The difference between your solar panel output and your household consumption at any given moment. When Solar Output > House Load, the excess energy is either exported to the grid (earning minimal credits) or can be diverted to charge devices like EVs, batteries, or robot mowers.
The Concept: Free Energy for Free Mowing
A typical residential solar array generates peak output during midday hours (10 AM – 3 PM). If your household load is 1,500W and your panels are producing 4,000W, you have 2,500W of surplus being exported to the grid at wholesale rates — often just $0.03–0.06 per kWh.
A robot mower charges at 80–150W. Diverting this tiny fraction of your surplus to the mower costs you nothing in real terms — the energy was being exported anyway. Over a full mowing season, this approach saves $15–25 in electricity and achieves a truly zero-operating-cost lawn.
What You Need
| Component | Purpose | Examples |
|---|---|---|
| Solar monitoring integration | Reports real-time solar output | Enphase Envoy, SolarEdge, Fronius |
| Whole-home energy monitor | Reports real-time household consumption | Emporia Vue, Sense, CT clamp on main panel |
| Home Assistant instance | Automation engine | HA OS on RPi 4/5 or mini PC |
| Mower HACS integration | Controls mower charge state | Husqvarna, Mammotion, or Worx HACS integration |
| Smart plug (optional) | Physical power control for charging station | Shelly Plug S, TP-Link Kasa |
Step 1: Set Up Energy Monitoring Sensors
Before building the automation, you need two sensor values available in Home Assistant:
- sensor.solar_power_output — real-time solar generation in watts
- sensor.house_power_consumption — real-time household consumption in watts
Most solar inverter integrations (Enphase, SolarEdge, Fronius) provide both values natively. If your inverter only reports solar output, add a whole-home energy monitor (Emporia Vue is the most cost-effective option) to measure household consumption separately.
Create a template sensor to calculate the surplus:
Step 2: Build the Automation Logic
The core automation uses a hysteresis band to prevent rapid on/off cycling when clouds pass. The mower starts charging when surplus exceeds 300W and stops when surplus drops below 100W.
Step 3: Add Safety Fallbacks
Two critical fallbacks prevent the mower from being stranded with a dead battery:
Minimum Battery Threshold
If the mower battery drops below 20%, override the solar logic and allow grid charging regardless of surplus status. This ensures the mower can always complete its return to the dock and have enough reserve for the next scheduled mow.
Rain Delay Integration
On rainy days when solar output is near zero, allow a single full charge cycle during the cheapest grid rate period (typically overnight). This ensures the mower maintains its schedule even during multi-day overcast periods. Use a weather integration (OpenWeatherMap) to detect rain conditions.
Step 4: Monitor and Optimize
Use the Home Assistant Energy Dashboard to track how much of your mower's energy consumption comes from solar vs. grid. After a few weeks, you will have data to fine-tune the thresholds. Common adjustments:
- Raise the surplus threshold if the mower charges and discharges too frequently (battery cycling).
- Lower the surplus threshold if the mower rarely gets a full charge before mowing time.
- Adjust the "for" duration — increase to 10 minutes if partly cloudy days cause too many state changes.
- Shift mowing schedule to start in the early afternoon, allowing maximum solar charging during the 10 AM–1 PM peak.
Expected Results
Based on user reports from the Home Assistant community, a well-tuned solar-sync setup achieves 85–95% solar-powered mowing during the April–September season in most US locations. The remaining 5–15% comes from grid fallback during extended overcast periods. The annual electricity cost for the mower drops from $15–25 to effectively $2–4.
Frequently Asked Questions
Most robot mowers draw 80–150W during charging, which takes 60–90 minutes for a full charge. Over a mowing season (April–October), total electricity consumption is typically $15–25 — a fraction of what a gas mower costs in fuel.
Yes, with a properly sized solar array (3kW+) and Home Assistant automation. In most US locations, a 5kW residential array produces 300–600W of surplus during midday — more than enough to charge a robot mower. The mower charges during solar surplus and mows on battery, creating a fully solar-powered mowing cycle.
You need: (1) a solar array with a monitoring API (Enphase, SolarEdge, or similar), (2) Home Assistant running on a dedicated device, (3) a smart plug or energy monitor on the mower's charging station, and (4) a HACS integration for your mower brand.
Configure a minimum battery threshold (e.g., 20%) below which grid charging is allowed regardless of solar status. This prevents the mower from being stranded mid-lawn if clouds persist. Most users set this as a fallback that rarely activates during the mowing season.
Interrupting charging cycles is generally safe for lithium-ion batteries. Modern charge controllers handle partial charging gracefully. The only concern is very frequent start/stop cycles (every few minutes), which the hysteresis band in the automation prevents.