What size solar water pump do you need?

Solar water pumps continue to grow in popularity, and for a good reason — they can handle any water supply project, from domestic supply to large-scale irrigation. That said, the wrong-size solar water pump can turn a wise investment into a big headache.

As a prospective solar water pump buyer, you need to familiarize yourself with all the necessary sizing info. Trust us when we say there’s more than enough incentive to do so.

Failure to purchase the correct size pump can result in unnecessary return trips to the store, shipping returns, and, worst of all, you being left with a pump that cannot fulfill your water demands.

Let’s take a closer look at how you can avoid those nightmare scenarios. 

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Solar water pump size: table

The following table highlights the water demands and corresponding solar water pumps for different size households.

Water needsQuantity of water per dayType of water pumpPump power (DC)Pump voltage (DC)
Small domestic (2 people)120 gallons (454 liters)Submersible50W-150W12V- 30V
Domestic (family of 4)300 gallons (1135 liters)Submersible50W-250W12V- 30V
Domestic+ (family of 5 + swimming pool)450 gallons (1703 liters)Submersible150W-270W12V- 30V
Circulation pump for swimming pool4500 gallons per hour (18m3/hour)Above ground500W48V
Agricultural irrigation5000 gallons per hour (20m3/hour)Submersible1500W110V

What size solar water pump do you need?

To determine the size of the solar water pump you require, carry out the following:

  • Minimum pumping flow: Calculate the minimum pumping flow from your daily water needs.
  • Total dynamic head: Calculate the total dynamic head from the depth of the well and the distance from the water source to your storage cistern or house.
  • Correct solar water pump: Source the correct pump with matching specifications.
Solar water pump diagram.
Solar water pump diagram.
Source: researchgate

1. Calculate the minimum pumping flow

To calculate your minimum pumping flow, you must estimate your water needs.

Let’s assume your solar pump will operate during peak sunshine hours — between 9 AM and 3 PM (360 minutes). If your household consumes 300 gallons daily, that’s 0.83 gallons per minute (300 gallons/360 minutes=0.83).

Variable power

As it is named, a solar water pump needs solar panels to function. Solar panels are variable power generators — their power output varies throughout the day and year. Consequently, a solar water pump’s water production also varies.

This can become problematic as your water needs are relatively stable, though there is often an increase during the summer months.

The most common solution to a variable water supply is to store water in a cistern. As a rule of thumb, we recommend a 7-day water storage capacity.

A family of 4 using 300 gallons daily would need a cistern of 2100 gallons.

2. Calculate the total dynamic head

To correctly size your solar water pump, you must consider the depth of your well and the friction from the pipes that slows the water flow. In other words: the deeper your well and the longer your pipes, the bigger your water pump must be.

Pump manufacturers commonly use the total dynamic head (TDH) to calculate the impact of friction loss on the water flow.

TDH = Static head + Friction loss

To calculate the total dynamic head, you’ll need the following:

  • Static head: This is the distance between your well’s water level and the top of your water tank.
  • Friction loss: This relates to your pipe’s type, length, and diameter.

Note to our readers: Estimate your friction loss here.

Example

Your well is at a depth of 70ft. Your cistern is at the same level, located 200ft away from your solar pump. Therefore, the TDH is:

 70 + 0.14 = 70.14 ft

In this configuration, you’ll have to look for a solar pump with a minimum flow rate of 0.83 gal/min and a dynamic head of 70.14ft.

3. Find a matching solar pump

You can select the correct water pump with the minimum pumping flow rate and the total dynamic head.

Solar water pumps are specially designed to work at variable power. You’ll have to look at their performance chart showing the water flow in the pump power and the dynamic head function.

With this configuration, you’ll have to look for a solar pump with a minimum flow rate of 0.83 gal/min and a dynamic head of 70.14ft.

Solar water pump flow chart.
Solar water pump flow chart.

In this example, the 12V DC water pump meets the water supply requirements (70ft, 0.83 gal/min) with a power of only 24W (2.0Ampsx12V=24W).


What size solar system does your water pump need?

Now it’s time to size your solar system. Doing so will help you provide your pump with the necessary amount of power.

First, gather the following information:

  • Pump power at minimum water flow and total dynamic head
  •  Solar panel hourly power profile

The flow chart in the previous section indicated that the pump is capable of meeting the water supply requirements — 0.83gal/min, total head of 70.14ft — at a power of 24W.

With a 30% extra power margin, the solar panel should supply at least 32W between 9 AM and 3 PM. Now, it’s time to select a solar panel that can satisfy this power requirement.

To do this, you must extract a solar panel’s hourly power profile at your well’s location. You can find this information using the solar calculator from Global Solar Atlas.

Enter your location and the solar panel’s power, and the calculator will provide a complete report of your potential solar production.

In this example, we tested a 100W solar panel in San Francisco. This solar panel will satisfy your requirements from 9 AM to 3 PM, even throughout winter. 

The hourly power profile of a 100W solar panel in San Francisco, California.
The hourly power profile of a 100W solar panel in San Francisco, California.
Source: Global Solar Atlas

In conclusion, to supply water for a family of 4 in the San Francisco area, you’ll only need one 100W solar panel coupled with a 12V solar water pump.


Do water pumps use a lot of electricity?

A DC solar water pump uses three times less electricity than an AC water pump of the same pumping capacity.

Naturally, the deeper your water well, the more electricity your water pump will use.

In the previous section, we demonstrated that for a family of 4 with a 70ft well, the water pump uses 24W for 6 hours. That’s only 144Wh per day (24Wx6hours). In comparison, a family-size refrigerator needs about 1200Wh per day.


Final thoughts

In summary:

  • To acquire the correct size solar water pump, you must estimate your minimum water flow and the water elevation (dynamic head) from the bottom of your well to your house or cistern.
  • By estimating your location’s hourly power output profile, you can determine the amount of solar power necessary to run your water pump.
  • DC water pumps don’t use a lot of electricity. 
  • The pump uses 8 times less electricity than a refrigerator to fulfill a family of 4’s daily needs,

If you have any further questions, please reach out to us in the comments section or follow us on Twitter.

Romain Metaye

Romain Metaye

Dr Metaye has a Ph.D. in chemistry from Ecole Polytechnique, France. He is a renewable energy expert with more than 11 years of experience within the research world. During his career, he supervised more than 150 projects on clean energy. Off-grid smart systems, solar energy, battery and the hydrogen economy are among his specialties.

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