power cleaner for all of my AV equipment, computers, etc, so I don't think the surge protection would benefit me either.




  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    Re: Capacitor in grid-tie system?

    Using a capacitor to correct your power factor can be a normal part of the tool kit used by utilities to "correct" for large inductive loads (such as motors) where the companies are charged for "poor power factor".

    But, since homes are not charged for power factor, but just the "real" energy used; the PF of your home is pretty much irrelevant for utility connected service.

    Many newer appliances (fridge/AC and such) can already have a "run capacitor" installed and have pretty good PF (near 1.0 is good; near 0.6 is "bad"). And adding a capacitor can create the "mirror image" that they are "solving" (capacitors and inductors are 180 degrees out of phase with each other--sort of like mirror images of each other's functions).

    Installing a PF Capacitor can make for some interesting demonstrations about saving "current"--but the truth is with AC electricity, they save very little (on the order of a couple dollars per year--if done correctly) on your power bill.

    If these guys are charging you a few tens of dollars for the capacitor--then at least they are probably honest folks. If they are charging you a hundred dollars or more--then either they are not knowledgeable or they are near scam artists.

    Check out this tread (starting at post 13) and read about the "Power Saver 1200"...

    And don't feel to bad if you don't completely understand the theory behind the Power Saver and why it "works" but does not really. AC Voltage/Current source/load relationships are complex. You almost have to accept that the math behind the claims of how the Power Saver (motor run capacitor) does not work as advertised as truth verified by others.

    The above is, of course, my personal opinion... It is possible that the company has measured the power factor of your home and is proposing to install "switched Run Capacitors" connected to each of your major loads"--and if so--at least is done with a valid purpose in mind. However, the fundamental issue still remains--that the power company probably does not charge you (assuming this is residential and not industrial/commercial) for "poor factor" so even if they do 100% PF correction--will will have near zero effect on your electric bill.


    PS: These PF Correction Capacitors are virtually useless at surge suppression too.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • benjam47
    benjam47 Registered Users Posts: 10
    Re: Capacitor in grid-tie system?

    Thanks for the detailed explanation Bill, you've confirmed my suspicion :)
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Capacitor in grid-tie system?

    There are also some potential damaging things that can happen from power factor correction on things like central air conditioning, refrig, and electric motors in general.

    Your air cond can have different PF depending on spot load on the system. How hot the outside temp, inside humidity and temperature, and finally variation in grid voltage. Getting too close to 100% PF correction with capacitors to cancel inductive loads can create resonance in the system resulting in damaging spike voltages to compressor motor.

    Also capacitors are placed on motor side of switch and this can put extra stress on relay contactor accelerating its failure rate.

    Like was said, you pay for true power, not appearent power, so there is little benifit to spending extra to correct large inductive loads.

    If you are running on TSW inverter on batteries there is some benefit to cutting down on inverter IR losses but don't push the correction further then about 90% PF or you run the risk of creating damaging resonances.