how much standby power does the openevse use ?

[Danny ter Haar]

I am participating in a pilot of SCE with evse charging.

I have the openevse to start at 21:05 and and at 11:55

So I have a window of 5 minutes to prevent accidentally charging in the expensive hours.

I am using the "wattbox" of Emotorworks to measure & provide the kWh of charging.

I was surprised the first month that I had used 1 kWh @ $0.36.

Now my 2nd month bill shows the same 1 kWh.

How much is the openevse (with huzzah module) drawing from 240 ?

9 hours per day "expensive" power x 4 watt x 30 days would be about 1 kWh

Is that about right ?

[chris1howell .]

OpenEVSE itself draws less than 0.5w with LCD. Huzzah draws 74ma @ 5v or 0.370 W. I would guess the total is less than 0.75W. 

How much does the wattbox draw?

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[Craig Kirkpatrick]

I can make some AC amperage measurements at 120VAC pretty easily.  I imagine the vampire-draw is going to be the same at 240VAC in terms of Watts.

I can do it with and without the ESP8266 module. I might not do it until tomorrow morning since baseball is about to start :-)

[Danny ter Haar]

On Monday, October 19, 2015 at 4:04:20 PM UTC-7, Chris wrote:

OpenEVSE itself draws less than 0.5w with LCD. Huzzah draws 74ma @ 5v or 0.370 W. I would guess the total is less than 0.75W. 

How much does the wattbox draw?

The wattbox has this meter in it:

 http://www.ekmmetering.com/ekm-omnimeter-ii-ul-v-3-universal-smart-meter-single-phase-or-3-phase-120-to-240v-50-60hz-up-to-5000-amps.html

Will measure usage tomorrow when my EV isn't charging ;-)

[Craig Kirkpatrick]

Ok, I wonder what the vampire kWh consumption is of the Wattbox.  I think that is the point Chris was making too.

I have a very nice Agilent handheld DMM that can measure AC milliamperes. Later today I'll put it in series in my 120V AC mains wiring to my bench V4 OpenEVSE.  I have a Huzzah module on hand that I can add to the V4's FTDI.  Right now I have the older RGB LCD RTC in place but I could also try the newer version that Chris ships lately though I wouldn't expect that to make much difference.

I'm not setup to try it at 240V but my thought is the Watts should be the same most probably.

[Craig Kirkpatrick]

I just made the measurements.  24ma AC idling with just the RGB LCD wRTC.  31ma AC with the Huzzah module with early firmware that did not use deep sleep mode.  The voltage was 116VAC.

This seems a bit on the high side of what was expected.  It would be interesting to repeat the measurements after removing the load resistor(s).  Is it OK to remove both R27 and R28 or is R28 the only one that makes sense to remove?

[chris1howell .]

Both can be removed, the only time the resistors are really needed is when using a large 12V DC Contactor like the one used in the Juicebox.

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[Danny ter Haar]

Wattbox (some arduino thingie & EMK meter) 40 mA according to my fluke meter (on 10amp range though so I wonder about accuracy) 

I couldn't find my other fluke that has a 200 mA range.

But it also adds some resistance in series so might paint a different picture.

On Tuesday, October 20, 2015 at 11:47:35 AM UTC-7, Craig Kirkpatrick wrote:

I just made the measurements.  24ma AC idling with just the RGB LCD wRTC.  31ma AC with the Huzzah module with early firmware that did not use deep sleep mode.  The voltage was 116VAC.

so:

openevse no huzzah L1: 24mA x116V = 2.8VA

openevse with huzzah L1: 31mA x 116V = 3.6 VA

just for comparison:

Wattbox  L2 40mA x 240V = 9.6 VA

Since the wattbox measures its own power and the openevse, it is safe to say that together they don't use more than the 4-5 Watt I calculated before. Should be some power correction factor at work there.

[Craig Kirkpatrick]

It is healthy to be skeptical of all measurements.  I can say that after spending almost thirty years working for HP/Agilent/Keysight assisting customers with all sorts of measurements.

I'm reporting the mill-amperage readings with a DMM that has a uA, mA, and a 10A range for either AC or DC.  I looked up the burden resistance in series for the mA range and it is 1 Ohm so I conclude that the meter has very little voltage drop and maybe my wires have more voltage drop than the meter does at these mA measurements.  So lets just say that the vampire Wattage of OpenEVSE is somewhat less than my numbers and also makes assumptions that it is the same vampire Wattage at 240V which is debatable too.  Still these are pretty interesting data points.

I removed R27 and R28.  Since it was easiest for me to use a new RGB LCD RTC for this second test I did make that one change but I don't think it matters.

I measured 13mA without the Huzzah.  Assuming 116V that is  1.5VA

I measured 19mA with the Huzzah.  Assuming 116V that is 2.2VA

The little power supply on OpenEVSE feels notably cooler with the load resistors removed.

-Craig K.

[chris1howell .]

The load resistors keep a 20% load on both 5v and 12v. Without, there is not a lot of draw at idle.

[Craig Kirkpatrick]

Probably each little wall-wart transformer in my home pulls more wattage at idle than OpenEVSE.  Well done Chris.

[Danny ter Haar]

I just found out about the city of Oxnard who operates a couple of L2 chargers.

They want to increase the price from zero to $2.85/hour [SICK]

They wrote a whole bunch of info in this document:

http://oxnard.granicus.com/MetaViewer.php?view_id=46&event_id=1405&meta_id=156463

So the city is able to buy L2 electricity for $0.18/kWh which I think is darn cheap, independent of time of day etc!

They are using clipper creek evse, they calculate that they use 6 Watt.

54kWh/yr = $29.16

Openevse is much _much_ more efficent.!

great job Chris!

[Ted Drain]

I'm trying to measure the draw on my new 50A kit (purchased from the store).  I put a meter inline between one leg of the AC input and the AC line input terminal on the board and get 4.9 mA.  So that's either 0.6W (*120V) or 1.2W (*240V) - I'm not sure which since I'm measuring one leg of a 240V circuit.

[Craig Kirkpatrick]

That would be 1.2W and is in line with what I was hoping.  When I did my measurement it was at 120V and I did not have a good way to check it easily at 240V.  Thanks for doing that!

[chris1howell .]

Yep, 1.2w the current measurement is not dependant on voltage to ground. For current your supply/return is 240v.