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Solar Irrigation Pivot Calculator

Enter your center-pivot pump size and irrigation season — get solar system size, battery for cloudy days, diesel cost comparison, and USDA REAP grant estimate.

Center-pivot irrigation system
ft
HP
hrs/day
months
ft
Solar irrigation pivot analysis
1,112 × 400W panels — 444 kW DC system
Pump motor draw149.2 kW
Daily kWh during season3,282 kWh/day
Seasonal kWh (annual)393,888 kWh/season
Battery for 2 cloudy days4,267 kWh
Annual grid electric cost (alt)$39,389/yr
Diesel pump fuel (annual)10,782 gal/yr
Annual diesel cost$45,283/yr
Gross system cost$3,597,356
USDA REAP grant (est. 40%)−$500,000
Net cost after REAP grant$3,097,356
Payback vs grid electric121.0 yrs
Payback vs diesel pump68.4 yrs
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How to Use This Calculator

Enter your pivot and pump specifications

Start with the pivot arm length — quarter-mile (1,320 ft) covers roughly 130 acres; half-mile (2,640 ft) covers about 500 acres. Enter the pump motor horsepower from the nameplate on your motor. Center-pivot pumps typically run 75-300 HP depending on well depth, pressure requirements, and acreage. Enable the end-gun toggle if your system has a gun sprinkler at the tip of the pivot arm — end guns extend coverage into field corners but add roughly 10% pump load.

Enter operating hours and season

Most center pivots run 20-22 hours per day at peak season — completing one full revolution per day (some operations do 2-day revolutions). Enter your irrigation season length in months: corn belt operations typically run April through August (5 months); Texas and Southwest operations can run 5-7 months. Water table depth affects pump head requirements, which is already factored into your stated HP.

Read the results

The calculator shows pump kW draw, daily and seasonal kWh, recommended solar system size (sized at 65% of peak load — a practical seasonal offset strategy), battery sizing for cloudy days, and full comparison against both grid electric and diesel pump alternatives. USDA REAP grant eligibility is calculated at 40% of project cost (maximum $500,000).

The Formula

Pump kW = HP × 0.746 (+ 10% if end gun enabled) Daily kWh = Pump kW × Operating Hours/Day Seasonal kWh = Daily kWh × (Season Months × 30 days) Solar System kW = (Daily kWh × 0.65 offset) ÷ Peak Sun Hours ÷ 0.80 Battery kWh = Daily kWh × 0.65 × 2 cloudy days Diesel Gal/Hr = (Pump kW ÷ 0.90 motor eff) × 3,412 BTU/kWh ÷ 138,500 BTU/gal REAP Grant = min(System Cost × 40%, $500,000) Net Cost = System Cost − REAP Grant

The solar system is intentionally sized at 65% of the daily load rather than 100%. This is a practical agricultural strategy: the array handles the majority of energy during sun hours, the utility grid or battery supplements during morning startup and cloudy periods, and the system is sized to an economical panel count. Full 100% offset would require an impractically large array for a load that may run 22 hours/day.

Example

Hendricks Farm — Quarter-mile Pivot, 200 HP, Lubbock TX

A West Texas cotton farm runs a quarter-mile center pivot with a 200 HP electric pump, 22 hours/day for 4 months during growing season. They're considering solar to reduce their $12,000/summer electric bill and qualify for USDA REAP.

Pivot length1,320 ft (quarter-mile)
Pump motor200 HP
Operating hours22 hrs/day
Season4 months (May-Aug)
LocationLubbock, TX (6.0 PSH)

Result

Pump draw149.2 kW
Daily kWh3,282 kWh/day
Seasonal kWh393,840 kWh/season
Solar system~446 panels, 178 kW DC
Battery (2 cloudy days)~4,267 kWh
Gross system cost~$503,000
USDA REAP grant−$201,000 (40%)
Payback vs grid~16 yrs
Payback vs diesel~5 yrs

The economics against a diesel pump are compelling — diesel at $4.20/gal for a 200 HP pump running 4 months costs over $100,000 per year. Farmers switching from diesel to solar-electric pivots can achieve payback in 4-6 years after REAP grants, with 20+ years of near-zero fuel cost operation thereafter.

FAQ

Solar can offset 60-80% of a center-pivot pump's seasonal energy when sized properly. Full 100% offset is theoretically possible but impractical for pumps running 20-22 hours/day — you'd need a very large array plus large battery storage for nighttime operation. The most cost-effective strategy is a solar array sized to cover daylight hours (6-8 peak sun hours), grid or battery for nighttime, and REAP grant funding to improve economics. Some farms use solar to power auxiliary loads (control systems, lighting, monitoring) and a separate solar+battery system for the main pump during daytime hours.
The USDA Rural Energy for America Program (REAP) provides grants and loan guarantees to agricultural producers and rural small businesses for renewable energy and energy efficiency projects. Solar water pumping systems for irrigation are explicitly eligible. Grants cover 25-50% of eligible project costs up to $1 million (recently increased). The application process involves a project proposal, energy audit, and documentation — typically handled by a solar installer with REAP experience. The program is competitive; applications with higher energy savings and rural impact score better. Apply early — REAP funding rounds are annual.
For new installations or diesel pump replacement, solar-electric wins decisively. A 200 HP diesel pump running 4 months costs $80,000-120,000 per year in fuel. A solar-electric system with REAP grant can be installed for $150,000-300,000 net cost — a 2-4 year payback. Diesel also requires expensive maintenance (fuel filters, injectors, engine overhauls every 3,000-5,000 hours). Solar-electric motors have minimal maintenance and 20-25 year lifespans. The main advantage of diesel is independence from the grid — important in areas with unreliable rural electric service, which is exactly where REAP grant-funded solar+battery systems make the most sense.
End guns are sprinkler heads mounted at the tip of the pivot arm that spray water outward when the pivot is in field corners, increasing coverage from the circular pivot pattern to cover more of the square field. When active, end guns require additional pump pressure and flow, adding approximately 10% to the pump's power draw. This calculator adds 10% to the stated HP when end gun is enabled. In practice, end guns operate intermittently (only in corners, typically 20-30% of pivot rotation), so the actual average power increase is closer to 2-4% — this calculator takes a conservative approach.
Battery sizing for agricultural irrigation depends on your risk tolerance for cloudy-day pump stoppage. The calculator sizes battery for 2 cloudy days of operation at the solar-offset fraction of load — this is appropriate for most operations where missing a day or two of irrigation is acceptable. For high-value crops (vegetables, orchards) that can't tolerate irrigation gaps, size for 3-5 days of backup. Battery costs for large irrigation systems can be significant ($550/kWh for LFP), so many farmers choose to maintain a grid connection as backup rather than relying purely on battery. The battery mainly fills morning and evening gaps when solar panels aren't at full output.

Related Calculators

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🏭 Off-Grid Solar Calculator
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  title="Solar Irrigation Pivot Calculator"></iframe>