Solar Water Pump Array Sizing

ScopeThe daily demand and the lift set the array, and the pump runs only while the sun is up. This calculator follows the USDA NRCS North Dakota Technical Note No. 1 method. The design flow is the daily demand divided by the design-month peak sun hours, not by twenty-four; the pump input power is 0.1885 W per gpm-ft of head and flow divided by the pump efficiency; the minimum array is that power divided by the panel derate, typically 0.85. The technical note's own inputs -- 1,000 gallons per day, 2.52 peak sun hours, 150 ft of total dynamic head, 60% pump efficiency -- give a 6.61 gpm design flow, 311.6 W of pump input, and a 366.6 W minimum array; the note reads 310 W off its pump curve, and the difference is the curve's efficiency at that operating point against the flat 60% entered, so the curve governs. The hydraulic workload, lift times daily volume in metric, is the NRCS screen for whether solar is the right tool: 173.1 m^4 here, well under the 1,500 m^4 where solar stops being the simple answer. Storage carries the night and overcast days; three days of demand, 3,000 gallons here, is the North Dakota requirement and the editable default. A field clinic well at 600 gallons per day, 4.5 sun hours, 180 ft, and 45% needs about a 197 W array. It does not select a pump or read a pump curve, compute head or drawdown, find peak sun hours, or size wiring, controllers, or batteries. The NRCS technical note, the pump and module manufacturers' data, and the AHJ govern.

Formula or tableQ (gpm) = daily demand (gpd) / (peak sun hours x 60) (Eq. 7.1); P_in (W) = 8.34 / 33,000 x 745.7 x TDH (ft) x Q (gpm) / pump efficiency, 0.1885 W per gpm-ft (Eq. 7.3); array = P_in / derate (Eq. 7.5); workload (m^4) = TDH (m) x daily volume (m^3) (Eq. 5.1); storage = days x daily demand.

Source and editionUSDA NRCS North Dakota Technical Note No. 1, Design of Small Photovoltaic (PV) Solar-Powered Water Pump Systems (July 2017), Equations 5.1, 7.1, 7.3, and 7.5 and the 3-day storage requirement, by name. The pump manufacturer's performance curve governs.

Free accessA free USDA technical note; the equations are restated and evaluated on the user's own demand, head, and efficiency.

What governsEstimate. Operator of record and primacy agency govern.

API field namesdaily_demand_gpd, peak_sun_hours, tdh_ft, pump_efficiency, panel_derate, storage_days, design_flow_gpm, pump_input_w, array_min_w, workload_m4, storage_gal

• Flat pump efficiency entered as one number; the manufacturer's curve sets the actual operating point and efficiencyNRCS ND Technical Note 1
• Panel derate 0.85 by default for temperature, soiling, and wiring lossesNRCS ND Technical Note 1 Eq. 7.5
• Three days of storage the North Dakota NRCS requirement, used as an editable defaultNRCS ND Technical Note 1