Understanding solar sizing for irrigation pumps
What size solar system a typical pump needs
A 15 HP pump running at 88% motor efficiency draws about 12.72 kW, and running it 6 hours a day uses roughly 76.3 kWh of energy. At 5.5 peak sun hours with an 80% system derate, that requires a solar system of about 17.3 kW to fully cover the pump's daily energy needs. If irrigation is one part of a broader farm energy picture, the agricultural solar sizing calculator covers the whole-operation view, and the solar panel calculator in our system sizing section can translate this figure into an actual panel count. Once you have a size in mind, the loan payment calculator in our Financing section can help you compare loan, lease, and cash purchase options for the system you just sized.
Why the solar system needs to be larger than the pump's rating
The pump only draws its rated power while running, but the solar array only produces its full rated output during peak sun hours, which is a small fraction of the day. The system has to be sized so its total daily energy production, not just its instantaneous power, covers what the pump consumes over its full run time, a distinction the commercial inverter sizing calculator also has to account for when matching inverter capacity to array output. The details behind this are in guide to going solar.
Whether battery storage is needed for irrigation
Not necessarily, if irrigation timing can be scheduled to run during sunny daytime hours, which is common for many crop and field setups. Battery storage becomes more important if irrigation needs to happen at specific times regardless of sun availability, such as early morning or overnight watering schedules. The battery storage sizing calculator covers how to size storage for that scenario, and the backup runtime estimator in our battery storage section runs a similar load-matching calculation for a different variable daily load.
How a variable frequency drive changes the sizing math
Yes, a variable frequency drive lets the pump run at less than full power during lower-demand periods, which can reduce daily energy consumption compared to a pump that only runs at full rated power. Sizing with a VFD in mind typically requires input from the pump manufacturer or a solar irrigation specialist for an accurate figure, similar to how the three-phase system sizing tool needs manufacturer specs for larger motor loads. USDA NRCS Technical Note No. 1 on designing small PV-powered water pump systems walks through this same pump-to-array matching process in far more technical depth, including worked examples with real pump curves. The Department of Energy's guide to going solar covers the more general sizing principles this tool builds on.