How to Size a Solar Water Pump: The Only 3 Numbers You Need

The short answer: to size a solar water pump you need three numbers — your well depth (total head), your daily water requirement, and your location’s peak sun hours. Match those to a pump’s head-flow curve, and you have your system. Everything else is detail.

Most buyers I talk to start by asking “what HP pump should I buy?” — but horsepower alone tells you almost nothing. A 3HP pump can deliver 5 m³/day or 40 m³/day depending on how deep your well is. The right question is always about your three numbers.

Step 1: Find your total head (well depth + losses)

Total head is not just how deep your well is. It’s:

• Static water level — how far down the water sits when the pump is off
• Drawdown — how much further the level drops while pumping
• Pipe friction losses — resistance in your pipes (usually 5–10% extra)
• Delivery height — how high you push water above ground (to a tank)

For a quick estimate, take your well depth and add 10%. A 80m well ≈ 88m total head.

Step 2: Calculate your daily water need

Add up everything you need in a day, then add 20% buffer for cloudy days.

Step 3: Check your peak sun hours

Peak sun hours (PSH) is how many hours of full-strength sun your location gets per day. Most sunny farming regions get 4–6 PSH. Your pump only produces full output during these hours, so:

Required pump flow = daily water need ÷ peak sun hours

Need 20 m³/day with 5 sun hours? Your pump must deliver 4 m³/hour.

Step 4: Match to a pump curve

Here’s a typical head-flow reference for two common submersible models (values at ~5 peak sun hours):

Read it like this: a 3HP pump that pushes 12 m³/h at 50m only manages 3 m³/h at 120m — same pump, very different output. That’s why your well depth changes everything. Message me your head and I’ll point you to the exact model.

Common sizing mistakes I see every week

1. Buying by horsepower alone — a “5HP pump” means nothing without knowing your head
2. Ignoring pipe losses — long horizontal runs and small pipes silently cut your output
3. No water tank — without storage, cloudy days mean no water; a tank lets you pump hard on sunny hours
4. Sizing for the worst month — size for your average sun, store water for the rest

When to ask for help

If your well is deeper than 100m, your daily need is over 50 m³, or you’re supplying a whole village — message me your three numbers on WhatsApp. I’ll check your sizing and suggest a system, free.