How the numbers are made

Sun hours

Radiation data is the National Solar Radiation Database (NSRDB), built by NREL (renamed the National Laboratory of the Rockies), read through PVGIS v5.2 from the European Commission's Joint Research Centre, which serves NSRDB for the Americas. Alaska falls outside the NSRDB grid and is served from ERA5. Each state is modelled at its largest city, on a plane facing due south at 20 degrees of tilt. The run is shared with our sister site SolarPanelCosts.com.

• Annual peak sun hours per day = annual irradiation on the plane (kWh/m2) / 365.
• Monthly peak sun hours = PVGIS monthly output per kWp / days in the month / the state's annual performance ratio (annual yield / annual irradiation). One ratio for the whole year slightly overstates winter and understates summer, because panels are more efficient when cold.
• Design sun: the darkest month (year-round), the darkest of April to October (grazing season), or the June to August average (summer).

Water needs

• Beef cattle (cow or finishing steer): 20 gal per head per day. NDSU AS1763: about 16 to 21 gal/day for mature and finishing beef cattle at 90F.
• Milking dairy cows: 35 gal per head per day. NDSU AS1763: 30 to 41 gal/day at 80 to 100 lb of milk.
• Horses: 15 gal per head per day. NDSU AS1763: 12.7 gal idle at 86F, 21.7 gal working at 95F.
• Sheep or goats: 2.5 gal per head per day. NDSU AS1763: 2 to 3 gal for a lactating ewe (goats not listed; we use the ewe figure).
• Finishing pigs: 3 gal per head per day. NDSU AS1763: 2 to 3 gal for 100 to 250 lb finishers.
• People in the house: 50 gal per person per day. EPA puts average US home use near 82 gal a person; off-grid households typically plan on less.
• Garden or orchard under drip: 0.089 gal per sq ft per day. 1 inch a week of water is 0.623 gal per sq ft, or 0.089 gal per sq ft per day.

Hydraulics

• Flow (gpm) = gallons per day / (pumping hours x 60). Pumping hours are the design sun hours for direct drive, or the chosen run time with a battery.
• Total dynamic head = depth to pumping water level + rise to the outlet + pipe friction x (1 + fittings allowance) + outlet psi x 2.31.
• Friction (ft per 100 ft) = 0.2083 x (100 / C)^1.852 x gpm^1.852 / d^4.8655 (Hazen-Williams), with C = 140 for Polyethylene (PE, SIDR) and 150 for PVC Schedule 40. Schedule 40 PVC inside diameters follow ASTM D1785 average dimensions; polyethylene is sold by inside diameter.
• Velocity (ft/s) = 0.4085 x gpm / d^2. The calculator warns above 5 ft/s.
• Hydraulic power (W) = gpm x head (ft) / 3,960 x 745.7. Daily hydraulic energy (Wh) = gallons x head x 0.003141.
• Pump input = hydraulic power / wire-to-water efficiency (default 35%; small DC pumps typically range 25 to 50%).
• Surface pumps: the calculator warns when suction lift exceeds 20 ft.

Panels and batteries

• Direct drive: array watts = pump input / 0.75, allowing for panel heat, soiling, wiring, mismatch and the low-light hours a direct-drive pump cannot use.
• Battery: array watts = daily electrical Wh / 0.9 / (design sun hours x 0.75).
• Battery capacity = daily Wh x days of autonomy / 0.8 (LiFePO4 usable fraction), divided by system voltage for amp-hours.
• Storage tank for direct drive: three days of daily demand.

Prices and products

Prices are read from the merchant's own store and printed with the date. Product specifications are the merchant's. We have not tested any product. The calculator lists a linked pump only when its rated maximum head is at least 1.25 times your total dynamic head and its rated maximum flow at least 1.5 times your flow; that is a screen, not a match, and the pump curve decides.

What the model leaves out

• Shading, snow cover and local weather that differs from the modelled city.
• Well drawdown that changes with flow; use a measured pumping level.
• Voltage drop in long cable runs; size cable for under about 3% drop.
• The real shape of a pump's efficiency across its curve.