What solar irrigation solves, and what it doesn’t
Solar irrigation replaces the wiring and the mains socket, not the judgement. A well-chosen solar kit waters a pot or a small vegetable garden for days without sun; it doesn’t water a large garden, it doesn’t water at night and it doesn’t replace the filter or the pressure regulator.
The real advantage is autonomy: on a plot with no electricity, on a terrace with no water supply nearby, or on a rural property where the electrical installation is far away, the solar kit makes the project viable. The cost is limited capacity, which depends on the sun at your location.
The three parts of the system
- Photovoltaic panel. Captures the energy and sets the production ceiling.
- Battery or accumulator. Stores whatever the panel produces in excess, because irrigation happens at night or on cloudy days.
- Charge controller. Regulates the battery charge. It’s the part that protects the battery and the one that decides whether the system can run the pump today.
On top of that, the low-voltage pump and the timer or valve. The whole set only works if the first three are sized together.
Sizing: start from consumption, not from the panel
The right order is the reverse of how it’s usually decided:
- How much water the system applies per day. It depends on the plants and the substrate, and it’s measured with the pot weight method. Multiply by the number of plants and by the number of days you plan to water.
- How much energy the system needs. If the pump moves, for example, one litre per second and runs fifteen minutes a day, the calculation is volume multiplied by lift and by pump efficiency. In practice, take the figure given by the pump manufacturer and add a 30 % margin.
- How much light there is. The useful sun hours of the site, not the theoretical sun hours. Data for your area is in AEMET OpenData, and FAO-56 explains how that radiation translates into water demand.
- Only then, the size of the panel and the battery. With margin for two or three cloudy days in a row, which is what happens in winter.
A typical mistake is looking first at the panel wattage offered by the deal. The panel doesn’t work magic: if the battery can’t hold the energy needed at night, it doesn’t matter how much energy is generated.
Low-voltage pump, no question
Garden pumps connected directly to the panel or to a battery pack are a bad deal: the starting current stops them and the damage is quick. For a solar system the sensible option is:
- 12 V pump with electronic control that starts smoothly, or
- a mains pump with a frequency drive, if the consumption justifies it.
In any case, check that the pump tolerates intermittent dry running and that it has dry-run protection: a solar system runs out of power exactly when it’s needed most.
Timer or sensor: what to put on top
A basic solar kit comes with a timer. It works, but it has two limitations worth knowing: it waters even if it has rained and it waters even if the soil is wet.
The jump in quality comes from replacing the timer with a soil sensor using the same board as the rest of your home automation. That way the system decides from readings, and the Home Assistant automation can skip irrigation when recent rain has already wetted the zone.
The trade-off is power consumption: an always-awake ESP32 node drains the battery in days. That’s why the rain sensor and ESPHome with deep sleep are the option when the battery is small.
Maintenance: the weak point of the system
- Clean the panel. Dust and leaves are the number one cause of a drop in production. With water available, a wash once a month or during the rainy season.
- Check the battery condition. A lead battery that spends months without a charge degrades, and then it won’t start the pump even if the panel works.
- Store the equipment over winter if you don’t irrigate, or at least make sure it doesn’t sit at zero for weeks.
- Protect the electronics. Everything that isn’t panel or battery goes in the shade and protected, as explained in the electrical safety guide for the garden.
When a solar kit isn’t worth it
- If there’s a water supply and a socket nearby: a normal mains pump is more reliable and cheaper.
- If the watering is constant and high-flow: the energy goes into pumping and the solar system can’t keep up.
- If the garden is in shade for most of the day: there’s less production than the catalogue suggests.
For a vegetable garden on a small balcony, the assembly with tank and zones is usually more useful than solar power.
Sources and next step
For calculating water demand from climate, FAO-56 is the reference; for local radiation data, AEMET OpenData; and for sizing the whole set, always use the datasheet from the pump and panel manufacturer, which is where the concrete flow, consumption and efficiency figures are.
External sources
- FAO: Crop evapotranspiration, Paper 56 — relationship between radiation, water demand and irrigation.
- AEMET OpenData — radiation and temperature data from the nearest station.