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.