Running air conditioning on solar is possible. Here is how many panels it takes

First, let's look at the energy consumption of an air conditioning unit itself. Is it all that different from a toaster or a computer? The sheer numbers show that it is. An average american house spends around 10,812 kW a year, according to the U.S. Energy Information Administration. An AC unit requires 20% of this amount — it uses around 2,000 kW every year. In hot places, like Arizona or Texas, this number goes even higher.

Exact energy consumption highly depends on the size and type of the AC unit you've chosen. The cooling capacity of an AC somewhat translates to its wattage like this: 1 ton of cooling power requires slightly more than 1,000 W. Central air conditioning systems that can take care of the whole house use around 3,500W. A medium-size AC unit requires around 1,000-1,500 W. Small units for tiny rooms can have a wattage of 500W.

To get a daily energy consumption of an AC unit you need to multiply its wattage by the estimated number of working hours. However, the whole calculation gets messy because:

• Presence of people and animals in the room where an AC unit is working increases the energy it consumes.
  
  
• An air con tries to cool every object in the room: the more there are different things around, the more electricity it takes.
  
  
• An AC unit requires extra energy to start working, but then spends much less energy maintaining the desired temperature, rather than actually cooling the air.
  

Therefore sometimes it is suggested to halve the amount of energy you've got from calculation. What you'll receive in the end is the power that additional solar panels would need to generate daily to support your air conditioning unit.

To understand the principle, let's think of the most simple situation: an AC unit works at the same time as solar panels. In this case, we'll just divide the power usage of the AC unit by the wattage of panels. For example, four 300 W panels would be enough to support a 1200 W air conditioning system. Now, let's move on to slightly more realistic and, therefore, more complicated examples.

We've decided to install a central air conditioning system in a house somewhere in LA. Its rated wattage is 3000 W. We'll set 6 hours as an estimated daily work time, assuming that it's going to be used only during the hottest hours of the day.

3000W * 6h = 18 kWh

Assuming that the AC unit spends most of the time maintaining the temperature, we'll divide this number by 2.

18 kWh ÷ 2 = 9 kWh

This is the amount of energy the AC unit needs every day and it is easy to find out its cost. Los Angeles residents paid an average of 20.8 cents per kWh in December 2020.

20.8 cents * 9 kWh = $1.87

This is the cost of running an AC unit for one day in Los Angeles. This amounts to $56 a month and $682 a year. Quite a lot. Given the fact that an average AC unit costs a little over $4,000, it's like purchasing a new one every 5-6 years. Solar panels in Los Angeles, to the contrary, pay for themselves in about 6 years. Running your AC on solar energy contributes to speeding up this process.

Well, to run the AC unit on solar we need to get approximately 9 kW from PV modules every day. The average number of peak sun hours in Los Angeles is 5.6 – this is the time when irradiance reaches 1000W/m2 and panels operate at their maximum.

9 kWh ÷ 5.6 h = 1607 W

You can generate this amount of power with six 300 W solar panels. In fact, they will produce 1800 W, which is even more than needed. This extra energy, however, is going to come in handy.

While we do expect the AC unit to spend less energy than we've calculated, the performance of solar panels can be affected by the weather and their positioning. Besides, batteries and a charge controller aren't 100% efficient, so some amount of current is inevitably going to be lost in the process. Therefore, it's better to have 20-25% extra energy for the AC unit just in case.

9 kWh * 1.2 = 10.8 kWh

Around 11 kW of solar power should cover our needs perfectly. Six 300 W panels generate slightly less than that:

1800 W * 5.6 h = 10.08 kWh

We can leave these panels as they are, or replace one or two of them with 330W modules to compensate for that difference. Don't forget that these six modules are intended to cover only the AC unit. When sizing a solar PV system, we need to take into account all the other appliances as well.

Imagine a house somewhere in Minnesota. There is a study where it gets really hot during summer days and it's impossible to work there. We install a small AC unit which requires 500 W. It works for 6 hours 5 days a week. Therefore, on paper this AC unit needs up to 3 kW each working day. The number of peak sun hours in Minnesota in summer is 5.4.

3 kW ÷ 5.4 h = 555 W * 1.25 (margin) ≈ 700 W

You can get those additional 700 W by extending your solar panel system with two extra 350W paneIs. But is it necessary to add them if they are not going to be all that useful in fall, winter and spring? Well, probably not.

The AC unit is only needed in summer when solar panels receive more than usual sunlight during the day and, therefore, produce more electricity. On average a solar panel system generates 50% more electricity in July and August than in December and January. So, if the existing solar array produces enough power to cover your energy needs in winter, leave it as it is: that small AC unit won't be much of a burden.