£200 to spend - where should I start

I have a long-service award of £200 coming my way and want to spend it here!

Does anyone have any suggestions on the best way to get started. I’m an electronic engineer who can programme and use Linux so I can handle the trickier stuff (I hope!).

I have:

  1. Solar PV on the roof with no current ability to use any surplus energy from it.
  2. An electric car that I’d love to charge on surplus power.
  3. An immersion heater that could be used for surplus energy (and a transformer to lower the power to 750W so I can use smaller surpluses).

Now, I could get a EmonTx, EmonBase, EVSE car charger, relay (for my immersion heater) and so on but £200 won’t buy all that.

So, does anyone have any recommendations on the best way to start my system for £200 ish? I would even quite enjoy a bit of self-build if that’s what you thought best! I could then grow the system in the future if Santa Claus wants to bring me some more bits.

Many thanks,


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  1. Just change your usage patterns? Change the mindset. Cheaper than any device. :slight_smile:

I say this because you might struggle to get options 2 and 3 for £200.

  1. Smart EV charging like Zappi? (only charge when generating etc) But that is £500.

Although you might be able to scrape back 75% of costs through govt subsidy.

  1. Solar iBoost? approx £300
    Solar iBoost+ - UK's Favourite PV Immersion Controller

Again, scrapes off spare solar and sends to your heater.

Obviously, the final option is invest in an emonPi so that you can monitor input and output that can help with option 1. But i’m not sure you’ll recoup that £200 in savings. I think that would be a tough ask. But you will have some nice shiny graphs to look at. :smile:

Options 2 and 3 would likely yield more benefit (and savings) in the longer term.

Hope this helps.

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You could do Robin’s or MartinR’s DIY energy diverter for water heating for a lot less than £200,
those will use any surplus if there’s anywhere to put it, and you might just stretch to an emonBase and emonTx with what’s left.

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Thanks guys, there are some useful ideas. I didn’t make it clear what I really want here! I don’t want to save any money, just spend it having fun with a project (if it saves any money, that’s good but it doesn’t have to). The Open Energy Monitor products look great fun to start a project that can be aimed at those issues starting small and then building up over time. I’m just looking for the best way to begin my journey with £200.

You can’t go wrong by starting with this. I run emoncms on a Pi zero - ok I have a rotating disk attached but you could do it on an SD card. Connected to this is an emonTx with Martin R’s software to divert all surplus PV into an immersion in a heat bank until it won’t take any more.

Been running for 3 years and gives us enough free hot water in the summer months and a bit of back up on sunny days in the winter months.

Great learning on both the hardware and software front. For example I’m now replacing my home brew esp based heat bank controller with one based on a sonoff device running the tasmota software.

You’d be well advised to spend the £200 on buying more time to work on all the things you could possibly do - sadly a couple of hundred wouldn’t stretch far in terms of buying more time.

But seriously, a home brew emoncms on a pi zero with an emonTx running Martin’s PV diversion code would be a brilliant way to start and you’d have enough over to buy a few beers to celebrate when you’ve got it all running.


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I can vouch for Robin’s diverter too. I have one of his prototypes. It saved me a considerable amount of money.

Prior to the installation of a net meter, the excess energy my PV system generated made my Wh meter
readings increase. i.e. I was paying for energy I was generating.

The diverter ran the excess energy into a dump load (55 gallon drum of water) yet I had the full output of my
PV system available when I needed it. e.g. if I wanted to run the clothes drier, or oven/stove or other large “energy eating” load, (I have an all electric house) the diverter made that possible, yet kept me from being
charged for the excess energy I generated when i wasn’t using the “energy eaters.” All without the need
to do anything else.

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A couple of further options for lower cost monitoring:

You could also try asking in the Community Marketplace category for used equipment Community Marketplace - OpenEnergyMonitor Community

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There are some really good ideas there guys, thanks for all your advice. I’m going to start down the energy diverter water heating route. I have to spend the money quite soon so I’m going to start out with the EmonTx and EmonBase with a good bunch of sensors and hey, let’s throw in the display too. That seems to be the most flexible and expandable starting setup. I can then deploy a fairly standard energy monitoring system while I find my feet with the hardware and software then progress onto the energy dumping into the hot water tank once I’ve got the hang of everything.

Good choice David!!


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You should add a few DS18B20 sensors to attach to the hot water tank if you can. Then you can watch the tank heating up in an emoncms dashboard.

If you haven’t got any sensor pockets on the tank, you can still attach them to pipes close to the tank. They’ll read a bit lower than the actual temperature in the tank but in a dashboard you’ll still be able to see the temperature rising while you sit there with your feet up.


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You could use Sodium Acetate in the drum to store the heat for later, and then release it with a shock - just like the hand warmers.


Hi Dave,

Storing the heat wasn’t needed. My goal was to avoid getting charged for the excess energy my PV system
generated, yet have the full PV system capacity available when the need to operate a large load came about.