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 total dynamic head (TDH), 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 the actual TDH. The right question is always about your three numbers. Here’s the full method, step by step.

Step 1: Find your total dynamic head (TDH)

Use the ground at the wellhead as the reference for water depth and delivery elevation:

• Static water level — water depth below that reference when the pump is off, not drilled depth.
• Drawdown — the extra drop at the intended pumping flow. Static level plus drawdown gives the dynamic (pumping) level.
• Delivery elevation — the outlet or tank water level above the same reference.
• Pipe and fitting losses — calculate at the required flow using actual internal diameter, total pipe length (including the riser), material/roughness, bends, valves and other fittings.
• Pressure head — any pressure required at the delivery point, expressed in metres of water (about 10.2 m per bar); zero for an open tank.

The method: TDH = static water level + drawdown + delivery elevation + pipe/fitting losses + pressure head. If you have measured the dynamic water level, use it directly instead of static level plus drawdown; do not add drawdown twice. Drilled depth determines pump placement and pipe length, not lifting head. Neither a fixed percentage of well depth nor a fixed horizontal-distance ratio gives reliable losses.

Hypothetical arithmetic example: static level 60 m, drawdown 8 m, delivery elevation 5 m, open tank (0 m pressure head). Suppose an independent pipe/fitting calculation at the design flow has supplied a loss of 5 m. Then TDH = 60 + 8 + 5 + 5 + 0 = 78 m. The 5 m loss is a given input for this example, not a calculation from pipe length; a real site needs its own loss calculation.

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

The table below is a hypothetical head-flow illustration, not verified data for real models. Hourly flow must come from the selected model’s measured curve at the specified speed and power; sun hours affect daily production, not the curve itself.

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. These illustrative numbers show why actual TDH matters. Message me your head and I’ll point you to the exact model.

Hypothetical worked example: using the 78 m TDH above, 20 m³/day over an assumed 5 equivalent hours at the design duty gives 20 ÷ 5 = 4 m³/h at 78 m. This is a target to check against a measured model curve, not proof that any 3 HP pump is suitable. The illustrative table only shows why flow falls as head rises. Confirm actual daily production for the local solar conditions, array and controller before choosing a model; 5 PSH does not guarantee 5 hours at a fixed flow.

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.

Before fixing the tank elevation, check gravity water distribution design so the stored water can deliver the required outlet pressure. If the house needs pressurized delivery instead of gravity alone, compare solar pump pressure tanks and switch settings before choosing the plumbing layout.

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. Using drilled depth as lifting head — use static water depth plus drawdown, or measured dynamic depth directly, then add delivery elevation, calculated losses and required pressure head
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.