How do you heat a house with solar power?

If you live in a northern or midwestern state in the US, space heating is probably the largest energy consumer in your home.

In most climates in the US, central heating is accomplished with the use of fossil fuels. Natural gas is most common, but oil heating is still used in many regions. Even coal hasn’t completely disappeared for home heating use.

But why heat your home with fossil fuels when the sun is available? At a minimum, the sun can provide supplemental heating to your home. If your heating needs are modest, or if you have a superinsulated home in a cold climate, solar energy can provide 100% of your heating needs.

Even if you have a rather ordinary house, it’s possible to retrofit a solar-powered heating system into nearly any existing setup.

Energy efficiency first

If you want to add solar-powered heating to your home, your first step should be to address energy efficiency. Your heating system will be smaller and less expensive if it has less work to do, and insulating and air sealing a home is usually the cheapest way to lower your energy bills.

We wrote an article with a few simple tips on energy efficiency upgrades you can do, but this is a pretty big topic. I’m a fan of This Old House, and the Energy Vanguard blog has lots of nerdy articles on the subject.

What’s the best solar heater for your house?

Solar thermal panels will capture more of the sun’s energy as heat, but for most homes in the United States, the best way to heat your house with solar power is to install a heat pump and photovoltaic solar panels. There’s a few reasons for this:

• Widely available expertise. There are a couple million solar PV installations in the United States, and thousands of local companies that install them. Heat pump installers are very common too, given that they are very similar to air conditioners. Solar thermal, on the other hand, is much less common, which means finding an experienced contractor will usually be harder.
• Large product selection. Solar PV and heat pumps are very popular around the world, so there’s a wide selection of manufacturers to choose from. Solar thermal, on the other hand, is popular in some parts of the world but is much less common in the US. Because of this, you have fewer solar thermal products to choose from; some homeowners even choose the DIY route and build their own thermal collectors.
• Solar thermal requires more maintenance. It’s possible to own PV panels for 25 years and never perform any maintenance. Heat pumps, like air conditioners, require periodic service checks. But solar thermal systems are based on plumbing, solar collectors, heat transfer fluids, pumps, and more. There’s much more mechanical equipment to take care of, and more components that can fail.
• You get heating and cooling. Heat pumps can heat and cool your home, taking the place of both a furnace and an air conditioner. There are few places in the US where you need heating but can’t also benefit from air conditioning. A solar thermal system, on the other hand, can only heat your home.

In other words, while a solar thermal collector will capture more solar energy to heat your home, other practical considerations mean that most people are better off using the more popular technology combination of photovoltaic panels and a heat pump.

That said, there are a wide variety of home solar heating systems, ranging from passive solar to active solar using ground or air-source heat pumps. Each of these techologies have benefits that might suit your home, so it’s worth learning about them.

Solar heating for your house

There are two types of solar energy heating: passive solar and active solar.

Passive solar relies on the design of the house to heat your home without the use of mechanical devices, while active solar uses various technologies such as photovoltaics and solar thermal heaters.

For most homes, active solar heating using photovoltaics and a heat pump is the best approach. While there are other types of solar heaters available, many of them are fairly exotic and rarely used. Finding an experienced installer may be difficult.

Heat pumps, however, are becoming commonplace and can often be installed by any contractor that already installs central air conditioners. Plus, many utilities are offering rebates on high efficiency heat pumps.

What is passive solar heating?

Passive solar heating means that a building has been designed to capture sunlight to heat the interior. In the Northern hemisphere, this means having lots of south-facing glass that lets the sunlight inside the building, and less glass on the north side so that better wall insulation can keep the heat contained.

Other features might include long overhangs designed to let light in during the winter months but shade the windows in the summer when the sun is higher in the sky. Landscaping can be part of it, such as using deciduous on the south side of the building that lose their leaves and let sunlight pass through in the winter, but block sunlight in the summer.

This technique is called passive because there is no use of electricity or mechanical technology. Still, high tech may be used, such as double or triple glazed windows or thermal mass in the walls or floor that help to retain heat.

While a fully passive solar home needs to be designed from scratch, any homeowner can still do simple things to maximize their solar heat gain in the winter and reduce it in the summer.

For example, you can save a surprising amount of energy simply by operating your blinds. If it’s a sunny day in winter and you have a lot of south-facing windows, leaving your blinds wide open can add heat equivalent to running a space heater. Conversely, keeping your blinds shut in the summer can reduce your air conditioning needs significantly.

Passive solar heaters for your house

The passive solar heating principle can be used to build heaters that use no technology except for a small fan. On Youtube there are many people showing off their DIY projects that use simple materials such as aluminum cans inside a frame of wood and plexiglas.

These simple heaters use sunlight to passively warm the air inside the heater, and a small fan to move the warm air inside a building. These can be effective and are similar in principle to the more sophisticated commercial solar thermal collector products mentioned below.

While uou might not want to attach a hacked together solar heater to your home, they might be a good option for a shed or detached garage. If you’re handy and want to do a DIY project, you can check out Youtube to find many solar heaters that people have built with simple materials like pop cans, plexiglas, and sheet metal.

What are active solar heating systems?

Active solar heating, in contrast, uses mechanical or electrical technology to add heat your building. There are a lot of different types of solar heaters out there - some of them fairly exotic.

Heating your home with solar thermal collectors

Solar thermal collectors directly capture the sun’s energy as heat and transfers it to a hot water tank in your home.

Unlike solar photovoltaic panels which use semiconductors to turn sunlight into electricity, solar thermal collectors are relatively simple devices. They work by using a dark colored collector to absorb the sun’s rays. The collector warms up, and the heated fluid inside travels along a loop to a heat exchanger that dumps the heat into a hot water tank. A system like this can supply the hot water needs for a home, or the hot water can be used for radiant space heating.

Here’s a great video from This Old House that shows the system in detail:

Types of solar thermal heaters for the home

There are different designs for solar thermal collectors. Utility-scale power plants, like the Ivanpah solar plant in the Mojave Desert, use fields of mirrors to reflect sunlight at a central collector, which uses the massive amount of collected heat energy to drive steam turbines.

Ivanpah is huge, with a nameplate capacity of around 400 megawatts. This type of design is possible in a desert where space isn’t a concern, but a homeowner needs something more compact.

That’s what a flat plate collector is. It’s simply a set of fluid-filled tubes inside a black colored collector with a glass cover. The glass cover allows sunlight to heat the pipes inside and also helps trap heat inside the collector. Depending on your climate and the amount of heat energy you need, multiple collectors can be connected together so that the final temperature is very high - even above the boiling point of water.

The nice thing about these collectors is that they are low profile. Each one takes up about the same space as a photovoltaic panel. There are two basic types:

• Flat plate collectors consist of copper tubes inside a glass-covered panel. Fluid moves through the copper piping and heats up before exiting the collector.
• Evacuated (vacuum) tube collectors work similarly, except instead of a continuous copper tube, the collector is made of several individual collectors that are vacuum sealed, like thermos bottles. The pipes are also exposed directly to the sun rather than sitting inside a collector box. This type of collector is a better choice for cold climates because less heat energy is lost to the atmosphere. The photo at the top of this article shows what an evacuated tube collector looks like.

Hot water from the sun

The heat from either of these types of collectors is dumped into a hot water storage tank. A couple collectors may be all you need to satisfy the hot water needs for the average home. If you have larger hot water needs, such as for a pool or hot tub, you can simply add more collectors.

Hydronic radiant floor heating

If you have a hydronic radiant in-floor heating system, your solar hot water can then be used to heat your home. Many hydronic systems use the same system to heat both the fluid for the in-floor heat and your hot water supply. This would allow solar energy to supply all of your heating needs.

Forced air heating

It’s even possible to use a solar thermal system to heat a home with forced air heating. This works with the use of a heat exchanger in the airflow of the forced air system. The heat exchanger has metal fins that allows the collected solar heat to be dumped into the air, where it’s circulated into the ductwork.

Standalone heat exchanger

Another option is to run the solar heated fluid into a standalone heat exchanger in your living space, such as a baseboard or wall mounted radiator. This has the advantage of not needing to modify any existing systems. This could be a good choice for a small house without an existing heating system, or if you want to add some heat into a space that isn’t already heated, such as a garage or basement.

Heating your home with a solar photovoltaic system

Solar photovoltaic panels can be used to heat your home as well. In many ways, this is a preferable approach because it’s a mechanically simpler approach, especially if you already use electricity to heat your home. With electric heating, there’s no separate system that carries heated fluid into your home, and no heat exchangers that need to be mechanically integrated.

In addition, it doesn’t matter if the sun isn’t shining. Your solar home will stay connected to the grid, so you will use grid electricity to heat your home when the sun isn’t shining. If your solar system is designed to generate 100% of your electricity in a year, you’ll probably generate excess electricity in summer that will be sent into the grid, banking you credits that you use in the winter.

The one main disadvantage of solar photovoltaics for heating is lower efficiency compared to solar thermal. While solar thermal efficiency varies greatly with the climate (solar thermal collectors lose more heat to the environment in colder climates), in the best case scenario they can be more than 70% efficient.

The best solar photovoltaic panels, on the other hand, have a nameplate efficiency of about 22%, and reach less than that in real-world conditions.

Even so, the advantages of solar photovoltaics tend to outweigh this disadvantage.

Air source and ground source heat pumps

Heat pumps extract heat from the outside environment and dump it into your home. They work by using a compressor and a coil filled with refrigerant, similar to your refrigerator or an air conditioner. In fact, heat pumps usually are able to work as air conditioners in the summer, making them an even more useful addition to your home.

One big advantage of heat pumps is that they are very efficient heating devices - much more so than electric resistance heaters.

Generally speaking, there are two types:

• Air-source heat pump. This type works by extracting heat energy from the air by using a fan to blow air over a compressor coil. The air warms up the coil, and then a compressor then squeezes the refrigerant liquid inside the coil, causing it to heat up. A second fan blows air over the heated segment of the coil, extracting the heat which is then used to warm up your house. This works even in very cold weather: some cold climate heat pumps will work down to 0°F. If you’re wondering how it’s possible to extract heat from cold air, think of a deep freezer. The average deep freezer chills your food down to around 0°F, but the coils at the back of the freezer are still warm.
• Ground-source heat pump. Instead of extracting heat energy from the air, a ground source heat pump relies on a refrigerant loop buried in the ground. A few meters below the surface, the temperature of soil doesn’t fluctuate very much, even in the depths of winter or the hottest days of summer. The more moderate temperature in the ground means that this type of heat pump is even more efficient than an air-source pump. The downside is that ground-source heat pumps are much more expensive to install.

How much electricity does a heat pump use?

The amount of electricity that an electric heat pump varies greatly with its size (rated in tons or BTUs), and also differs by manufacturer and model. To find out how much electricity your model uses, check with the owner’s manual. You will want to add up the electricity usage that is listed for the compressor motor, compressor fan, and outdoor fan.

Keep in mind that the number of amps listed in the manual for the motor and fans are just nominal ratings, and the actual electricity usage will differ depending on the real world load that the unit is under.

Still, to help size your solar photovoltaic system, the numbers you get from your heat pump’s manual are a good enough estimation. To help you out, we took a look at some split system heat pumps from major manufacturers (Rheem and Goodman) and averaged the electricity usage by unit size. We’ve rounded these to emphasize that these are just approximate numbers.

How many solar panels do I need to run my heat pump?

If you already have a heat pump, just look at your electric bill during the heating months to see how much more electricity compared to months that your heat pump doesn’t run.

If you’re planning to add a heat pump system in the future and want to estimate the electricity usage, you’ll need to do calculations to estimate the correct size of heat pump and its run time.

The maximum wattage of a heat pump only tells you how much electricity it uses when it’s running full throttle. How much electricity it will use to keep your house warm depends on the temperature outside, how well your house is insulated and air sealed, and what the square footage of your house is.

Calculating your whole house heating or cooling load is complicated, and requires an HVAC professional to perform a Manual J calculation. This calculation is hard to do correctly, so be sure to choose a contractor with a lot of experience in this area.

Once you have the calculation done by a qualified professional, you’ll have a good estimate of the correct size heat pump for your house and climate, and the approximate run time to expect. This will give you the number of kilowatt-hours that your heating system will use, which in turn will tell you how many solar panels you will need (by using our solar calculator to estimate the power generation for your climate and roof).

Electric resistance heating

An electric resistance heater uses electricity to heat up high resistance wires, just like in a toaster or hair dryer. Small space heaters, the kind you might place beside your desk in a cold office, use electric resistance.

Because this type of heating is very inefficient, it’s rarely used for whole house heating, except in climates with few heating degree days.

A small space heater uses about 1,800 watts. If you have an electric furnace, read your electric bill during your heating months to estimate how much additional power you’re consuming to heat your home.

Lots of options for heating your home with solar

As you can see, there are a lot of different ways that you can heat a home. Fortunately, solar power is a completely viable way to provide that heat, no matter what type of heating system your house currently uses.

Both solar thermal and solar photovoltaic technology have pros and cons. Which you choose depends on the type of heating (and cooling) system that you want to use. For most homeowners in the United States, solar photovoltaic panels paired with a heat pump is the way to go, but solar thermal is popular in Asia and many European countries.

These choices can be technically complicated, so we recommend working with an experienced HVAC professional to correctly size your heating system and recommend the best way to integrate either a solar thermal or photovoltaic system into your HVAC mechanical system.

One service that The Solar Nerd provides is getting multiple quotes from qualified solar contractors. This service can help you find an experienced professional who will properly install your solar thermal or photovoltaic system, and choose a system that is correctly sized to provide your heating needs.

Further reading

• Heat pump systems (Energy.gov)
• Active solar heating (Energy.gov)
• Solar water heaters (Energystar.gov)