How do I heat water automatically with solar surplus?

The hot water cylinder is the cheapest energy store you already own — 200 litres holds roughly 10 kWh as heat. There are three tiers of getting surplus into it: a simple timer (free, crude), a surplus diverter (precise, hardware cost), and forecast-driven automation via a smart relay (precise and predictive, cheapest hardware if you already run Home Assistant).

Why this is the first automation to build

Water heating is a big, fully shiftable load with built-in storage: nobody cares when the water was heated, only that it is hot at shower time. Diverting surplus that would export at a poor rate into the cylinder effectively multiplies that energy’s value by your import/export price ratio — typically two- to three-fold, per day, all summer.

The tiers differ in what they know. A timer guesses (“11 am is usually sunny”) and heats from the grid on grey days. A diverter (Eddi/iBoost-class, or PV-ready heat pump cylinders via SG Ready) measures live export and modulates the element to consume exactly the surplus — but reacts only to the present moment. Forecast-driven automation adds the future: heat at noon when the forecast promises surplus, fall back to the cheap overnight window when tomorrow looks grey, and never heat twice.

What to do

1. Check the cylinder and immersion element — any electric immersion works; three-phase or heat-pump cylinders just need the right relay or SG Ready input.
2. Start with the dumb timer today — midday slot in summer; it captures a surprising share of the value for zero cost.
3. Upgrade to measurement or forecast — either a diverter, or a smart relay plus an automation that compares forecast surplus against cylinder needs (the Home Assistant self-consumption answer shows the pattern).
4. Add the grey-day fallback — if the forecast says no surplus, heat in the cheap tariff window instead; that single rule is most of what a commercial diverter cannot do.