Solar Panel Wattage, Battery Storage and Pump Duty Cycle: How the Three Must Be Matched

A solar irrigation kit has three power components: a solar panel that harvests energy, a battery that stores it, and a pump that consumes it. If these three are not matched — if the panel is too small for the battery, or the battery is too small for the pump, or the pump runs longer than the battery can sustain — the system fails. Not dramatically. It simply stops watering after a few days, and the customer assumes the product is defective.

For buyers, understanding how these three components interact is the difference between sourcing a kit that works reliably and one that generates support tickets.

Why Panel Wattage Alone Does Not Determine Runtime

Panel wattage is the rate of energy harvest under standard test conditions (1000 W/m² irradiance, 25°C panel temperature) — conditions that rarely exist in real-world balcony or garden installations.

What matters is the daily energy harvest: how much energy the panel actually puts into the battery over the course of a day. This depends on actual sunlight hours at the installation location, panel orientation and tilt, shading from buildings or trees, charge controller efficiency, and temperature effects on panel output.

The buyer cannot estimate daily harvest from the panel wattage alone. The actual harvest must be measured at the installation conditions — or at minimum, estimated from location-specific solar irradiance data and derated for real-world losses (shading, orientation, temperature, controller efficiency).

Battery Energy in Wh and the Usable Operating Window

Battery capacity is often listed in mAh, but mAh alone does not tell the full story. The energy stored in a battery is:

Energy (Wh) = Capacity (Ah) × Voltage (V)

• A 3.7V 2200 mAh battery stores 3.7 × 2.2 = 8.14 Wh
• A 11.1V 2200 mAh battery stores 11.1 × 2.2 = 24.42 Wh

The 11.1V battery stores three times more energy than the 3.7V battery with the same mAh label. This is because the 11.1V battery is a three-cell pack (3 cells in series, each at 3.7V), while the 3.7V battery is a single cell. The mAh number is per-cell capacity and does not account for the voltage difference. For buyers, this means two kits both labeled “2200 mAh” can have significantly different runtime depending on whether the battery is 3.7V (single cell) or 11.1V (three cells) — always compare Wh, not mAh.

This Wh calculation gives the nominal energy — the theoretical maximum. The actual usable energy is less, due to discharge cutoff (the BMS prevents full discharge to protect the battery), controller overhead (standby current), and conversion losses. The buyer should measure the actual usable energy rather than assuming the nominal Wh is all available.

Pump Power, Duty Cycle and Controller Losses

The pump consumes energy when running. The pump’s power consumption depends on:

• Operating voltage — Must match the battery voltage (or be regulated by the controller).
• Current draw — How many amps the pump draws under load. The actual current draw should be measured on the sample — do not rely on spec sheet values without verification.
• Duty cycle — How long the pump runs per watering cycle, and how many cycles per day.

The energy consumed per day is:

Daily pump energy (Wh) = Pump power (W) × Duty cycle (hours/day)

The controller also consumes energy — even when the pump is off, the controller circuit draws standby current. The actual standby current should be measured on the sample. On small systems, the standby drain can be a meaningful fraction of the daily energy budget — measure it rather than estimating.

Charging Conditions and Low-Light Margin

The system is sustainable when the daily energy harvest exceeds the daily energy consumption:

Daily harvest > Daily pump energy + Daily standby energy

The low-light margin is the ratio of daily harvest to daily consumption. A high margin means the system can tolerate several cloudy days before the battery depletes. A low margin means one cloudy day may be enough to stop the system.

For winter use (short days, low sun angle, frequent clouds), the low-light margin is critical. A system that works in summer may not work in winter — this is not a defect, it is the physics of solar energy. But the customer needs to know.

What Data a Supplier Should Provide for a Matched Power System

To evaluate whether the power system is matched, the supplier should provide:

• Panel rated wattage and voltage
• Battery voltage and capacity (and total energy in Wh — V × Ah)
• Pump operating voltage and current draw (measured, not just rated)
• Controller standby current (measured when pump is off)
• Estimated daily energy consumption — Based on a typical watering schedule. The buyer should verify this estimate against measured consumption.

If the supplier cannot provide all of these, the buyer can measure the missing values directly with a multimeter and a stopwatch. The key is to verify that the daily harvest exceeds the daily consumption by a comfortable margin — what constitutes a sufficient margin depends on the target climate and expected weather variability. Measure the actual harvest and consumption at the target installation conditions, and use those measured values to determine whether the system is adequately matched.