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Do I Need Purified Water for Cleaning Solar Panels?

What Type of Water Is Best for Cleaning Solar Panels?

Solar panel manufacturers often claim that rainwater is enough to keep them clean. However, that’s like saying rainwater is enough to keep your car clean. Rainwater is not enough to get rid of not only dirt, but other problems like bird droppings, dead bugs, pollen, and pollution. Plus, just like with window cleaning, rainwater and regular tap water can leave spots.

For solar panel cleaning to be done correctly, you need to use a pure water system, just like you use for water-fed pole window cleaning. Achieving pure water can be done using either a deionization (DI) unit or a reverse osmosis/deionization (RODI) unit.  Determining which system is best for you will vary based on the size of the job you are doing and what equipment you already own. In a perfect setting, using only DI is the best approach because it gets the water cleaner, but DI resin gets expensive. However, since solar panels are not transparent like windows are, your total dissolved solids (TDS) can have a little higher parts per million (ppm). While we recommend 10 TDS for window cleaning, you can go up to 30 TDS for solar panels.

Determining which pure-water option for solar panel cleaning is best will vary based on the job size you are doing and your equipment. If you already own a DI-only system, you will achieve the purest of water, but it may become too costly depending on the size of solar panel cleaning projects you take on. If you are only doing a few residential solar panels occasionally, then DI-only may be cost-effective. 

While reverse osmosis alone does not provide as pure of water as using RODI or DI only system, you can probably get by with only running through your RO system or solar panel cleaning. In a commercial application where you are cleaning thousands of solar panels, it's not easy to cost-effectively purify the volume of water needed. In this case, you may want to clean the panels using regular tap water, then switch to DI for rinsing only. We have plenty of customers who scrub panels with a washer, then rinse with DI water to help you conserve DI filter costs. 

How to test for TDS

A TDS meter is a quick and easy way to test for the amount of total dissolved solids in the water you’ll be using. For drinking water, the EPA’s secondary drinking water regulations recommend 500 ppm (parts per million) as the maximum amount of TDS. With window cleaning, anything above just 10 ppm needs to be filtered either using a DI system, or an RODI system. For solar panels, however, you can go up to 30 ppm since you do not have to see through them. With a system like the H2Pro (below), you can even turn off your DI filter and use RO water only, which saves you money. 

You can check your water’s TDS easily with a TDS water tester. Simply remove the cap, turn it on, and stick the correct end in the water you’re going to be using. The TDS tester has an electrical probe in the bottom of it that runs electricity through the water. The easier the electricity can travel through the water, the more minerals it contains.

The Unger Hydro Power DI system 

  •  Good option for those getting started in WFP/solar panel cleaning who only plan to do a few smaller jobs per week
  • Good choice for those in areas with 150 TDS or lower
  • Produces commercial-grade zero TDS (Total Dissolved Solids) pure water for site glass cleaning and other applications that utilize deionized water.
  • QuickChange Resin Canisters allow users to change resin in less than 30 seconds.
  • FloWater Technology ensures more efficient water flow, improving resin efficiency and increasing its lifetime
  • FastLock Opening for quick and easy opening and locking of the filter. Safe, self locking mechanism has a pressure release valve

ProTool Eco Cart

The ECO 3 stage RODI cart is the best pure-water bang-for-your-buck system out today, and the perfect entry-level RODI option.

  • 40in RO Membrane offers great water flow
  • Easy to Replace Carbon Filter
  • Large DI Resin Cartridge is Refillable
  • Stainless Steel Construction
  • Easy to Use, Simple to Maintain


One of the cool things about the H2PRO for solar panel cleaning is that it has the option of turning off the DI filter and running RO only, which saves you money. Solar panels aren't transparent, so your water doesn't need to be absolutely pure, and the RO membrane will get you clean enough. You can also use this option for any shiny surface that's not transparent, like car washing, awning cleaning, Alucobond surfaces, and so on.

The H2PRO Max is THE best RODI cart for use with larger projects or regular use. Featuring the largest filters on the market, you'll get longer life to help you stay more productive on the job. This professional yet easy-to-use cart-based water purification unit offers great performance in a compact format. The commercial pump offers a boost in psi where water pressure is low.

  • 500% longer Carbon filter life for fewer changes
  • Best 40"x 4" RO membrane flow in the industry
  • 75% more Resin than most systems for longer filter life and lower Resin replacement cost