How To Integrate Air Conditioning With Solar

If you want to control your air conditioner properly with solar, it helps to understand how the thing actually works. As an ex-control systems engineer, this is right up my alley. Let’s break it down.

How Air Conditioners Control Temperature

Old-school “bang-bang” control

Early air conditioners used to work on a crude method called bang-bang control. You set a temperature (called the set point) on the remote.

• In cooling mode: if the room is hotter than the set point, the compressor kicks in at full power. Once the room drops below the set point, it switches off.
• In heating mode: same idea, but flipped.

It’s literally all-or-nothing.

Modern inverter control

These days, air conditioners use inverter control. Instead of being 100% on or off, the compressor can run anywhere between 0–100% power. That means:

• Better temperature stability
• Higher efficiency
• Longer lifespan (less hammering the motor on/off)

The Brains Behind It: PID Control

So how does the aircon know how much power to pull at any given moment?

It uses a bit of maths called a PID algorithm. I won’t bore you with the equations (I’ve literally written three books on it), but here’s what you need to know:

1. Not all PIDs are equal. Bad implementations behave no better than bang-bang. Good ones keep the room at a steady temp with smooth adjustments.
2. The algorithm reacts to “error”. Error is simply the difference between the set point and the actual room temperature. The bigger the error, the more power the system pulls.

Example: your room is 12°C and you want 22°C. The system starts at full power, then gently ramps down around 20°C so it doesn’t overshoot, eventually holding the room steady at 22°C. It might settle at, say, 63% power.¹

Electrical vs Thermal Power

Here’s where confusion often creeps in.

• A unit might be marketed as a “14 kW air conditioner”.
• That number refers to thermal output (how much heating or cooling it can push into a room).
• The actual electrical input is much lower, thanks to heat pump efficiency.

For example:

• A 14 kW aircon with a coefficient of performance (COP) of 3 only needs about 4.7 kW of electricity to pump out 14 kW of thermal energy.
• If the COP is closer to 4, then it’s even better – about 3.5 kW electrical input for 14 kW thermal output.

So when you size your solar, always work from the electrical rating, not the flashy marketing number.

Covering Aircon With Solar

Let’s assume your aircon pulls 7 kW electrical at full power.

• A 7 kW solar system only hits its peak on a perfect day at midday.
• Losses (shading, inverter limits, panel orientation) reduce that further.
• So if you want your aircon always covered by solar, you’ll need way more than 7 kW on the roof.

The good news:

• With decent PID control, your system won’t run flat-out all day.
• Once the room is at temp, power demand drops sharply.
• A well-sized battery can also smooth things out – topping up the shortfall when the aircon starts hard or when clouds roll over. Remember you need to think about:
  • battery power: kilowatts (kW) to cover initial power (and surge currents if you want it to work off-grid)
  • battery energy capacity: kilowatt-hours (kWh) to get you through the evening once the temperature is maintained.

Smarter Use: Pre-Cooling and Timing

Because solar is strongest during the day, it’s smart to:

• Pre-cool (or pre-heat) the house while the sun is out.
• Get the temperature stable by late afternoon.
• Then let your battery take over for the evening.

This avoids smashing your battery with a full-tilt aircon load right after sunset.

Unfortunately, most Aussie homes have such a poor thermal envelope that the pre-cooling doesn’t last long. To improve your home’s thermal envelope, address:

• gaps
• glazing,
• insulation.

Then, unless you live in the tropics, add thermal mass to your home. Use dense materials like brick inside, and for bonus points, swap your plasterboard for something more substantial.

Bricks inside for thermal mass and lime render instead of Gyprock works a treat.

What To Discuss With Your Installer

If you’re serious about covering aircon with solar, sit down with your installer and talk through:

• Electrical size of your air conditioner (not the thermal rating on the box).
• Your usage profile. Cooling in summer is easy – sun is abundant. Heating in winter (especially in southern Australia) is harder, so you’ll want a generous solar array.
• Your habits. Do you want the unit running all night, or just until bedtime? Do you blast it in the morning before sunrise?
• Data. If you’ve got smart meter data (5-minute resolution), you can usually see aircon loads clearly. If not, consider a dedicated meter on the circuit.

From there it’s about sizing:

• Solar array large enough to cover daytime loads.
• Battery sized to stretch that coverage into the evening or early morning.

Wrapping Up

That’s the basics of matching solar with air conditioning. In short:

• Don’t confuse thermal and electrical power ratings.
• Expect higher demand at startup, but lower demand once the room stabilises.
• Solar covers the bulk during the day, batteries smooth the rest.

For verdicts on which air conditioners are worth getting and which brands are best to avoid, check out our new aircon review hub.