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 (litres/day) — total volume needed each day. Typical UK targets: 45–70 litres/day per cow at pasture, 6–10 litres/day per sheep, 25–35 litres per laying hen flock of 100, 4–6 litres per square metre per week for irrigated horticulture in summer, 80–120 litres per person per day for off-grid mains-equivalent supply.
2. Total dynamic head (m) — vertical lift from water surface to discharge plus pipe friction plus any required discharge pressure. For a typical borehole feeding an open trough, this is the pumping water level in the borehole in metres.
3. Peak sun hours/day — annual-average daily irradiance for your site. Typical UK figures: Brighton/Portsmouth 3.0, Bristol 2.9, Midlands 2.8, Manchester 2.7, Glasgow 2.5, Inverness 2.4. The PVGIS portal from the European Commission’s Joint Research Centre gives a precise value for any UK postcode.
4. Pump wire-to-water efficiency (%) — overall efficiency of the pump end-to-end, leave it at 45% for a submersible if you don’t have a manufacturer pump curve.
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% on a south-facing roof with poor airflow.
6. Panel wattage (W) — your chosen panel. The 2026 standard MCS-listed module is 400–435 W; 540 W bifacial modules are common in 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 is usually no battery 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 midday, 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 overcast UK weather while a grid-style pump would short-cycle.

The pump itself is typically a brushless DC submersible (for boreholes) or a surface-mounted DC pump (for streams, ponds, and shallow wells). Grundfos SQFlex, Lorentz PS2, and Shurflo are the dominant brands in UK rural water applications. Helical-rotor pumps from Lorentz are preferred for high-head, low-flow applications — moving 500–3,000 L/day from 60–150 m depth.

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 metre), gravity (9.81 m/s²), volume, and height.

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. The electrical energy the pump must draw is determined by wire-to-water efficiency and further system losses:

E_electrical_Wh = E_hydraulic_Wh / (η_pump × η_system)

The PV array size in watts-peak is the electrical energy divided by peak sun hours:

PV_Wp = E_electrical_Wh / PSH

Worked example

A 3,500 L/day smallholding livestock supply at 30 m of head, in Devon (PSH 3.0 annual mean), with a Grundfos SQFlex pump (45% wire-to-water), 85% system efficiency, 400 W panels:

• V_m3 = 3.5 m³
• H_m = 30 m
• E_hyd = 3.5 × 30 × 2.725 = 286 Wh
• E_elec = 286 / (0.45 × 0.85) = 748 Wh
• PV needed = 748 / 3.0 = 249 Wp
• Panels = ceil(249 / 400) = 1 panel for annual mean

But UK December PSH is closer to 1.0, so the array sized for the worst month would be 748 / 1.0 = 748 Wp — two 400 W panels. Most UK installers oversize on this basis, accepting that summer output goes to filling a larger header tank.

UK sizing rules of thumb

For solar-direct pumping with no batteries and a storage tank in the UK climate:

• Design the storage tank to hold 4–7 days of demand. UK winter irradiance is highly variable and you want comfortable margin during November–February.
• Size the array for the worst sun month at your location (typically December or January), not the annual average.
• Add 25–50% PV oversizing to the worst-month calculation.
• For UK livestock pumping at typical 30 m boreholes, expect 0.5–0.8 Wp per litre-per-day at annual mean PSH, or 1.5–2.5 Wp per litre-per-day if you size to December alone.

Pump types compared

For most UK smallholding livestock applications under 10,000 L/day from a borehole under 60 m, a Grundfos SQFlex or Lorentz PS2 submersible is the standard choice. For deeper boreholes (90 m+), a Lorentz PS2 with the helical-rotor cartridge maintains flow at low input power where a centrifugal would stall.

UK incentives and grants

• Countryside Stewardship (Defra) — water-management capital items occasionally include solar pumping for off-mains livestock drinking water. Item codes change each scheme year; check the current Countryside Stewardship Grants list.
• Catchment Sensitive Farming (Defra/Environment Agency) — pays for water-quality improvements that displace pumping from sensitive watercourses. Solar pumping to replace stream-fed troughs has been funded in priority catchments.
• Sustainable Farming Incentive (SFI) — the SFI replacement for BPS has water-management actions; check whether your land qualifies through the Rural Payments service.
• Smart Export Guarantee — does not directly cover solar pumping, but if your pump array also exports to the grid, the SEG tariff applies to exported units.
• VAT relief — solar PV installations for residential use carry 0% VAT under HM Treasury’s 2022 measure, valid through 2027.

Common mistakes that hurt performance

• Sizing to annual-average PSH instead of December. A system sized to the annual mean of 2.8 PSH will deliver only 35% of demand in a typical UK December.
• Using static water level instead of pumping level. Drawdown can be 5–15 m on Cretaceous chalk or Permo-Triassic sandstone aquifers; sizing to static under-sizes the array.
• Skipping the tank. A solar-direct system without storage delivers nothing on cloudy days. A 5,000 L IBC plus a smaller pump beats a larger pump with no tank every time.
• Cheap pumps that aren’t MPPT-compatible. A 12 V DC pump from a marine catalogue without a proper MPPT controller wastes 25–35% of the panel output across a normal sunny day.
• Sub-standard wire sizing. UK Building Regulations Part P doesn’t cover DC pump wiring in detail, but BS 7671 applies — voltage drop above 3% on the pump string causes nuisance trips during low-irradiance starts.

Sources

• Energy Saving Trust — Solar PV technical resources — UK irradiance and PV sizing data
• MCS — Microgeneration Certification Scheme — accredited installer database
• Solar Energy UK — industry guidance and cost data
• PVGIS — European Commission JRC photovoltaic geographical information system — peak sun hour data by UK postcode
• Defra Countryside Stewardship Grants — water-management capital items
• Lorentz PS2 sizing manual — pump curves and selection charts referenced for efficiency defaults