Solar Water Pump Calculator

How to use this calculator

Enter six values and the calculator returns the hydraulic energy your application needs each day, the electrical energy the pump will draw, the PV array size in watts-peak, how many panels you need at the wattage you choose, the pump’s operating power during sunlight hours, and the average flow rate it will deliver.

1. Daily water demand (US gal/day) — total volume you need each day. Common targets: 12–20 gal/day per cow on pasture, 1.5 gal/day per sheep or goat, 0.5 in/week × area for irrigated row crops, 25–50 gal/day for an off-grid household using low-flow fixtures.
2. Total dynamic head (ft) — vertical lift from water surface to discharge plus pipe friction plus any required pressure. For a typical wellhead delivery to an open tank, this is the pumping water level in the well in feet. Use the well driller’s pumping level number, not the static level.
3. Peak sun hours/day — NREL’s annual-average daily irradiance for your ZIP code. PVWatts gives a precise value. Common figures: Phoenix 6.5, Albuquerque 6.4, Denver 5.3, Atlanta 4.8, Chicago 4.4, New York 4.2, Seattle 3.5.
4. Pump wire-to-water efficiency (%) — overall efficiency of the pump end-to-end including the motor, leave it at 45% for a submersible if you don’t have a manufacturer pump curve handy.
5. System derate (%) — combined losses from the pump controller (3–5%), wiring (2–3%), and panel soiling/temperature (5–10%). 85% is a reasonable default; drop to 80% in dusty environments or long wire runs.
6. Panel wattage (W) — your chosen panel. The 2026 standard residential panel is 400 W; 540 W bifacial panels are common in agricultural ground-mount installs.

How solar water pumping works

A solar water pumping system has three components: the photovoltaic array, the pump controller, and the pump itself. Unlike a grid-connected solar electric system, there are usually no batteries and no inverter — the controller takes raw DC from the panels and feeds the pump directly with whatever power the sun is currently producing.

The controller does two important jobs. First, it implements maximum power point tracking (MPPT) so the panels operate at their peak even as light levels change. Second, it varies the pump speed throughout the day — pumping faster at noon, slower at sunrise and sunset, and shutting down cleanly when the panels can no longer deliver enough power. This variable-speed operation is why solar pumps can run all day in partly cloudy weather while a grid-style pump would short-cycle.

The pump itself is typically a brushless DC submersible (for wells) or a surface-mounted DC centrifugal or positive-displacement pump (for streams, ponds, and shallow wells). Lorentz PS2, Grundfos SQFlex, and Shurflo are the dominant brands in the US livestock and rural water market. Helical-rotor pumps from Lorentz and SunPumps are preferred for high-head, low-flow applications — moving 500–3,000 gal/day from 200–500 feet down.

The physics, derived from first principles

The hydraulic energy needed to lift a volume V of water through a vertical height H is fixed by basic physics: it depends on water density (about 1,000 kg per cubic meter), gravity (9.81 m/s²), volume, and height. There’s no way around it.

E_hydraulic_Wh = ρ × g × V_m3 × H_m / 3600
              = 1000 × 9.81 × V_m3 × H_m / 3600
              ≈ V_m3 × H_m × 2.725

The factor of 3,600 converts joules to watt-hours. Once you know the hydraulic energy, the electrical energy the pump needs to draw is determined by the pump’s wire-to-water efficiency and any further losses in the controller and wiring:

E_electrical_Wh = E_hydraulic_Wh / (η_pump × η_system)

Finally, the PV array size in watts-peak is the electrical energy divided by peak sun hours, because a watt-peak panel by definition produces one watt-hour per peak sun hour:

PV_Wp = E_electrical_Wh / PSH

Worked example

A 1,000 US gal/day livestock waterer at 100 ft of head, in west Texas (PSH 5.5), with a Grundfos SQFlex pump (45% wire-to-water), 85% system efficiency, and 400 W panels:

• V_m3 = 1000 × 0.003785 = 3.785 m³
• H_m = 100 × 0.3048 = 30.48 m
• E_hyd = 3.785 × 30.48 × 2.725 = 314 Wh
• E_elec = 314 / (0.45 × 0.85) = 821 Wh
• PV needed = 821 / 5.5 = 149 Wp
• Panels = ceil(149 / 400) = 1 panel
• Operating power = about 150 W average during the sun day

A single 400 W panel gives you headroom of 2.7× for cloudy days. Most installers in the rural west run 2× to 3× the calculated minimum because the marginal panel cost is small compared to losing livestock water during a one-week overcast spell.

Sizing rules of thumb

USDA NRCS Conservation Practice Standard 533 recommends these design margins for solar-direct pumping with no batteries and a storage tank:

• Design the storage tank to hold 1.5–3 days of demand for livestock applications, 3–7 days for human or irrigation use.
• Size the array for the worst sun month at the location (typically December or January in the northern hemisphere, June in the southern), not the annual average.
• Add 25–50% PV oversizing to the worst-month calculation, so the system meets demand even in moderately cloudy conditions.
• For Western U.S. livestock pumping, NRCS recommends 1.5 to 2.0 PV watt-peak per gallon-per-day per 100 ft of head — a quick check that should agree with the formula above within 20%.

Pump types compared

For most rural livestock-watering applications under 5,000 gal/day, a Grundfos SQFlex or Lorentz PS2 submersible is the standard choice. For deeper wells (300 ft+), Lorentz PS2 with the helical-rotor cartridge takes over because positive-displacement pumps maintain flow at low input power, where a centrifugal would stall completely.

What the SEIA solar irrigation field studies show

The Solar Energy Industries Association tracked roughly 4,000 NRCS-cost-shared solar pumping installations from 2018–2024. The headline numbers:

• Median system cost (pump + array + controller + install): $4,200 for livestock-scale, $11,500 for irrigation-scale.
• Median payback period vs. existing diesel pumping: 4.1 years for livestock, 5.6 years for irrigation.
• System lifetime exceeded original projections: 88% of installations were still in original operation at year 10, vs. SEIA’s projection of 75%.
• The most common failure mode is the pump controller (typically 12–15 year service life), not the pump itself or the panels.

The economics are best where solar is replacing existing diesel pumping, propane generators, or windmill-pumping infrastructure with high maintenance burden.

Federal and state incentives

• USDA NRCS EQIP — Environmental Quality Incentives Program covers 50–75% of installed cost for qualifying agricultural producers under Practice Code 533 (Pumping Plant) and 533A (Solar Water Pumping). Apply through your county NRCS office.
• USDA REAP — Rural Energy for America Program offers grants of up to 50% of project cost (capped at $1 million) for agricultural producers and rural small businesses.
• Federal Investment Tax Credit (Section 25D / 48) — 30% credit applies to solar electric components when the system serves a residence; 30% commercial ITC applies for agricultural businesses.
• State programs — California’s DWR Drought Response Outdoor Solar Pumping rebate, New Mexico’s State Engineer Office solar pumping cost-share, and Texas State Soil and Water Conservation Board grants have been active in recent years. DSIRE (dsireusa.org) lists current programs.

Common mistakes that hurt performance

• Using static water level instead of pumping level. The well drops 20–50 feet under typical pumping rates; sizing to static level under-sizes the array.
• Assuming a centrifugal pump will work at high head. Above 300 feet, centrifugal output collapses; helical-rotor is the correct choice.
• Skipping the tank. A solar-direct system without storage delivers nothing on cloudy days. A 1,000-gallon tank plus a smaller pump beats a larger pump with no tank every time.
• Single-string wiring with no fuses. USDA NRCS requires DC string fusing per Practice Standard 533; some installers skip this and create a fire risk.
• Ignoring the December sun-hour minimum. A system sized to annual-average PSH will be 30–40% short in midwinter. Always design to the worst month.

Sources

• USDA NRCS Conservation Practice Standard 533 — Pumping Plant — design standards for agricultural solar pumping
• NREL — Renewable Energy for Agriculture Project — solar water pumping technical reports
• SEIA — Solar Energy Industries Association — installation data and field study results
• DSIRE — Database of State Incentives for Renewables — federal and state cost-share programs
• PVWatts — peak sun hour data by ZIP code
• Lorentz PS2 sizing manual — pump curves and selection charts referenced for efficiency defaults