How Much Power Does a Drip Irrigation Pump Need

How much power does a drip irrigation pump need? For the worked example on the calculator page, 600 litres per hour at 15.7 metres of total head needs about 51 watts at 50 percent efficiency. Your answer will differ if your flow, your head or your efficiency differs, so use the formula below with your own numbers.

The Power Formula in Plain Words

Pump power in watts comes from four numbers. The first is flow, in litres per second. The second is total head, in metres of water. The third is the constant 9.81, which comes from gravity. The fourth is efficiency, written as a decimal. The formula is power equals 9.81 times flow times head divided by efficiency.

Flow is the water your largest zone needs each second. Head is the total resistance the pump works against. Efficiency is the share of input power that becomes useful water power. A pump at 50 percent efficiency needs twice as much input power as the water power it delivers. Total head is lift plus pressure head plus friction. Lift is height from the water surface to the highest emitter. Pressure head gives the drip line its pressure. Friction is head lost in the supply pipe. To use the formula, divide litres per hour by 3600, multiply by 9.81 and by head, then divide by efficiency.

The Worked Example From the Calculator Page

The calculator page works one example in full. It uses 600 litres per hour at 1 bar, with 3 metres of lift and 50 metres of pipe at 5 percent loss. Pressure head is 1 times 10.2, which is 10.2 metres. Friction is 50 times 0.05, which is 2.5 metres. Total head is 3 plus 10.2 plus 2.5, which is 15.7 metres.

Divide 600 by 3600 to get 0.1667 litres per second. Multiply 9.81 by 0.1667 to get 1.635. Multiply 1.635 by 15.7 to get 25.67. Divide 25.67 by 0.50 to get 51.3 watts. The page states this as about 51 watts. Choose a pump giving at least 720 litres per hour at 19 metres. That is about a 20 percent margin on both numbers. Flow rises from 600 to 720, which is 600 times 1.2. Head rises from 15.7 to 19, close to 15.7 times 1.2, or 18.84, rounded to 19.

Pressure head is 10.2 metres of the 15.7 metres total, about two thirds. Lift adds 3 metres and friction adds 2.5 metres. Higher pressure, lift or pipe length each raise total head, and so raise power.

How Much Power Does a Drip Irrigation Pump Need at Different Efficiencies

Take 600 litres per hour at 15.7 metres, and work it at three efficiencies. At 40 percent, power is 9.81 times 0.1667 times 15.7 divided by 0.40, which is 64.2 watts. At 50 percent, power is 51.3 watts. At 60 percent, power is 42.8 watts.

The 40 percent result is about 25 percent higher than the 50 percent result. The 60 percent result is about 17 percent lower. A more efficient pump needs fewer watts for the same job. Efficiency does not change flow or head. It changes only input power. The 50 percent figure is used only inside the worked example. It is not a default efficiency. The calculator gives no default efficiency. Use the efficiency of the pump you are considering, or work at more than one efficiency to see the range.

Efficiency	Power for 600 L/h at 15.7 m
40 percent	64.2 W
50 percent	51.3 W
60 percent	42.8 W

How Power Rises With Flow

Hold head at 15.7 metres and efficiency at 50 percent, and change only flow. At 300 litres per hour, power is 25.7 watts. At 450, power is 38.5 watts. At 600, power is 51.3 watts. At 900, power is 77.0 watts. At 1200, power is 102.7 watts. Doubling flow doubles power. A zone needing twice as much water needs twice as many watts. Work out flow for the largest zone first. The Drip Flow Rate Calculator gives flow from emitters, and the Drip Emitter Calculator helps with the emitter side.

Flow	Power at 15.7 m and 50 percent efficiency
300 L/h	25.7 W
450 L/h	38.5 W
600 L/h	51.3 W
900 L/h	77.0 W
1200 L/h	102.7 W

How Power Rises With Head

Hold flow at 600 litres per hour and efficiency at 50 percent, and change only head. At 10 metres, power is 32.7 watts. At 15.7 metres, power is 51.3 watts. At 20 metres, power is 65.4 watts. At 25 metres, power is 81.8 watts. At 30 metres, power is 98.1 watts. Raising head from 10 to 20 metres doubles power. Each extra metre adds 3.27 watts at this flow. Head rises with lift, pressure or friction. Small pipes and many fittings add more friction.

Total head	Power at 600 L/h and 50 percent efficiency
10 m	32.7 W
15.7 m	51.3 W
20 m	65.4 W
25 m	81.8 W
30 m	98.1 W

Watts for Common Zone Flows and Heads

The table below uses 50 percent efficiency throughout, because that is the efficiency in the worked example. It is not a default. Each cell is 9.81 times flow in litres per second times head divided by 0.50. Find your flow in the first column and your head in the top row. The number where they meet is watts at 50 percent efficiency. If your numbers fall between cells, work the formula with your exact numbers. Lower efficiency means higher watts. Higher efficiency means lower watts.

Flow	10 m	15 m	20 m	25 m
300 L/h	16.4 W	24.5 W	32.7 W	40.9 W
600 L/h	32.7 W	49.0 W	65.4 W	81.8 W
900 L/h	49.1 W	73.6 W	98.1 W	122.6 W
1200 L/h	65.4 W	98.1 W	130.8 W	163.5 W

Imperial Rounded Equivalents

Metric and imperial examples are kept separate here, and imperial figures are rounded equivalents, not exact conversions. Watts do not change when you change units. The metric example is 600 litres per hour at 15.7 metres, needing about 51 watts at 50 percent efficiency. The rounded imperial equivalent is about 52 feet, needing the same about 51 watts.

One bar equals about 10.2 metres, or 33.5 feet, of head. One psi is 2.31 feet of head. Multiply psi by 2.31 to get feet of water. Gravity gives only about 0.1 bar, or 1.4 psi, per metre, or 3.3 feet, of height. A barrel raised by one metre gives only about 0.1 bar. The page says to use low pressure drip kits or a small pump in that case. Ten metres is about 33 feet, 20 metres is about 66 feet, and 25 metres is about 82 feet. The pump choice, 720 litres per hour at 19 metres, is about 62 feet. Work power in metric first, then state the imperial equivalent.

How to Use the Result When You Choose a Pump

A watt figure alone does not choose a pump. It tells you how much power the job needs. Next, match that need to a pump curve. Pick a pump whose curve gives the flow at the head shown, with a margin. Check flow at your head, not maximum head. Maximum head is head at zero flow. A pump with a high maximum head may still give too little flow at your working head.

For the worked example, choose at least 720 litres per hour at 19 metres. Start from your calculated flow and head, then look for a pump giving at least that flow at at least that head, with room above both. The page states no fixed margin rule, so do not treat the about 20 percent margin in the example as a rule for every pump.

Friction deserves a second look. It is an allowance, entered as a percent loss on pipe length. Small pipes and many fittings add more. A narrow pipe or many bends can push head higher than the simple figure suggests. Wells need a pump rated for the water depth below ground. That rating is separate from watts. The Drip Irrigation Pump Calculator runs the same sequence with your own inputs.

What the Calculator Does Not Tell You

The calculator gives flow, total head and power. It gives no cost figures, so this guide gives none. It gives no brand or model, so this guide names none. It gives no pump curve data, only the flow and head to look for on a curve. It gives no default efficiency, no imperial worked example, and no pipe diameter friction table. Friction is a user entered percent allowance. It also gives no run time, emitter count, emitter spacing or zone count. If you need run time, the Irrigation Run Time Calculator covers that step. Work flow and head first, work power next, then move to run time.

FAQs

Q: How much power does a drip irrigation pump need for 600 litres per hour at 15.7 metres of head?
A: About 51 watts at 50 percent efficiency. The calculation is 9.81 times 0.1667 times 15.7 divided by 0.50, which is 51.3 watts, stated as about 51 watts. At 40 percent efficiency the same flow and head need 64.2 watts. At 60 percent efficiency they need 42.8 watts.

Q: What is the formula for drip irrigation pump power?
A: Power equals 9.81 times flow in litres per second times head in metres divided by efficiency. Flow in litres per second is litres per hour divided by 3600. Total head is lift plus pressure head plus friction. Pressure head is pressure times 10.2 metres per bar. Friction is pipe length times loss percent.

Q: Does a higher head need more pump power?
A: Yes. Power rises in direct proportion to head when flow and efficiency stay the same. At 600 litres per hour and 50 percent efficiency, 10 metres needs 32.7 watts, 20 metres needs 65.4 watts and 30 metres needs 98.1 watts. Every extra metre adds 3.27 watts at that flow and efficiency.

Q: Does a larger zone need more pump power?
A: Yes. Power rises in direct proportion to flow when head and efficiency stay the same. At 15.7 metres and 50 percent efficiency, 300 litres per hour needs 25.7 watts, 600 litres per hour needs 51.3 watts and 1200 litres per hour needs 102.7 watts. Size the pump for the largest zone flow at total head.

Q: Is 50 percent efficiency the standard efficiency for a drip pump?
A: No. The figure of 50 percent efficiency is used only inside the worked example on the calculator page. It is not a default efficiency and it is not a typical efficiency. The calculator gives no default efficiency. Work the calculation at the efficiency of the pump you are considering, or at more than one efficiency to see the range.

Q: How do I convert psi to feet of head?
A: Multiply psi by 2.31 to get feet of water. One psi is 2.31 feet of head. One bar is about 10.2 metres, or 33.5 feet, of head. Gravity gives only about 0.1 bar, or 1.4 psi, per metre, or 3.3 feet, of height.