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steve30's Home Energy Monitor Project

Posted: Sat Aug 31, 2019 10:09 pm
by steve30
I had a solar PV system installed a couple of years ago, and ever since I've wanted to build something to monitor how much power is generated, used, exported etc. At the moment, I'm limited to the inverter's front panel display and an old (and not very good) Owl-brand energy monitor.

I've been working on a system on my BBC and am now making some good progress. I thought I'd show it off here, since its not Arduinoy enough for a lot of forums :D.

I have two current transformers outside in my meter cabinet. One measures how much solar power is being generated, and the other measures how much power my house is using. A cable goes from those, up the outside wall, through a hole in my bedroom wall, round some furniture, and to my desk, where it enters a breadboard.

The breadboard contains a couple of LTC1966 True-RMS to DC converters. The current waveform picked up by the transformers will not be a straight forward sine wave, so the LTC1966 measures the true RMS, and gives a DC output proportional to the true RMS. This DC output is then boosted a bit by an LM324 op-amp so it can be measured by the BBC Model B's analogue port.
The software then uses these values to calculate how much power you are using/generating/exporting/importing. I am now working on the bit that calculates how much energy this is in kWh. I have also recently added a clock to the screen.

Here is a screen shot taken now and one taken the other day when it was sunny. It should be obvious which is which.
Its a work in progress, but its coming along nicely, and trying to figure out how to write the programme is keeping my brain occupied :).

Re: steve30's Home Energy Monitor Project

Posted: Sun Sep 01, 2019 7:17 am
by Elminster
Interesting. I guess there won’t be an IOS/Android version, but you could hook up beeb to upload somewhere via a serial to internet adaptor.

Re: steve30's Home Energy Monitor Project

Posted: Sun Sep 01, 2019 12:42 pm
by julie_m
You will need to measure the voltage, and the phase difference between the voltage and current waveforms, to get the power and power factor. The frequency will come for free, of course :)

Re: steve30's Home Energy Monitor Project

Posted: Wed Sep 11, 2019 4:17 pm
by steve30
I decided against measuring the power factor and voltage. I think the voltage is stable enough for it to not affect the accuracy too much, although I may add it at some point if I feel like it. I'm not bothering with PF at the moment as we don't get billed for it.

There certainly won't be an iOS/Android version. I don't even know what programming language they use :). It may well get ported to a Pi or something similar. But the BBC does seem to work nicely for this project.

In the near future, I'm hoping to get it to save the data to a disk, and draw graphs.

Re: steve30's Home Energy Monitor Project

Posted: Wed Sep 11, 2019 4:44 pm
by julie_m
You are right that residential users only get billed for real power. But you do need to measure the Power Factor; otherwise, by multiplying the measured current by a guessed voltage, you will be estimating the vector sum of reactive and real power.

Also, the voltage is less stable than you think.

Re: steve30's Home Energy Monitor Project

Posted: Wed Sep 11, 2019 5:06 pm
by BigEd
Nice project! Any idea, those in the know, as to how much difference the power factor business makes - is it more or less than 10% for example?

Re: steve30's Home Energy Monitor Project

Posted: Wed Sep 18, 2019 5:13 am
by steve30
BigEd: It would depend on what your load is. For example, if you are using nothing but incandescent light bulbs and electric heaters, then you will have a unity power factor, and the real power and apparent power will both be the same.

But if you are using lots of motors, you will have a poor power factor, then the real power and apparent power will be different. Which one you measure depends on which one you get billed for.

TBH I always get incredibly muddled up with different kinds of AC power. How I managed to pass the assignments about it at college when I studied electronic engineering, I don't know.

Regarding this project, it is important to note that my benchmark is a fairly inaccurate and imprecise 'Owl Energy Monitor' (which also assumes the voltage, and just measures the current).