My Cunning Plan

Hi All, My names Paul, I live near Manchester in the UK, I've had a 3kw on grid Solar set up with Aurora power-one inverter for the past 7 years, and recently noticed the surge in DIY powerwalls, which grabbed my attention.

I'm a software engineer, have several raspberry pi's lying around doing nothing, and found this guys site< diytechrepairs.nu> which uses raspberry pi's with influxdb,grafana,graphite .... all tools I use in my day to day work, so I got quite excited about employing them on my solar kit.

However ... being in the north of england, with only 3kw , I was concerned that forking out for a battery storage system may not be a sensible thing to do , so thought I'd set up a cheap DIY monitoring system first, need to get an accurate picture of how much my house is using, how much I'm exporting to the grid etc.

I've just ordered a handful of RS485->USB converters off ebay, then. noticed the SDM630 power monitor he's using on diytechrepairs costs around 100. Surely there must be a cheaper alternative out there ?

Also keep going round in circles on AC vs DC coupling . My solar set up has 2 strings of 7 panels in series , so at the inverterI'm seeing 300Vdc @5amp per string. Can you even use a DC buffer solution when set up like this? Is there some BMS that can handle 2 300V strings ?

My overall goal is to have a system which can detect when drawing current from the grid, and increase the battery output to compensate.

Any advice you guys could give me would be most. welcome.

It's not cheap, but you can use IoTaWatt ( https://iotawatt.com/ ) to monitor and log power usage of your various circuits. I don't have one yet (ordered yesterday), but I've heard good things about it.

3kW of PV is certainly not much. But if you have a time-of-use plan, esp Octopus Energy's Agile plan, a small Powerwall put in use at just the right times could prove useful. Perhaps you can install more panels on the walls, shed, balcony, ...? Lots of cheap used panels going around nowadays.

High voltage DC buffering is generally not done for residential use. The usual way is to use a solar battery charger to reduce the DC voltage to 12V/24V/48V first. Even so, most devices support only 100V~150VDC on the input side.

You may find my system interesting. I have PV arrays dedicated to charging my powerwall batteries, which feed special grid tie inverters that produces just enough AC power to offset my usage and not feed back power to the grid. Though there probably are legality issues with such an inverter in your area.

Thanks AJW , I'll go have a read of your setup... I've got to do something, just worked out my grid usage for the past year 9100 kWh !! Bleeding kids ! I see the iotawatt uses current clamps, and have read some people complaining of up to 15% error on the CT clamps ? Is. that true ? Was looking at this fella... https://www.ebay.co.uk/itm/RS485-Mo...869039?hash=item2ae7570fef:g:B~8AAOSw4mReokhc. . Doesn't state whether it can measure negative power though, but looks interesting for the money.

Do you have a link to the grid tie inverters you're using ? I do have a bit of a south facing cliff at the back of the house, would require quite a long wiring run, maybe 50M

I think cheap handheld CT clamp meters can be quite inaccurate.
The IoTaWatt apparently supports calibration of all 14(?) CT clamp ports, and also samples voltage (via AC transformer, also supports calibration) to calculate power factor, etc. I think the accuracy will mostly depend on the accuracy of the calibration.
Using the V reference, it can deduce the direction of flow, so it will correctly calculate consumed and back fed energy. But ideally you'll want to have one CT dedicated to capture the PV output, and one or more to capture consumption(s)... you'll get a much better picture of your requirements.