One of the roles of my Home Energy Management System (HEMS) is to switch the control state of my Powervault storage system. Many users would probably leave the system in Normal where it always either charges or discharges proportionally to solar surplus or deficit respectively. Other users on time of use tariffs might also program charging on cheap night time electricity in a fixed time window. However my HEMS integration allows for automatically charging in varying cheap windows only when solar production won’t be enough to fully charge the battery.
The Powervault has six Control States covering different combinations of charging and discharging behaviour. Only three of these are used in my implementation. The other states could be more relevant if I was paid to export to the grid, and the value of that export started to exceed bought electricity costs; but in my case I’m not paid for export and thus any export is an error state.
| Control state | Charge | Discharge | When used |
| Force discharge | No | Full power | unused |
| Only discharge | No | Proportionate | unused |
| Normal | Proportionate | Proportionate | High electricity cost |
| Only charge | Proportionate | No | Medium electricity cost |
| Force charge | Full power | No | Low electricity cost |
| Disabled | No | No | unused |
While I could use the HEMS to load a full daily schedule into the Powervault, instead I store all my schedules in the HEMS itself and switch the state of the Powervault (and indeed my other devices) every half hour as dictated by the schedule. This approach also gives me some flexibility to nuance Control State as a function of state of charge which isn’t available via the Powervault’s own scheduler.
The HEMS thus has three roles:
- Creating the daily schedule from future electricity prices and the solar forecast.
- Switching the states of the devices every 30 minutes following the schedule.
- Real time data display and data upload to Solcast the solar forecasting service.
