Modeling a 1.5 m³ garden spa with dual 3 kW heaters in Predbat

[mpi]

Hi!
I’m using Predbat to optimize my Home Assistant energy flows. It works great! I’d like to extend the config to model our garden spa (essentially a 1.5 m³ water tank with two 3 kW heaters) as part of Predbat’s optimization, so that:

1. I can set a target water temperature.
2. I can set a deadline when that temperature must be reached.
3. Predbat avoids draining the house battery to power the spa heaters—ideally using cheap import rates in winter and excess PV in summer.

I'm already able to control water heaters target temperature via Home Assistant and I am monitoring energy and power via Shelly energy meter.

I’ve considered three options within Predbat:

1. predheat
2. iBoost diverter
3. modeling the spa as an EV (battery_capacity = heat of water × volume, soc = current_temp / max_temp, target_soc = target_temp / max_temp).

However, none of these seem designed for my use case. Could you suggest the recommended way to model and control this spa load in Predbat? Are any of the above approaches viable, or is there another mechanism better suited to time-and-temperature-constrained heating loads?

Many thanks for your guidance!

[gcoan]

I would think predheat is the closest to your requirement because concepts like energy loss with outside air temperature and energy required to reach a target temperature are already built in.

However as you say, it is bending preheat to make it fit your requirement. I don't think iboost is the right answer as this manages using excess solar to heat your hot water or heat hot water when electricity is cheaper than gas.

Additional option you could try is export triggers https://springfall2008.github.io/batpred/apps-yaml/#triggers but again that is focussed on excess solar.

[trullock]

I'd like to do the same thing, but also consider the gas boiler to heat the water.

e.g. if the plan for today won't heat the water sufficiently, turn the gas boiler on to meet the deficit

Is this possible?