How to Calculate Total Dynamic Head (TDH) for a Solar Pump

① Static water level

The distance from the ground surface down to the water surface when the pump is not running. Usually stated in your well driller’s report. If you don’t have a report, lower a weighted string into the well until it gets wet, then measure.

② Drawdown

When the pump runs, it pulls water faster than the aquifer can replenish locally, so the water level drops. The difference between the static level and the pumping level is drawdown. This varies by aquifer — it can be 2m or 30m. Without a pump test, assume drawdown = 15–20% of static level.

③ Vertical pipe friction loss

Water moving through pipes loses energy to friction. The losses depend on pipe diameter, length, flow rate, material, and number of elbows. Quick rule: for a typical farm setup (2” PVC, under 100m vertical), add 15% to the sum of static + drawdown + delivery.

④ Horizontal pipe distance

This is often overlooked. If your water tank or irrigation point is far from the wellhead, the horizontal pipe run adds significant friction loss. The standard rule:

Horizontal pipe loss = Horizontal distance (m) ÷ 10

Every 10 meters of horizontal pipe adds roughly 1 meter of equivalent head loss. For example:

• 50m horizontal run → +5m head
• 100m horizontal run → +10m head
• 200m horizontal run → +20m head

This applies to typical 2”–3” pipes. For smaller pipes or very long runs (>200m), the loss is proportionally higher — consult a pipe friction calculator.

⑤ Delivery height

The vertical distance from the ground surface to the top of your storage tank (or the highest point water needs to reach). If your tank sits on a 5m tower, your delivery height is 5m.

The complete formula

TDH = (Static level + Drawdown + Delivery height) × 1.15  +  Horizontal pipe length ÷ 10

Or with a diagram:

Vertical head  = Static + Drawdown + Delivery
Vertical friction = Vertical head × 15%
Horizontal loss = Horizontal pipe meters ÷ 10
TDH = Vertical head + Vertical friction + Horizontal loss

Quick shortcut (only works if horizontal pipe is short, <30m):

TDH ≈ Well depth × 1.15

If you have significant horizontal pipe distance, you must add it separately. Many failed pump installations are caused by ignoring horizontal pipe loss.

Example calculations

Example 1: Farm well, short pipe to tank

→ Choose a pump with rated head ≥ 87m

Example 2: Deep well, long horizontal run

→ This is a challenging deep well with long pipe run. Send me your details on WhatsApp — I’ll confirm the right model.

Example 3: Shallow well, elevated tank, medium distance

→ A 1.5HP submersible or surface pump will work.

Why getting TDH wrong is expensive

If you underestimate TDH:

• The pump can’t reach the required head → flow drops to nearly zero
• Motor overheats running at the edge of its curve
• You waste money replacing with the right pump

If you overestimate TDH:

• You buy a bigger pump than needed → higher cost, more solar panels
• The pump runs off its best efficiency point → shorter lifespan

Rule of thumb: it’s better to have a pump rated 10–20% above your TDH than right at the limit. Pumps are most efficient and longest-lasting at 60–80% of their maximum rated head.

Quick reference

Next step: once you know your TDH and daily water need, use the sizing tool to find the right pump in 30 seconds — or message me on WhatsApp and I’ll calculate it for you, free.