How Long Does it Take for a Solar Hot Water Heater to Heat Up?


Whether you’re looking to lower your electricity bill or live off the grid, a solar water heater is an excellent choice for your family homestead.

You’re not alone if you’re wondering how long it takes for a solar water heater to heat up.  Many people hesitate at the idea of purchasing a solar hot water heater because they arenʻt too sure about its effectiveness.  I am here to tell you, the worry is over.

The average solar hot water heater with a tank of 26 gallons takes about 4-6 hours of direct sunlight to heat up completely. Cloudy days, season and temperature will affect the heat up time. Typically a backup on-demand water heater is installed as a backup, heating water in minutes.

Five to six hours may sound like a long time for you to wait for your shower to be ready, but that is not exactly how it works.  Once warm, your Solar Hot Water Tank should keep the water hot for at least 48 hours, depending on what type of Solar Hot Water System you have.

Real World Test On Solar Hot Water Heat Up Times

I tested 3 different water heaters, owned by my neighbors, in order to get some real world numbers.  Each one was a passive Solar Hot Water Heater since I live in Hawaii.  I tested them on sunny days and cloudy days during the winter in Hawaii and here is what I came up with.

I tested to see how long the water would reach 120 degrees, which is the average temperature for hot water used in a home.

Solar Hot Water Heater #1 (Unknown)Solar Hot Water Heater #2 (Sunbank)Solar Hot Water Heater #3 (Sunearth)
Sunny Day Average6 hours 18 minutes4 hours 25 minutes4 hours 43 minutes
Cloudy Day Average7 hours 5 minutes5 hours 58 minutes6 hours 9 minutes
Rainy Day AverageNeeded BackupNeeded BackupNeeded Backup

The tests I ran measured how long the solar hot water panel took to heat water without the need for a backup water heating source. 

Some of these numbers may change based on where you live, but they should give you a good basis on how long it will take for you to heat up water in your location.  I took these measurements in the winter here in Hawaii to mimic cooler climates.  Heating times can be much lower in the summer.

The Different Types of Domestic Solar Hot Water Systems

Solar hot water systems typically consist of solar collectors (usually thermal or PV cells) and storage tanks. 

Solar hot water systems can be either active or passive. Active heating systems include controls and circulating pumps, whereas passive heating systems do not have any additional moving parts and use convection to heat water.

Active solar hot water systems have many moving parts and are the most efficient converters of solar energy to heat your water. The downside is they’re more expensive and cost more to maintain. Most active systems require electricity as well. They are usually installed in colder climates where circulating water keeps everything from freezing.

Passive systems are cheaper to install and maintain, often last longer than active systems. Passive systems don’t need power to work, but they are much more weather dependent. In the tropics, there is often plenty of sunlight to go around, and passive systems might be the best choice for you. 

Keep in mind that passive hot water systems will take longer to heat up on overcast days than active hot water systems.

Weather Factors that Influence Heating Times 

Solar water heaters will heat up your water rain or shine, but it might take longer. Here are some things that can affect heat up times.

  1. Season

Depending on where you live in the world, seasons can affect your solar water heating. The sun intensity might go down, and you may have more overcast days that will extend heat up times. 

If your winters are sunny and cold (or sunny and warm), you’ll get sufficient diffused radiation to heat up your water. Sun is the important factor here, not temperature. 

  1. Cloud/Rain

A day of clouds and rain won’t make your hot water cold. However, it might increase the time it takes to heat up if it’s over a long period of time. 

Just like you can still get a sunburn on a cloudy day, your solar hot water system can get enough solar energy from the sun to keep your water warm. Extended overcast days, though, will likely make heat up times slow and the water not as hot. 

Use an electric backup on those days.

  1. Shade

Like clouds and rain, shady days shouldn’t affect your hot water heater if it’s only for a day or two. It’s only prolonged dark days that may be a problem. You can combat this with an electrical backup.

  1. Sun Intensity

The way your hot water heater works depends on the type you have. For some, a hot, sunny day will mean faster heat-up times. For others, it might not make as much of a difference. Ultimately, as long as your system is getting direct sunlight, it will heat your water efficiently.

A Good Hot Water Tank Will Store the Heat 

Purchasing a good hot water tank will mean you can be almost completely reliant on the sun to warm up your water — even when you have cloudy days.

An insulated tank prevents the heated water from cooling down too much before use. A good hot water heater will keep your water warm for up to 48 hours.

Extended days without sunlight might require back up heating from electricity or gas. A backup heating unit simply adds a little extra heat in order to keep the water warm and will still be cheaper (and more sustainable) than ditching solar hot water all together.

Does a Solar Hot Water Heater Work at Night?

With your insulated hot water tank, water temperatures rarely drop significantly during the night. If you notice a big difference, it’s useful to hook up a back-up gas or electric booster so it’s always ready in rain or shine or emergency.

Benefits of a Solar Hot Water Heater

If you’re not using solar, solar hot water heaters are one of the best ways to ease yourself into renewable energy.

As we saw above, solar hot water heaters effectively heat up water. They’re also a fraction of the cost of electrical or power to run. For many families switching to solar hot water saves them thousands of dollars every year, even if they have an electric backup for cloudy days.

In fact, the payback period can be as little as 2 years!!

Solar hot water heaters are also cheaper to install than solar panels. Solar panels have to convert the sun’s energy into electricity which then can be used to power a house and an electric hot water heater.

A solar energy hot water heater does not need to do this conversion. The solar energy is used directly to heat up your water, foregoing the electricity conversion.

Additionally, solar water heaters take up much less roof space than full solar systems. Space-constrained homeowners may be better off utilizing solar energy with solar hot water heaters. 

Are you ready to use a solar hot water heater? Check out our other articles about solar hot water systems here.

Sean Jennings

Sean has been living simply Off-Grid in Hawai'i for over 18 years. He lives debt free on Hawai'i Island with his family and over 40 chickens. When he's not tinkering around the homestead, he's off exploring the shorelines for fish & surf.

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