Solar Irrigation System Calculator
Size a solar irrigation system for UK horticulture, polytunnels and small holdings using ETc, MCS-aligned PV data and Energy Saving Trust guidance.
Solar Irrigation System Calculator
How to use this calculator
Enter eight values and the calculator returns daily water demand, hydraulic energy, electrical energy, the recommended PV array in watts-peak, the number of panels at your chosen wattage, pump operating power, and average flow rate during sun hours.
- Field area (hectares) — area actually irrigated. For polytunnels, this is the covered ground area; for outdoor crops, the canopy area.
- Crop ETc (mm/day) — daily reference ET times crop coefficient. UK peak-summer ETc: potatoes 4.6, brassicas 4.2, soft fruit 4.0, polytunnel tomatoes 4.4–4.8. Met Office MORECS gives ET₀ per 40 km grid square.
- Irrigation efficiency (%) — 90% drip, 85% microsprinkler, 75% sprinkler, 65% travelling gun. Use AHDB measured distribution uniformity if available.
- Total dynamic head (m) — vertical lift from borehole pumping water level or surface source to discharge plus pipe friction plus emitter pressure (drip 7–14 m, sprinkler 28–55 m).
- Peak sun hours/day — annual or worst-month average. Typical UK PSH: London 2.8, Bristol 2.9, Manchester 2.5, Edinburgh 2.4, Norwich 3.0, Brighton 3.1, Belfast 2.5. Met Office MIDAS data and PVGIS feed the figures.
- Pump wire-to-water efficiency (%) — 45% for a typical solar submersible (Grundfos SQFlex, Lorentz PS2) without a specific pump curve.
- System derate (%) — combined controller, wiring and soiling losses. 85% is the recommended default.
- Panel wattage (W) — 400 W is the 2026 UK residential standard; 540 W bifacial dominates field-scale ground-mount.
How solar irrigation works in the UK
A solar irrigation system uses a PV array, MPPT pump controller, DC pump and an irrigation network. The array sends raw DC to the controller, which tracks the maximum power point and varies pump speed throughout the day. The pump moves water from a borehole, river abstraction, rainwater cistern, or reservoir into filtration and then the field distribution network.
UK growers typically buffer the system with a header tank because British weather produces frequent cloud-passage events that would otherwise cycle the pump on and off all day. A 5,000-litre tank elevated 4–6 m above field level provides gravity emitter pressure and decouples pumping from irrigation scheduling. Water is pumped during the day and applied early morning or late evening when ET is lowest.
Borehole abstractions over 20 m³/day require an Environment Agency licence in England; below that, no licence is needed but the abstraction must still be registered. Wales, Scotland and Northern Ireland have parallel regimes through NRW, SEPA and DAERA.
The physics, first principles
Water demand from crop ET and irrigation efficiency:
V_L_day = ETc_mm × Area_m² / efficiency_fraction
V_m3_day = V_L_day / 1000ETc in millimetres per day times area in square metres equals litres per day directly, because 1 mm depth over 1 m² is 1 L.
Hydraulic energy to lift that water through TDH:
E_hydraulic_Wh = 1000 × 9.81 × V_m3 × H_m / 3600 ≈ V_m3 × H_m × 2.725Electrical input through pump and system losses:
E_electrical_Wh = E_hydraulic_Wh / (η_pump × η_system)
PV_Wp = E_electrical_Wh / PSHWorked example — half-hectare polytunnel salads, Lincolnshire
- Area = 5,000 m² (0.5 ha), ETc = 4.0 mm/day, drip efficiency 90%
- V = 4.0 × 5,000 / 0.90 = 22,222 L/day = 22.22 m³
- TDH = 30 m (borehole pumping level 18 m + 4 m filtration + 8 m emitter pressure)
- E_hyd = 22.22 × 30 × 2.725 = 1,817 Wh/day
- Pump η 45%, system η 85%: E_elec = 1,817 / (0.45 × 0.85) = 4,750 Wh/day
- PSH 2.8: PV = 4,750 / 2.8 = 1,696 Wp → five 400 W panels (2,000 Wp, 18% headroom)
Add another 25% PV oversizing for UK cloud days and that block needs six 400 W panels.
UK irrigation efficiency by method
| Method | Distribution efficiency | Pressure required |
|---|---|---|
| Subsurface drip | 88–95% | 7–14 m |
| Surface drip | 85–92% | 7–14 m |
| Microsprinkler | 80–88% | 14–21 m |
| Solid-set sprinkler | 75–85% | 28–55 m |
| Hose reel rain gun | 60–70% | 45–70 m |
| Furrow / sprayline | 50–70% | 5–15 m |
For solar pumping where PV cost scales with pump power, drip is the right answer for most UK horticultural applications. Hose reels with rain guns need 5–8 times the array of an equivalent drip system because both efficiency and operating pressure are unfavourable.
UK climate and seasonal planning
UK peak summer ETc is 4–5 mm/day in the south, dropping to 3–4 mm/day in the north. The active irrigation season runs May to September for outdoor crops and February to November for protected cropping. Three planning realities shape solar sizing:
- Worst-month PSH is the design constraint for protected growing. December PSH in Lincolnshire is 0.5; even with a 5× sized array, solar-direct pumping doesn’t work in winter. Protected-cropping systems need a mains backup or a battery-buffered system.
- Field-scale crops are summer-coincident. Peak ETc occurs in June and July when PSH is at its annual maximum, so solar-direct works well for outdoor potatoes, brassicas, salads and soft fruit.
- Variable summer cloud is the operational issue. A 20% PV oversizing reserve plus a 2-day header tank handles British weather well; without one, the pump cycles 200+ times per day in unsettled conditions, halving its expected service life.
UK incentives and grant support
- Defra Farming Investment Fund — Farming Equipment and Technology Fund and the Farming Transformation Fund have funded solar pumping equipment for water-stressed horticultural holdings. Open through Defra and the Rural Payments Agency.
- Annual Investment Allowance — £1 million capital allowance gives 100% first-year tax relief on solar pumping equipment for businesses.
- VAT 0% on energy-saving materials — solar PV components attracting the 0% relief through March 2027 under the energy-saving materials policy.
- Smart Export Guarantee — applies when surplus PV is exported to the grid, but solar irrigation systems are usually too small and too off-grid to qualify.
Common UK-specific mistakes
- Sizing to annual-average PSH. A pump sized to 2.8 PSH is 30% short in April–May when growers actually start irrigating. Use the lowest month of the active season, not the year.
- Ignoring borehole draw-down. UK chalk and limestone aquifers in the south can show 10–20 m of draw-down under typical horticultural pumping rates. Use the pumping water level from the abstraction test, not the rest level.
- Forgetting filtration head. A typical media filter for a drip system adds 6–10 m of TDH at design flow. UK borehole water often has manganese and iron that need additional pre-filtration, adding another 4–8 m.
- Treating polytunnel growing as outdoor irrigation. Polytunnel ETc is 15–25% lower than outdoor for the same crop because of reduced wind and saturation deficit; over-sizing to outdoor figures wastes pump capacity.
Sources
- Met Office MORECS evaporation service — daily ET₀ for UK grid squares
- AHDB Horticulture irrigation guides — crop coefficients, distribution uniformity benchmarks
- Environment Agency water abstraction licensing — borehole and surface abstraction rules
- FAO Irrigation and Drainage Paper 56 — reference ET methodology
- Energy Saving Trust — UK solar PV performance data
- MCS — solar PV installer standards — UK PV system certification framework
- Lorentz PS2 sizing manual — solar pump curves and efficiency defaults