I've been using DESS "trade mode" for a while now and since a significant bug affecting my tarrif (where buy price is lower than sell price for some of the day) was fixed a month of so ago it now works pretty well.
That said, there is specific issue that should be an easy fix which is costing me money and will result in me moving back to a 3rd-party solution if there is no way to fix or tune this.
As conext, I am on a fixed tarrff with a very good overnight import price (7.5p) and a higher daytime price (24p). Export is a fixed 15p.
With such a tarrif, and when using trade mode, it makes sense to:
- Fully charge battery overnight at 7.5p
- Export excess PV directly to the grid during the day to avoid inverter/battery losses (assuming enough in battery for the evening).
- End the day with battery at 10%, ready to fill-up at 7.5p
- Can also sell (and then recharge) overnight if time permits.
With the bug caused by my weird tarrif fixed, DESS is actually doing a great job of all all of the above. The problem is the discharge strategy, which is too optimistic. If PV and consumption forcasts were 100% accurate it wouldn't matter when the is discharge happens (at 6am or 11pm), but the reality is that they are not accurate and vary with weather and usage. If DESS has chosen to discharge to the gird early in the day and then the weather changes (or consumption is higher than forcast) then you end up importing the shortfall in the evening at the high tarriff. If discharge is delayed (and caulated based on current battery % at the time) then this issue can be easily mitigated. Obviously you can't leave all discharge for last 1hr due to limits in discharge power, so it's not that trivial, but generally delaying discharge as much as possible within fixed periods like this is preffered and would avoid unnecessary imports and costs.
Here is an example from yesterday which resulted in evening import:
What I've noticed though is that this behave seems to have changed recently. A month ago this wasn't perfect and there was a small amount of evening import but it seemed to at least be trying to delay most of the discharge until the evening. Recently, however, it seems to be deliberatly discharged early in the day!
Do we know what stategy DESS is using to decide when to discharge in this scenario and if this have changed at all recently?
