I have a Watts Clever electricity monitor (EW4009) with 2 wireless inputs - house electricity usage and solar panel output. These wireless CT inputs send a pulse to the monitor about every 10 secs. The monitor has a RS232 to USB output. On average about 30% of the solar generation is surplus to our consumption and we get bugger all for it. I would like to automatically switch on devices to consume any surplus power (eg a heater, pool pump, beer fridge etc).
Is this feasible to do with Arduino? Any pointers would be appreciated. I'm brand new to Arduino but experienced at DIY and computers.
Probably... But, you'll need to figure-out how the communication is working. RS-232 would probably be easier to "tap into" than the wireless or USB. Do you know if any special drivers were loaded for the USB connection? Making a similar special-driver for the Arduino would probably be impossible. 
Or, are you thinking about leaving a computer connected to communicate with the Energy Monitor, and then connecting the computer to the Arduino to control relays, etc.? That might be possibe too, but you'd have to get the data out of their application somehow...
There must be some sort of relay, or indicator light, or something "obvious" that "happens" when it switches between solar and power-company (if that's the kind of solar system we're talking about). It might be easier to tap-into that signal, even if you have to set-up a separate wireless system to communicate with the Arduino. If you can run a wire, it's getting even easier...
Hi Doug,
Thanks for prompt reply.
- Yes special drivers were loaded for the USB connection
- I wanted the controller to be standalone and simply connected to the Energy Monitor unit. Arduino would accept input from it and then control relays to switch the items on/off
- I'm confident its also possible to get a wired RS232 feed from the inverter. However there's no way of getting a measure of house power being used (other than via this energy monitor unit)
'Assuming' I could decipher the RS232 pulsed signal from the monitor, I would see Arduino comparing the 2 inputs and then, based on there being sufficient surplus, operating a relay to cut in say a heater or pump. The output control side I would imagine to be easy. My main concern (assuming I can decipher the RS232 signal) is that the inputs would be at roughly 10 sec intervals. Can Arduino accept RS232? Can it store inputs and then make decisions based on these stored values?
Can Arduino accept RS232?
It does have "regular" serial inputs & outpus. But they are not the correct voltage for RS-232, and RS-232 has a couple of handshaking lines. If you want to do it "right", you'll need to add another "little" chip. One such chip is the [u]MAX232[/u].
Sometimes, the handshaking isn't used, and sometimes it "works" at 5V, even though that's not to spec. The handshaking can be done with a couple ofthe other digital inputs/outputs. Even if you can get-by without the special ship, you would have to somehow protect the Arduino from the (higher) RS-232 voltages, but that's no big deal.
There are lots of options & variables in the RS-232 protocol... It's a lot simpler for the programmer than USB, but if the software doesn't automatically set-up the baud rate, stop-bits, and handshaking on both ends, RS-232 can be more difficult than USB for the user.
Can it store inputs and then make decisions based on these stored values?
Yes! It's very good at that! Computers (and microcontrollers) can "make decisions" by doing different things according to [u]if-statements[/u] along with [u]logical AND & OR operators[/u].
BTW - Here in California there is a law requiring power companies to buy unused energy from people with solar panels. I know a couple of people with solar panels and their electric meter actually runs backwards when they are generating "excess" energy!!!
Thanks. Sounds like I need to do some investigation of the Energy Monitor unit to figure out the RS232 bit. There is an active group pvbeancounter that I might investigate on that subject.
One question - what is a RS232 'shield' that I see mentioned? Is this the sort of interface component I could use?
Here in the state of New South Wales in Australia we are on 'net' metering for the solar, and 'time of day', aka smart metering, for our consumption. Any extra that we generate goes back to the grid and we get paid a miserly amount for it. In winter our house tends to be cold so I was particularly interested in having a control system that would auto cut in a small fan heater whenever we had sufficient surplus capacity. Then cut it out again when say a cloud came over, or the freezer or fridge cut in etc.