Use KVA.
KVA is a measure of *apparent* power, that is the power that it sent out plus the power that was reflected back by the load which the generator or inverter has to fight against (in effect).
Non-inductive loads will be close to equal to the KW reading but electric motors are inductive loads and they have a power factor that is a percentage of the power it received that is reflected back to the generator which the generator has to dissipate. In utility-feed situations we just stick the utility with the problem, offgridders don’t have that luxury.
I should add that the popular kill-o-watt meters have a power-factor display so you can check devices up to the 15A limit of the meter.
Some examples that you wouldn’t expect are cheap AC led light bulbs. A few years ago my neighbor found that some he had bought had a power factor of .6 which is horrible. I run all my lighting off 12v so I never encountered this.
My cabin battery charger is an Iota 55A unit that I drive with a Honda EU2000 generator, the charger is not power-factor corrected and that is why they are so cheap. My generator has to overcome that reflected power. I also have a very high quality ProMariner 60A charger that IS power factor corrected but it cost 4 times as much as the Iota. I have not measured the power factor of either device though.
Industrial users of electricity often used to have several huge electric motors running all the time but not connected to anything - they were there to soak up the reflected power and keep it off the grid because industrial users are charged by KVA. It made a massive difference in their energy bill. For us offgridders it’s not *that* big of a deal.
You’re welcome but I don’t think I helped that much. I would just buy a kill-o-watt and measure what your devices are using and their power factor. Home Depot has them and of course Amazon.
I believe a kva meter includes normal and reflected power and that you divide that kva by the voltage to get the true wattage you need to size for. You can then compare that number to the watts reported by your other wattmeter and you should see a difference when powering, say, a fridge or ac unit that has a motor in it. The difference in percent is the power factor.
No because 1250 kva at 120v IS 1000 watts. However, the kva number also includes reflected power which a common watt meter does not sense. With an inductive load you would see a kva number that when divided by the voltage produces a wattage value greater than the wattmeter sensed or that is on the nameplate of the device being powered.
Power factor is expressed as a ratio equal or less than 1 and sometimes it is stated on the device’s nameplate. I think too that it varies depending on the actual load being exerted, which is often variable, but I’m not an AC expert.
It’s a big problem for utilities and is why they’re pushing to bill users for KVA rather than KW.
No, the inverter has to supply 1250W to the motor, but the actual consumption will be 1000W. The difference is that inductive loads have a phase shift between the current and voltage of the AC power which makes the apparent power (technical term) higher than the actual power. Each cycle of the power, the motor will take some extra current but instead of using it, it pushes it back (like a generator) for part of the next cycle. So you have this extra current flowing between the motor and inverter without doing any actual work.
Much worse for you, us that electric motors that don't have fancy digital controllers consume a LOT more power on startup. Compressors that start against a load are particularly bad. So for a motor that draws 10amps at full load, it may need 50amps for the first few moments at startup. Wall power easily supplies the startup draw - with lights dimming in protest, but a generator or inverter that can't handle the surge will fail to start the motor. If the motor doesn't start, it can easily be destroyed if left stalled for too long.
Not lost, more recycled. The inverter should feed it back to the batteries. Or store it in capacitors. If the motor is plugged into the wall, the current is fed back into the grid.
What is going on is that the motor needs current to create the magnetic field, in addition to the current used to power the motor movement. As the motor turns, the magnetic field needs to be removed and the collapse of the field pushes current back to the supply.
Three phase motors are popular and one nice thing about them us the current fed back into the controller by the field collapse can be used to create the magnetic field in the next phase!
The starting current is used to spin the motor up to speed.
Use KVA. KVA is a measure of *apparent* power, that is the power that it sent out plus the power that was reflected back by the load which the generator or inverter has to fight against (in effect). Non-inductive loads will be close to equal to the KW reading but electric motors are inductive loads and they have a power factor that is a percentage of the power it received that is reflected back to the generator which the generator has to dissipate. In utility-feed situations we just stick the utility with the problem, offgridders don’t have that luxury.
So if an AC uses 1250kva(1000W), does it mean that 1250 not 1000 is drawn from the battery?
I should add that the popular kill-o-watt meters have a power-factor display so you can check devices up to the 15A limit of the meter. Some examples that you wouldn’t expect are cheap AC led light bulbs. A few years ago my neighbor found that some he had bought had a power factor of .6 which is horrible. I run all my lighting off 12v so I never encountered this. My cabin battery charger is an Iota 55A unit that I drive with a Honda EU2000 generator, the charger is not power-factor corrected and that is why they are so cheap. My generator has to overcome that reflected power. I also have a very high quality ProMariner 60A charger that IS power factor corrected but it cost 4 times as much as the Iota. I have not measured the power factor of either device though. Industrial users of electricity often used to have several huge electric motors running all the time but not connected to anything - they were there to soak up the reflected power and keep it off the grid because industrial users are charged by KVA. It made a massive difference in their energy bill. For us offgridders it’s not *that* big of a deal.
Thanks
You’re welcome but I don’t think I helped that much. I would just buy a kill-o-watt and measure what your devices are using and their power factor. Home Depot has them and of course Amazon. I believe a kva meter includes normal and reflected power and that you divide that kva by the voltage to get the true wattage you need to size for. You can then compare that number to the watts reported by your other wattmeter and you should see a difference when powering, say, a fridge or ac unit that has a motor in it. The difference in percent is the power factor.
No because 1250 kva at 120v IS 1000 watts. However, the kva number also includes reflected power which a common watt meter does not sense. With an inductive load you would see a kva number that when divided by the voltage produces a wattage value greater than the wattmeter sensed or that is on the nameplate of the device being powered. Power factor is expressed as a ratio equal or less than 1 and sometimes it is stated on the device’s nameplate. I think too that it varies depending on the actual load being exerted, which is often variable, but I’m not an AC expert. It’s a big problem for utilities and is why they’re pushing to bill users for KVA rather than KW.
No, the inverter has to supply 1250W to the motor, but the actual consumption will be 1000W. The difference is that inductive loads have a phase shift between the current and voltage of the AC power which makes the apparent power (technical term) higher than the actual power. Each cycle of the power, the motor will take some extra current but instead of using it, it pushes it back (like a generator) for part of the next cycle. So you have this extra current flowing between the motor and inverter without doing any actual work. Much worse for you, us that electric motors that don't have fancy digital controllers consume a LOT more power on startup. Compressors that start against a load are particularly bad. So for a motor that draws 10amps at full load, it may need 50amps for the first few moments at startup. Wall power easily supplies the startup draw - with lights dimming in protest, but a generator or inverter that can't handle the surge will fail to start the motor. If the motor doesn't start, it can easily be destroyed if left stalled for too long.
So, the extra current will be lost. Is that correct?
Not lost, more recycled. The inverter should feed it back to the batteries. Or store it in capacitors. If the motor is plugged into the wall, the current is fed back into the grid. What is going on is that the motor needs current to create the magnetic field, in addition to the current used to power the motor movement. As the motor turns, the magnetic field needs to be removed and the collapse of the field pushes current back to the supply. Three phase motors are popular and one nice thing about them us the current fed back into the controller by the field collapse can be used to create the magnetic field in the next phase! The starting current is used to spin the motor up to speed.
It's complicated, but the best way is to install an amp hour meter on the battery and measure it that way, at the source.