Monthly Archives: July 2019

HEMS Wiring Update #2

Today I’ve further refined the wiring of the relays on the HEMS. At the time that I’d originally wired it I didn’t have small enough flex, or indeed multi core, which created an unnecessary number of cables (one per used relay) of over large size (and thus difficult to insert into the terminals). During the week I acquired some smaller gauge multi core allowing me to wire all three relays with a single cable containing one live feed and three switched live returns.

Revised HEMS wiring with multi core to HEMS relay outputs

Of the 5 incoming / outgoing cables at the bottom (left to right):

  1. Incoming mains (live / neutral / earth) from mains plug
  2. Live and switched live to / from ImmerSUN output relay to activate car charger.
  3. Live and switched lives to / from HEMS to activate car charger and water heating.
    • Red – live to contacts
    • Green – switched live for car charger direct – charge in response to price
    • Black – switched live for car charger indirect via ImmerSUN relay output – enable proportional charge in response to surplus PV
    • White – switched live for water heating – heat in response to price
  4. Outgoing mains (switched live / neutral / earth) to RF solutions radio transmitter to activate car charger, and on the second cable clamp..
  5. Outgoing switched live and neutral to ImmerSUN Boost relay input to enable immersion heater.

The revised wiring diagram looks like this..

Revised HEMS wiring #2

All of this still leaves one unused relay on the HEMS (HAT #3) and one unused proportional output on the ImmerSUN (#2; available for future expansion.

Initially even my smallest boot lace ferrules would not fit into the terminals on the HAT. Fortunately, once the ferrules has been crimped around the new cables, and flattened by squeezing in pliers, then the ferrules could be persuaded into the terminals.


I’ve been on my dynamic smart tariff for some months now, so I thought it would be a good time to see what I’m actually saving. My actual tariff rate changes each half hour, but for the purposes the supplier calculates the weighted average of what I’ve actually paid for the invoice. In principal that should be monthly, but I’ve had some bills combined over more than one month.

Smart Meter WAN
MonthYear 2018/19Year 2019/20
November14.27 p/kWh8.96 p/kWh
December10.17 p/kWh6.51 p/kWh
January6.86 p/kWh
February5.59 p/kWh
March5.02 p/kWh
April8.35 p/kWh
May4.48 p/kWh
June7.03 p/kWh3.49 p/kWh
July8.20 p/kWh
August6.29 p/kWh (part)
5.99 p/kWh (balance)
September5.63 p/kWh
October6.86 p/kWh

Over the course of the last few months my electricity price has reduced very significantly. I suspect that this is down to a combination of several factors including:

  • With the development of my HEMS (including its control of the battery storage) I’m getting slicker at optimising my purchase price
  • As we move into the summer the energy price is dropping with reduced demand and more renewable power available.

For reference the Energy Saving Trust reckons that the average UK price for electricity is 15.75 p/kWh on a flat rate tariff, or 19.0 on days and 9.1 p/kWh on nights for Economy 7. Thus my average electricity price in a month always beats their day rate and often beats their night rate. Of course the EST figures are the average market rates, so both better and worse deals will exist with different suppliers.

The 7.03 p/kWh for June 2019 seems to have been received with incredulity elsewhere so here’s the relevant part of the bill..

June 2019 average unit rate

I’ve never felt so engaged with my electricity supply, and am very pleased to have made the move from the dual-rate Economy 7 tariffs that I’ve used for around 30 years.

You can switch to Octopus yourself and earn the current referral bonus.