Solar Pump Sizing & Array, Panels & Pump HP

Start from the water you need each day and the total head it must lift against, work out the hydraulic power that demands, divide by the pump's wire-to-water efficiency to get input power, then add a derate for heat and dust to fix the solar array in watt-peak. Finally divide by your panel rating to get the panel count. This tool runs the whole chain for you.

The pump only runs while the sun shines, so the flow it must deliver is the daily water need spread over the usable sun hours. Operating flow Q = daily water (m³) ÷ (sun hours × 3600), which gives litres per second. For example 30 m³/day over 7 sun hours is about 1.19 L/s, or roughly 4.3 m³/hr.

Hydraulic power is the useful work of lifting water: P = ρ · g · Q · H, with water density ρ = 1000 kg/m³, gravity g = 9.81 m/s², Q the operating flow in m³/s and H the total head in metres. It is the power the water actually carries; the pump and panels must supply more to cover losses.

Wire-to-water efficiency is the share of electrical power that ends up as useful hydraulic power, after motor, pump and controller losses. Solar pumps typically sit around 35–45%, so input power = hydraulic power ÷ system efficiency. A lower efficiency means a bigger array for the same job, which is why pump quality matters.

Panels rarely deliver their rated watt-peak in the field. Heat, dust, wiring losses and off-perpendicular sun cut real output, so the array is oversized by a derate factor — here array Wp = input power ÷ 0.8. The 0.8 derate (a 20% margin) keeps the pump fed through normal field conditions rather than only at lab test conditions.

Divide the required array watt-peak by the rating of one panel: panels = array Wp ÷ panel Wp, rounded up. Larger modern panels (550–600 Wp) cut the count versus older 330 Wp panels. The tool reports the array size and the panel count for the rating you enter so you can plan the mounting structure.

Cutting the head is the most effective lever. Use a wider delivery pipe to lower friction loss, site the tank closer or lower, and avoid unnecessary lift — every metre of head removed shrinks the hydraulic power and therefore the array. Improving pump efficiency and spreading the draw over more sun hours also help.

A modest oversize is wise. On overcast days output falls and the daily volume drops, so adding 10–20% array or a little storage buffers the crop through dull spells and early-morning or late-afternoon low light. Avoid gross oversizing, though — it wastes money and can over-pump a weak borewell.

You should enter the total head — the static lift from water level to discharge plus friction losses in the pipe and any pressure needed at the outlet. Using only the static lift understates the work and undersizes the array. If you know the friction loss, add it to the static head before entering it.

They are sound design figures for planning an installation. Real performance varies with the specific pump curve, panel temperature, borewell drawdown, pipe sizing and local solar resource. Treat the array Wp, panel count and pump HP as a well-grounded starting point and confirm against the pump manufacturer's curve and a site survey.