energy usage on low supply voltage (low shedding)

mahendramahendra Solar Expert Posts: 177 ✭✭✭
hi there i need some clarification on an issue concerning consumption. i sometimes use grid power which is 120v nominal but we all know that we don not receive 120v ,in fact when using grid power i suffer from under voltages(brownouts/low shedding /low voltage) all the time( i receive 85-100 volts on average) and it seem to me, cant verify for sure, but i think i am using fewer kwh per day on solar(off grid) than when using grid power, so i am assuming that when experiencing under voltage conditions one can end up using more energy is my assumption correct based on the details given ?or is there something else at work?.thank you.


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: energy usage on low supply voltage (low shedding)

    off hand i can't think of any reason why you would be drawing more power while on the grid as opposed to solar. maybe a mental thing that you take advantage of the grid power while it's available or even differences in power meters if using different meters for each.
  • BB.BB. Super Moderators, Administrators Posts: 33,327 admin
    Re: energy usage on low supply voltage (low shedding)

    Many of the loads we put on AC power are "constant power" loads...

    For example, most electronic devices need a certain amount of Watts to run... P=V*I, if you have falling Voltage, then the device will draw more current to operate.

    Motors--A fan will use less power as the voltage falls... But a compressor on a refrigerator will use more current to keep the pump running.

    Also, power factor can change with voltage (and between Line power vs Inverter power). Your utility meter should measure kWatt*Hours... If you have a current meter to measure your energy usage, amps is not exactly equal to power usage:

    Power = Volts * Amps (simple equation)
    Power = Volts * Amps * Cosine (I vs V wave form) = V*I* Power Factor (ranges from 0.0 to 1.0, with 0.6 to 0.95 being typical values)

    So, unless you have a true kWH meter, you are usually measuring Volts and Amps with a standard meter of some sort--Which will overestimate the actual kWH used vs that measured by a utility (or other) energy meter.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mahendramahendra Solar Expert Posts: 177 ✭✭✭
    Re: energy usage on low supply voltage (low shedding)

    both were measured with a kwh meter but yes different meters,also its was on the refrigerator usage it was noted on.since that is on 24hrs
  • mahendramahendra Solar Expert Posts: 177 ✭✭✭
    Re: energy usage on low supply voltage (low shedding)

    also equipment runs smoother off grid since proper voltage is present,i dont know if the power factor has anything to do with it.thanks for the help though.just trying to understand what going on.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: energy usage on low supply voltage (low shedding)

    Lower Voltage to a motor will make it spin slower, draw more current, and be less efficient in the mechanical output it creates from the 'same' amount of electrical input. As such on lower Voltage the 'frige may have to run longer to achieve the same amount of cooling. There's your power difference. It's hard on motors too.

    If you're in this kind of situation you might want to consider doing a constant conversion UPS system. Even though they chew up power it could be worth it to gain the 'missing' 20 Volts and assure longevity from equipment.
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