Using solar for home space heating: questions answered

If you’re interested in a heating a whole house with solar power, that’s a pretty big topic. There’s a few ways solar can provide heat, from photovoltaics that power a heat pump to solar thermal panels that capture heat directly.

You can check out my article on solar heating if you want to get into the details. However, I’ve noticed that there’s consumer interest in heating just a single room with solar power, and even some products marketed for this purpose.

As it turns out, it is practical to use solar energy to heat a single room, but in most cases the best method is perhaps unexpectedly simple: simply open your blinds when it’s sunny. The amount of heat you can receive through a window can be surprisingly substantial. There’s a little more to know, including the types of glass to maximize solar heat gain, but the bottom line is that most products marketed for heating a room with solar power either aren’t worth it or aren’t practical.

Typical electric space heaters

If you walk into any hardware store you’ll probably find a wide selection of electric space heaters, often for less than $30. While they may vary widely in price and have different features, they all work on the same principal and have the same maximum heat output.

Heaters like this work using electric resistance. An electric current passes through a wire with high resistance, which means that the electricity isn’t able to move easily. This causes the electricity to be converted into heat, which the space heater then distributes into your room using a fan or ceramic plate that slowly radiates the heat outward.

This is basically the same as a hair dryer or toaster. It has the virtue of simplicity and being cheap, but it’s not very cheap to operate in the long term. This is because a space heater converts electricity into heat on a 1:1 basis, which means that one watt of electricity is converted into one watt of heat energy. A space heater is limited (in North America) by the typical 15 amp 120 volt outlet. For safety, a heater will be limited to about 12.5 amps or 1,500 watts. That works out to about 5,100 BTUs of heat per hour. (By the way, a more expensive electric space heater might have more features but can’t generate more heat than that.)

A more efficient way to generate heat with electricity is with a heat pump. Instead of electric resistance, a heat pump moves heat from outside your house to the inside - even when it’s cold outside. It’s the same principal that your fridge and freezer operate on.

Using this method, a heat pump will generate heat on a 3:1 basis or better when it’s not too cold outside, or 2:1 when it’s very cold. That means a heat pump can be more than 3 times efficient as a space heater. Again, this is possible because a heat pump doesn’t convert electricity into heat, but uses electricity to move heat from one place to another.

Because of this, for most people the best way to heat a whole house with solar power is to have photovoltaic panels and a heat pump. Sized appropriately, the solar panels can generate as much electricity (averaged over the course of a year) as your heat pump consumes.

You can install a single room heat pump, but it’s not the same as a cheap space heater

In general, heat pumps are intended to heat your entire home. There are types of heat pumps that are zoned, which means you can send heat to specific areas of your home. You can even get single-zone heat pumps to heat just one room.

However, that’s not exactly a replacement for a $25 space heater. A single-zone heat pump for one room will cost about $1,000, and that doesn’t include installation. A space heater is a cheap and easy solution when you need a little added warmth in one room. Unfortunately, there’s no heat pump equivalent - at least not yet.

The best solar heat collectors in your home are your windows

If you’re trying to find out if there’s a practical solar-powered device that’s equivalent to a low-cost electric space heater, unfortunately there isn’t. There are single-room solar thermal collectors available, and you can even build your own, but there are shortcomings.

Instead, if you want to heat a room with the sun, just open your blinds to let as much sunlight in as possible. This might seem too simple, but this method with the right type of window glass will capture as much heat from the sun as the most sophisticated solar thermal collectors.

How much heat can you gain through a window? Depending on where you live, a single south-facing window can collect between 2,600 and 7,700 BTUs of heat per day in the middle of winter. Remember that an electric space heater puts out 5,100 BTUs per hour. This means that if your room has multiple windows, keeping your blinds open during a sunny day can be equivalent to running an electric heater for a couple hours. That’s quite a bit of free heat!

The data for this is based on an average January day, which includes cloudy days too. A sunny day will you’ll get a lot more heat.

Solar heating in the winter depends a lot on climate and latitude

The amount of electricity you can generate with photovoltaic panels depends on your local climate and the orientation of your roof. The same principal is true for the heat energy you can capture through your windows.

Climates with more cloudy days, like cities in the Northwest, don’t get as much sun in the winter as cities in Colorado. There’s a great blog post at Green Building Advisor by Robert Opaluch with a summary of how much sunlight and heat energy you can expect to capture through a window for cities in various parts of the United States in January.

Another good resource is the monthly solar DNI maps by the NREL. For example, if you look at the DNI maps for winter months, you can see that the southwest, parts of the midwest, and parts of the northeast have more sunlight than the Northwest or Great Lakes regions.

Windows for higher solar heat gain

The blog post at Green Building Advisor makes an important assumption, which is that the solar heat gain coefficient (SHGC) of the windows is 0.5.

SHGC describes the amount of solar energy that can pass through the window. It varies between 0 and 1, where 0 means that there is no solar heat gain and 1 means that all of the solar energy can pass through the window and be captured as heat. A SHGC of 0.5 means that half the energy is captured as heat.

SHGC varies a lot between windows. In many cases, windows are sold as low-E, which means they are intentionally designed to have a low SHGC. This is helpful in the summer because it means that your air conditioner needs to do less work. However, low-E glass is detrimental during heating months in the winter.

Low-E glass might have a SHGC somewhere in the range of 0.25, which means that 75% of incoming solar heat energy is blocked. The opposite of low-E glass is high SHGC glass. In my own home, I have windows with SHGC 0.52 glass - double that of low-E glass.

Even though I do use air conditioning in the summer and high SHGC glass means that I gain more heat, this is more than balanced out by the benefits of heat gain in the winter.

Plus, the solution to summer heat gain is simple: I close my blinds.

If you’re shopping for windows, you’ll be able to find glass with SHGC anywhere between about 0.2 and 0.7. Determining whether high or low SHGC glass is best for your climate depends on several variables, but in general homes in cold climates with a lot of heating days will benefit from high SHGC glass.

You can use the window selection tool at efficientwindows.org, which takes into account your climate and the age of your building, to help you decide.

Automated shades can make this easier

If it seems like too much trouble to open and close your shades to take advantage of solar heat gain, there are products out there that can automate all of this for you.

You can get automated shades that work on a timer, which will let you schedule them to open during the day and close again at night. Closing shades, especially cellular shades that have air pockets in them, can help reduce the amount of heat lost through your windows after the sun has gone down.

There are even Wi-Fi blinds that you can control through an app.

I don’t have specific product recommendations, but if you do a search for automated or smart window blinds, you can find plenty of results.

DIY solar collectors

If you’re looking for a product that’s somewhat analogous to a cheap space heater, the closest thing would be a DIY solar collector.

It’s a device that consists of a frame, a glass or plexiglass front, and cans or tubes painted black. The transparent front traps solar energy inside, heating up the air in the cans or tubes. With piping and a small fan, the heated air can be blown inside your home.

You’ll find plenty of results if you search “DIY solar air heater” on the web or YouTube. While these devices certainly do work, I don’t think they’re a good solution for most people.

First of all, I doubt that most homeowners would want to mount a big, flat box on the outside wall of their house. Another reason is that if the installation is done poorly, you can lose heat through the hole you need to cut in your wall.

If you want to use solar thermal to heat your home, you would be better off going all-in and buying a commercial product and having it professionally installed.

Passive solar is a big topic

The bottom line is there really isn’t a solar-powered product that can replace a $25 electric space heater.

The good news is that there’s something even better and free. Simply open your blinds when you want to capture the sun’s warm, and close them at night to help keep the heat in. In the summer, keep your blinds closed to help keep your home cooler.

On a sunny day, you’ll gain a surprising amount of heat this way. In fact, homes can be designed such that solar heating is the only source of heat they need. This type of design is called passive solar, and it’s a whole topic unto itself. I haven’t written much about passive solar, but if you’re interested there are plenty of resources on the web, including the Green Building Advisor website I linked to earlier.