One of the features of the way my HEMS works is the inverse relationship between electricity demand and electricity price – electricity demand increases as the price falls and falls as the price rises. This effect helps to contribute to my low average cost for electricity – 5.63 p/kWh ex-VAT most recently (Sep/Oct 2019). The effect exists because my different controlled loads (battery, car and water heating) each need different times to achieve full (approximately 8, 5 and 2 hours respectively).
![](https://greening.me.uk/wp-content/uploads/2019/11/HEMS-increasing-power.jpg)
This chart is illustrative only because day-to-day needs for the different loads may change, and also the electricity price varies half-hour-by-half-hour and day-by-day but typical recent behaviour is shown. The actual rules for each device may be summarised as follows:
- House baseload – support house from battery when cost greater than 10 p/kWh (limit adjusts automatically to reflect pricing on the day), otherwise take baseload from grid.
- Storage battery charging – charge from the grid in cheapest 5 hours over entire 24-hour period.
- Car charging – charge for a total period of so many hours within a window between arrival time at home and departure time the next day, and also any hours during day at equivalent price or lower.
- Water heating – charge for the cheapest 2 hours when electricity price falls below gas price (did not occur on day illustrated below).
![](https://greening.me.uk/wp-content/uploads/2019/11/HEMS-illustration-11-11-2019.jpg)
These same loads may also be enabled in priority order when the solar panels are in surplus. Priority is:
- Baseload
- Storage battery charging
- Car charging
- Water heating.