Solar air conditioner

Two types of efficiency improvements give solar cooling a future

A solar air conditioner might be a sensible way to cool your home, if you can determine that the up-front costs will be recovered in reduced operational costs, or that the ecological benefits are worth the extra money.

In my original article on solar powered air conditioning I argued that the high costs of solar power made active solar cooling a much less sensible alternative than investing in a better building envelope, at least for anyone grid connected.

A solar air conditioner is basically a combination of solar electricity generation and air conditioning for the same building, such as a home.

The idea is that the hot summer sun is the main cause of unwanted solar heat gain in a building, so capturing some of that solar energy to generate electricity to run an air conditioner might make sense.

Also, since electricity prices are often highest when cooling demand is highest, a solar electric module can reduce your overall electricity bill most when it is used for cooling.

The trouble with investing in a solar air conditioner is that the up front costs are quite high: air conditioners consume a lot of power, so you’ll need a fairly extensive (and expensive) solar electricity module; and the air conditioner itself can be quite pricy. If you live in an older home with limited space to install solar modules (for example, just the roof on a two storey home) there may not be enough room to get enough solar panels to fully power your air conditioning unit.

There is a sweet spot between the efficiency of the solar air conditioner, and the capacity of the solar electricity module: the more efficient the air conditioner, the less solar generation capacity required, but unfortunately more efficient air conditioners tend to have a higher up front cost.

Luckily, over the years solar electric prices have plumetted, while central air conditioners have ratched up efficiency considerably. If you live in an area where abundant sun and frequent heat happen in combination, a solar air conditioner may be the way to go.

Solar power cost changes

The cost per watt of solar generation capacity has dropped from roughly $7.50 in 2010, to $3.36 in 2015, to $2.71 in 2020. (This includes the full system cost, not just the cost of the panels themselves). While the price drop is not as steep in the most recent figure (2020 is 80% the cost of the 2015 cost, whereas 2015 was basically half the cost of 2010) prices are continuing to fall. Looking just at the price per watt of solar panel capacity, the price has dropped from $2.32 in 2010, to $0.68 in 2015, to $0.34 per watt in 2020, to $0.26 per watt in 2022. That’s almost a 9x decrease over the 12 year period.

On the other hand, costs for solar power are not expected to drop nearly as quickly over the next decade as they have in the past 13 years, so if you are interested in setting yourself up with a solar air conditioner, waiting for prices to drop further is probably not going to help.

Air conditioner efficiency

Improvements in air conditioner efficiency over the past decade years have also made the option of couping solar panels with an efficient air conditioner a better option. Efficiency has in some cases gone up dramatically due to new types of cooling technology.

At the lower level, consider the US DOE standards on cooling efficiency. The standard change in 2006 raised the Seasonal Energy Efficiency Ratio or SEER for home air conditioners was raised from SEER 10 to SEER 13, meaning a 30% increase in minimum mandated efficiency. Fast forward to 2023, and the minimum SEER for ENERGY STAR is 15.2, with some units achieving a seasonal energy efficiency rating as high as 25.8.

But actual average efficiency typically lags a change in standard by several years, because the new minimum standard is typically established by looking at the most efficient systems currently available, and setting a minimum efficiency a little below that.

As the new standard comes into play, manufacturers already at the standard get to sell more units, while others play catch-up; eventually competition drives everyone’s efficiency up, so there can be a lag of 2-5 years between a new standard and a major improvement in efficiency.

An example – my house

To illustrate the changing economics of a solar air conditioner, let’s consider my own house, which is a 1500 square foot home, including the basement. If I wanted to cool my home by 20F throughout the summer – say, from 90F down to 70F – here’s where things would have stood in 1996, 2008, 2012 and 2023 given the changing cost of solar power modules (expressed in $ per watt of generation capacity), and the improved efficiency of air conditioners (expressed in the standard EER or Energy Efficiency Ratio):

You can see that over the course of even just the past 9 years, the value proposition of solar powered air conditioning has substantially improved.

But I still question whether this is the most sensible investment for someone trying to save energy while staying cool. Before you install a solar air conditioner, you should address all of the following first:

Insulation: Make sure your home is properly insulated and sealed. Have a home energy audit done and address as many of the audit findings as possible. You may be able to decrease the capacity of the air conditioner you need (and therefore the solar electric panels) if you can make your home more efficient first.

Attic ventilation: Pay special attention to your attic. If you don’t have adequate attic ventiliation, your attic will create a huge amount of heat for the air conditioner to extract from the home. The attic should have vents at the base of the roof (such as in the soffits) and at the peak, to keep air flowing through it.

Attics can hit temperatures of 150F if not properly ventilated, and much of that heat will make its way down (even through insulation) into your home. That’s a lot of extra work for the air conditioner.

Solar panel siting: Make sure you have a suitable location for the solar panels. I’ve had too many neighbors tell me they’ve been talking with a solar panel salesperson to have a kit installed on their roof. I walk by their house and spot trees to the south, southeast, or southwest of the home. Shade on a solar panel for even part of the day will significantly decrease its output. Even the shadow of a twig on one 4×4″ cell can cut a huge amount of power from the panel the cell is a part of. (Mind you, as of 2023 solar panel prices have come down so much that some homes are having them installed on the north slope of the roof as well as the south, east or west – since even the limited sunlight that hits a northern exposure panel early and late in the day can provide enough generation to make the panels economically viable.)

Look out for government subsidies

The other factor to consider is the availability of government subsidies for solar power generation. In the US with the Inflation Reduction Act, up to 30% of the cost of solar panels can be refunded. In Ontario, we no longer have the MicroFIT program which paid an above-utility rate for the power you generate; instead we have Net Metering, which means your meter runs backwards when you generate, forwards when you consume, and at the end of the year the best case scenario is you pay nothing for electricity (you don’t get any cash for the excess you generate over what you consume). (The MicroFIT program still runs for those who signed up; for example, I get 38.5 cents per kilowatt hour for what m solar panels generate, until my contrac runs out in May 2036.)

If you are committed to having solar electric panels but can access a subsidy like the above, you will probably be financially better off feeding the power you generate into the grid, and pulling out cheaper electricity to run your air conditioner.