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. Here’s the full method, step by step.

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

Total head is not just how deep your well is. It’s the sum of four things:

• Static water level — how far down the water sits when the pump is off (not the total well depth)
• Drawdown — how much further the level drops while pumping (more for sandy or low-yield wells)
• Pipe friction losses — resistance in your pipes, usually 5–10% of the vertical head, more with long horizontal runs or small-diameter pipe
• Delivery height — how high you push water above ground (to a tank or tap)

The quick method: take your well depth and add 10%. A 80m well ≈ 88m total head. Good enough for a first pass.

The accurate method: static water level + drawdown + (horizontal pipe length ÷ 10) + delivery height. Example: static level 60m, drawdown 8m, 50m horizontal run, 5m delivery height → total head = 60 + 8 + 5 + 5 = 78m.

If you don’t know your static level, measure it with a simple weighted string when the pump has been off for a few hours. It’s the single most valuable measurement you can take.

Step 2: Calculate your daily water need

Add up everything you need in a day, then add 20% buffer for cloudy days. For irrigation, count all your planted area — a common mistake is sizing for current crops and ignoring expansion plans or dry-season planting.

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.

Here are typical PSH values by region (annual averages):

Use the worst reasonable month, not the yearly average, if you need water year-round. If you have a tank, you can size for the average and store water for the bad weeks.

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.

Worked example: You need 20 m³/day, your total head is 78m, and you get 5 PSH. Required flow = 20 ÷ 5 = 4 m³/h at 78m. Looking at the table, the 3HP model delivers 7 m³/h at 80m — comfortably above 4 m³/h. The 3HP is your pump, and you’ll draw only ~7 × 5 = 35 m³ on a good day, meaning you can pump fewer hours or fill a tank faster. If your head were 120m instead, the same 3HP would only give 3 m³/h — under your 4 m³/h target — and you’d need the 5HP model.

Step 5: Size the panels (it follows the pump)

Once the pump is chosen, panels are simple: pump input × 1.3–1.5. A 3HP pump with ~2,200W input needs ~3,000W of panels, typically 6 × 550W. Full details in my solar panel guide.

Step 6: Add a water tank — don’t skip this

The tank is what makes solar work. It decouples “when the sun shines” from “when you need water”:

• Pump fills the tank during sunny hours (4–6 hours)
• You draw from the tank all day and night
• Cloudy days still deliver water because the tank was filled yesterday

Tank size = 1–2 days of your daily need. For 20 m³/day, a 20–40 m³ tank. This is cheaper than batteries, lasts decades, and needs no maintenance. On most sites I’d rather see a bigger tank than a bigger pump.

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
5. Measuring well depth instead of static water level — the pump works against the water level, not the drilled depth
6. Forgetting the delivery height — pumping to an elevated tank adds real head that some buyers never count

When to ask for help

If your well is deeper than 100m, your daily need is over 50 m³, you’re supplying a whole village, or your water is sandy (which changes pump material choice), message me your three numbers on WhatsApp. I’ll check your sizing and suggest a system, free — and I’ll tell you straight if your numbers point to something you didn’t expect.

Questions about sizing or models? Message me on WhatsApp — free, no signup.