Solar control for a water heater
Solar control optimizes a water heater so it runs when conditions are favorable and avoids excessive grid draw. This page explains the fields and a typical example.
Start here
- Begin with conservative limits so the heater does not switch on and off too often.
- Use only one active time window first and watch the behavior for a few days.
- Tighten the thresholds only after you see when other household load increases grid draw.
What does solar control do?
Solar control decides when the heater relay can be on. The decision is based on grid power draw (W), an optional time window and safety limits (max on-time, min off-time). The goal is to heat when the grid is not heavily loaded and prices are favorable.
It does not require PV to work, but it fits both PV and spot pricing: when other load is low, the heater can run; when load increases, control stops the heater.
Key fields in the profile
- Name and description: profile identifier (e.g. ?Day control 3 kW?).
- Heater device & output number: Shelly device and relay channel controlling the heater.
- Meter device: device used to read grid power. Empty = same as heater device.
- Heater power (W): nominal heater power (e.g. 3000 W) used to estimate load.
- Start when grid draw ≤ (W): if household grid draw is below this, the heater may start.
- Stop when grid draw ≥ (W): if grid draw rises above this, the heater is stopped (usually higher than start threshold).
- Start/stop delay (min): how long the condition must hold before starting/stopping (prevents oscillation).
- Active time (from/to): optional time window (e.g. 08:00–20:00).
- Max continuous on-time (min): safety limit for one heating run.
- Min off-time (min): prevents too frequent on/off switching.
Typical example: 3 kW water heater
Detached house with 3 × 25 A main fuses and a 3 kW water heater. Goal: run mainly during daytime when there is spare capacity, and avoid overloading.
- Heater power: 3000 W
- Active window: 08:00–20:00
- Start when grid draw ≤ 1500 W
- Stop when grid draw ≥ 5000 W
- Start delay: 5 min, stop delay: 5 min
- Max on-time: 180 min, min off-time: 30 min
With these settings the heater runs when other load is low. When cooking or EV charging raises consumption, control stops the heater to free capacity.
Tips for choosing settings
- Start conservatively and monitor for a few days.
- Keep the stop threshold clearly above the start threshold to avoid oscillation.
- Use min off-time to prevent rapid cycling (useful especially with contactors).