The night time consumption of my grid connect inverter

Night time “sleeping” load / consumption of my grid connected inverter(s)

Friday, August 5th, 2011 Inverter, My PV Solar Stuff 18 Comments

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

18 Comments to Night time “sleeping” load / consumption of my grid connected inverter(s)

Great page .

Keep up the good work.

chris on August 26th, 2011

I have experienced the same problem withe a Fronius inverter it takes 76w of imaginary or apparent power at sundown. I used a cheap 13 amp power meter to read no watts and near zero power factor. Email to installer of PV and Fronius produced no comment.
Can one buy true watt meter remote monitors like the Wattson which I use.

Warren Richards on September 8th, 2011

Hi Warren, I’ve not seen any remote monitors like the Wattson or CurrentCost devices that measure and display true power.

I guess the other obstacle with them is that they need connecting to an actual live point to measure the voltage that would technically take away the DIY component and probably means more testing and approvals for each market.

Steve on September 8th, 2011

great pages and very interesting. I have a 2.2kw Solar intallation (10 x 190w panels + 2.2kw inverter). I also have the current cost clamps and ENVIR monitor. Bugger me if I cant get a reading of Solar power generated from the inverter (using the clamps). I’ve stuck them on the “AC Out” from the bottom of the inverter , but it reads ZERO, despite the inverter telling me about the watts it is generating. Is there another place to put it?

Appreciate any help you have provide. I have two clamps: one to measure household usage (gross) and the other to see how much of that is coming from my roof…just cant get the thing set up.

adrian scott on September 17th, 2011

Hi Adrian,

The current cost clamp has to go around just the active conductor coming out of the inverter, you won’t get a reading if you have it around all three conductors (i.e. the active, neutral and earth flex). The clamp should go on the active conductor at the circuit breaker “Solar Main Switch” at the switchboard.

If you do have the clamp on the active only, then it might be a pairing issue with the EnviR or a faulty clamp / transmitter. (If you swap the gross clamp and transmitter with the solar one, see what happens).

I’ll send you an email in a while with a photo of where I have my clamps.

Steve.

Steve on September 17th, 2011

I have the same problem – two inverters are together reading 0.07kw at night on an Owl monitor (with a clamp sensor on the line conductor). I understand power factor and had assumed this was the cause, although it is not clear why such a large current is being taken, even if out of phase with the voltage. I’m also getting some earth leakage and had to take them off the RCD side of the consumer unit to avoid trips at dawn. These PV inverters (mine are by Ever-Solar) don’t seem terribly well-behaved.

Stuart on October 10th, 2011

Just as well you took them off the RCD as it’s illegal to have an inverter downstream of an RCD although I’m not sure why you’d have earth leakage problems with them.

Mine perform pretty well and the EverSolar inverters aren’t the only ones drawing apparent power when shut down. Some other makes draw even more 🙂

Steve on October 10th, 2011

Hi Steve, why do you say it is illegal? In the UK, if the cable was buried in the wall and thus vulnerable (it isn’t) it would be mandatory to have one!

Stuart on October 17th, 2011

Here we can’t have an inverter downstream of an RCD. An RCD has to trip within a certain time (i.e 30ms)if you have an inverter downstream of the RCD and there is an earth fault, the inverter will take longer to disconnect due to anti-islanding requirements so there goes your 30ms trip time (if there is something wrong with the anti islanding, then the circuit will still be live even after the RCD has tripped).

Some houses have an RCD as a main switch. This has to be changed when a solar PV system is installed and then RCD’s fitted to the outgoing circuits (except the inverter circuit).

The inverter downstream of an RCD is also an issue when the inverter is wired to a sub-board too. Same reasons, too long for the circuit to become dead on an earth leakage current.

Steve on October 17th, 2011

Hi, I have a GroWatt 4.2kw inverter and 3kw of solar panels from True Solar. On a sunny day 35 degrees the solar panels generate 2.3 to 2.4kWph during mid day. however at night when there’s no sun, i’m not sure if it’s costing me to keep the inverter running, i can see about 200watts being generated around 6pm when there’s no sun, but i’m still concerned whether that’s more than what the inverter is actually using. I might have to give True Solar a call or Origin energy to find out a little more.

Ben on February 28th, 2012

Hello Steve,
Very good information,
I have an installation with 20 Micro Inverters on a 5kWH system. The night time reading from my “CurrentCost” device is around 350W. When I put a clamp ammeter on the solar circuit it reads around the 1.2 amp. I wonder if the consumption multiplies by the number of inverters? It would be interesting to know what the actual consumption is.
Cheers.

John Froehlich on October 13th, 2012

Hi, my 2kW inverter is not shutting down at night anymore, and it wont go over 0.6A, is it broken?

Ion Dumitrache on February 7th, 2013

I have read your blog on this after having a 2nd system and inverter fitted and having a “Elite Classic 3.0 Wireless monitor” attached to both to show me what I produce.
I noted of a night time it still showed 0.170kW, and wondered why as both inverters had powered down and appeared to be off.
I did the trick of turning them off at the fuse box and it dropped to “0”, so I curious. I turned the newer one on and the moniter showed 0.015kW usage, I turned the second one on and it went to 0.170kw, I turned the first one off so it just showed me my old system and it showed “0.150kw”, I tried this several times as the difference is HUGE, and all returned roughly the same readings.

Now that is $250+ a year (15hrs a day (winter time) off x 0.170kw x $0.2779 x 365 days)
I know you have said in your Blog that the kW they show is not actually kW, but for one to show 0.015 and the other 0.150, this seems wrong?

Peter on April 10th, 2013

Hi Peter,

Are the inverters the same brand and size or are they different?

Different capacitors / EMC filters will give different readings. But it’s only “apparent power” and it’s not costing you anything at all.

Steve on April 11th, 2013

Hello Steve,

Thanks for the reply, the older inverter is a 3.0kW AeroSharp 2 years old ( a real old heavy item) it is the one that the monitor shows as using 0.150kWph , the newer one it a 3.0kW cheap China one but that only actually draws 0.015kW.

You are sure the 0.150kWph is not actually costing me money? as a layman you can see my concern as it would add up if it was

Peter on April 11th, 2013

I am very sure it is not consuming any ‘real’ power. The only way to be absolutely sure would be to hook a wattmeter up to it that can read, true power, apparent power, power factor, voltage and current etc.

I don’t have access to a 3kW Aerosharp inverter to check for myself though.

Steve on April 12th, 2013

Great article,

Thanks.

Malcolm on July 15th, 2015

Interesting article thanks Steve. I was referred to your blog by the ANU Solar group. The EMC cct must have failed in my inverter as there is a spike on the mains now (last 6-9 months) that turns on touch lamps on that phase. I thought there must be some filter.
ANU has a solar monitoring project and uses a simple clamp, hence the query about the residual ‘power’ on dark.