We have recently installed solar panels (photovoltaic). One of the challenge is to increase your auto-consumption. One of the main consumer we have is the water boiler. It used to run at night to benefit from off-peak tariff. We decided to build a system to pilot the water boiler in a way that it would improve the auto-consumption.

The system runs Axitec solar panels with a Fronius Symo inverter. Fronius offers the Ohmpilot in order to do this. However, it does not take into account weather forecast and the cost is rather high. We wanted to make something DIY and more flexible.

In order to keep the cost low, we didn't want to continuously adjust the output power like the Ohmpilot does. Instead, we are going to control separately the 3 resistances that are in the boiler.

• Wattpilot can control from 1 to N loads, switching them on and off based on the exported power.
• A schedule can be define which is meant to warm the water during the off-peak period.
• Just before the scheduled period, the weather forecast for the coming day will be fetched from OpenWeather and if the sky is going to be very cloudy, the water will be warmed during the scheduled period.
• The next scheduled period can be set manually to be triggered in any case.
• A force mode allows the user to start/stop the heating manually at any time.

• User interface. There will be a simple web based interface to monitor, control and configure the system.
• Temperature feedback. It would be useful to have the water boiler temperature as feedback. This could allow for a dynamic threshold (e.g. heat the water at higher temperature on solar) and also to stop the relays whenever the threshold has been reached. The system currently does not know if the heater is actually heating or not.