I’ve seen several threads and posts on various forums where people have mentioned their concerns about the apparent power their grid connect inverters use of a night when the inverter is shut down or sleeping.
I highlighted the term “apparent power” as that is all it is when using a basic clamp on device such as a Current Cost EnviR and the likes to measure the solar PV system generation.
On my EnviR my EverSolar inverters show a power consumption of 37W each when they are shut down, but in reality it is only 0.4W
The apparent power consumed by the inverter when it is dark and the PV panels are not producing power is not the full story and is not what the consumer is being charged for.
I’ll try and keep the first part of this post simple.
A basic clamp-on power measuring device is really only measuring the current of the load (or generator) in Amps. It is displayed as Watts /Kilowatts and Kilowatt-hours (kWh). It is only one third of the story. There is the actual voltage (that is not really a fixed value, but the devices use a fixed value) and there is power factor (more on power factor later).
The EnviR is showing “Apparent Power”. We, as residential / domestic customers currently don’t get charged for Apparent Power (kVAh). The VA that the basic clamp meter devices display is VA and not Watts even though the display might say it’s Watts & kWh
My EverSolar TL1500AS inverters show 37W each on the EnviR when shut down / off line (for a total of 74VA (not Watts as the EnviR would have me believe)).
The schematic for the TL1500AS inverter shows that the EMC filter is after the anti islanding disconnect contactor.
I used some test equipment I have at my disposal and got some real readings to help clarify the numbers observed.
Measured Volts = 251.4VAC
Measured Amps = 0.154A
Measured Power factor = Leading 0.01
Measured Watts = 0.4W
For VA = V * A = 251.4 * 0.154 = 38.7VA
The EnviR doesn’t quite get the VA right as it uses a fixed value for the voltage i.e. 240V (So EnviR 240V * 0.154A = 36.96VA that the EnviR displays as 37W).
Now for the Real Power / True power (i.e. NOT EnviR / Clamp device Apparent Power is displays as True Power).
Watts = V * A * p.f
Watts = 251.4V * 0.154A * 0.01p.f = 0.38W
My inverter consumes 0.38W when shut down of a night, my Watt meter showed it as 0.4W and the EnviR shows it as 37W.
Ignore your basic clamp meter devices when looking at your inverter consumption of a night.
I’m sure there are a few out there that aren’t aware of Apparent power (VA) and true power (W) or know of Power Factor. I know about it because I’m an Electrician and have to deal with both. The Current Cost devices do not measure or display true power which is acceptable to most, it’s only when you get into the nitty gritty that you need to be aware of the difference.
I’m no teacher. I understand Power Factor, but I’m not sure if I can teach others about it.
Put any inductor or capacitor (or a combination of both i.e EMC filter) on an AC supply and it will draw current / amps and will show up on a basic device like the EnviR. But the power factor is terrible, the current is way out of phase with the voltage.
If you put an actual real Watt meter on it, the Wattage is minimal. I’ll try a basic example: A capacitor, it charges and draws current from the supply as the AC sine wave voltage is increasing. As the voltage decreases on the sine wave the capacitor returns the energy back. Net value of Wattage used = zero (apart from some small unavoidable losses, like resistance as there is no perfect conductor, so a little heat is generated). You can measure the current with a clamp meter, and the EnviR will incorrectly display it as Watts. But no Watts are being consumed.
The same goes for an inductor, as current flows through the windings it creates a magnetic field, as the voltage falls, the magnetic field collapses and returns the energising energy back into the lines. Net value of Wattage used = zero (apart from some small unavoidable losses, like resistance as there is no perfect conductor, so a little heat is generated). You can measure the current with a clamp meter, and the EnviR will incorrectly display it as Watts. But no Watts are being consumed.
Anyone that is really concerned about the inverter wattage consumption when it is shut down should NOT be using a device that uses a clamp only like a Current Cost device. Get a REAL watt meter installed on it. You can get cheap ones for a couple of hundred dollars that will display, Volts, Amps, Power Factor, True Power and Apparent power as minimum, extra for installation – and the clamp only solution isn’t so bad.
Currently, as far as I’m aware, no domestic premises are charged for kVAh, only for kWh. Some industrial premises are and they run power correction centres to try and keep their power factor as high as possible. So if you hooked up a large capacitor to a 3 pin plug at home and the EnviR etc showed you using 1kW, you’d actually pay $0.00 as the EnviR is actually showing kVA that you aren’t charged for.
Here is decent description of power factor on WikiPedia en.wikipedia.org/wiki/Power_factor