Solar Ac – Running an Ac Unit With Solar Panels

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How to run solar ac

How many panels would you need to run solar AC?

How many solar panels does it take to power an air conditioner?

Home appliances such as food mixers and TVs pull a steady current from the supply, making it relatively easy to caclulate its power rating in watts.

You need this before you can size solar panels to generate enough power to supply the load.

AC units, also freezers and refrigerators, are different – they incorporate compressors with motors.

This means that the current drawn varies depending on which part of the cooling cycle the appliance is at.

When a motor is constantly starting, stopping and resting, instantaneous power is always changing so average values need to be used.

How many solar panels are needed to run an ac?

A 1 Ton AC unit would need 1500 watts of solar panels, with irradiance of 4 kWh/m2/day. 500Ah of battery backup would also be needed for 1 Ton for every hour the AC runs when there is no sunshine. If only running in daytime, a 50Ah battery is required to provide a power reservoir for motor surge currents.

What you need to know when sizing solar panels for solar AC:

  • Why rate an AC unit in tons?
  • What is the average AC power rating in kilowatts?
  • Why is the compressor motor surge current important?
  • How many hours and what part of the day will the solar AC unit run?
  • Are batteries needed to run your solar AC?
  • Which batteries are most suitable for solar energy storage?
  • How many watts can your solar panels generate?
  • What size solar panels will be used?
  • How to size the inverter and why you need one

Video – Can you power an AC unit with solar panels?

 

How to run ac off solar – the calculations

When considering d.c. loads it’s relatively easy to calulate power. Power in watts is found by multiplying volts (V) with amps (A).

Watts = Volts x Amps

For example, say a DC load is supplied from a 12 volt supply and it pulls a current of 12 amperes, then you would find the power with the formula below:

12V x 12A = 144W

Unfortunately, in AC circuits voltage multiplied by current isn’t always the same as power in watts. Let’s see why …

Howe to find load power in solar AC

Watts in solar AC calculations doesn’t always equal volts x amps!

Home appliances work on AC power, so watts is calculated in a different way. V and I values are constantly changing, taking the form of a sine wave.

Motors have copper windings which are inductive loads.

In this type of load the current waveform lags behind the voltage waveform and this causes the power calculation not to be strictly valid.

If V and I are maximum at the same point in time, they are said to be in phase. When this occurs V x I does equal power in watts.

If V and I are not in phase, then the equation doesn’t hold true. The result is that more solar power is required to supply extra watts.

How much electricity does an ac unit use

Why is ac measured in tons?

AC units are rated by their efficiency, and they all vary. If electrical kw ratings were used to compare them, it wouldn’t give an accurate assessment of their comparative cooling capacity.

Two different air conditioning units rated at 3kW would probably have different cooling capacities. It all depends on their efficiency rating.

So what is a ton of ac cooling capacity? It is the amount of cooling required to completely melt 2000 pounds of ice in twenty-four hours. It’s also called a short ton.

As a general rule, the higher value ac tonnage, then the bigger air volume can be cooled each hour.

A short ton of AC capacity is the same as 12,000 British Thermal Units/hour BTU/hour, which is in turn equivalent to 3.5 kW.

So a 1 Ton air conditioner has a cooling capacity of:

(12000 x 1055)/3600 = 3516 Joules per sec = 3516 Watts

What is EER rating for ac units?

The initials EER represents ‘energy efficiency ratio’ and it tells how well an air conditioner can cool.

Dividing an AC unit’s BTU value by it’s power rating in Watts , the ratio tells you how efficient it is.

A really efficient air conditioning unit could have an EER value of 11 or 12, for example. Air conditioners with high EER ratings are recommended for use in hot climates with high humidity.

The EER efficiency rating system uses an outdoor temperature of 95°F as a benchmark, with an inside temperature of just 80°F, and humidity level 50%.

The SEER rating is often used nowadays, as it reflects an air conditioner’s cooling capacity over a range of outdoor ambient temperatures. That’s what the ‘S’ stands for – it’s ‘seasonal’. 

The chart below the text shows the SEER/EER ratings for several popular home AC units.

Note: Lennox are coming up with new AC designs which include solar panels and these could well become a solar standard in the future.

Table – Comparing SEER and EER ratings

AC Brand and Model

SEER Rating

EER Rating

Notes:

Lennox Signature Series XC25

26

16.5

Solar option

Amana AVXC20 & Daikin DX20VC

24.5

14

Lennox Signature Series XC21

21.2

15

Maytag M1200 PSA1BG

20

13.5

Carrier Infinity 20 24VNA0

20.5

15.5

How much does it cost to run air con per hour?

This is driven by the following factors:

  • How efficient is the air conditioner?
  • How often does it run?
  • How many hours will the AC unit run per 24 hours?
  • What is the exterior ambient temperature?
  • How much does your electricity cost per kWh?

It can get a bit complicated but is this the best way to approach it? Which of these things are the most important?

AC installers often use rough guides for sizing air conditioners (supplied by solar). Such rules include the rule-of-thumb that an ac unit pulls around 7 amperes for every Ton of cooling capacity.

This isn’t a constant value but varies as the Tonnage increases:

  • 2 ton pulls 15 amperes
  • 3 ton pulls 18 amperes
  • 4 ton pulls 21 amperes

For estimating when installing AC systems, it’s a great rule. Let’s look a little deeper.

The term RLA (Rated Load Amps) is another thing that can be confusing. Look for it on the AC nameplate – see the image below:

What does RLA mean for AC units?

RLA value is the maximum current during an AC cooling cycle

The RLA current in amps is not the running current, but the maximum current pulled by the compressor motor when the AC unit is in the cooling part of the cycle.

Don’t confuse it with the LRA (Locked Rotor Amps). This current can occur if the motor gets stalled  (locked rotor). The motor can’t turn and it pulls a big current trying to turn.

This value is equivalent to the inrush current taken when the compressor motor starts at the beginning of the cooling cycle. The continuous running amps depends on other things.

See: https://www.energy.gov/eere/buildings/appliance-and-equipment-standards-program

What is meant by AC surge current?

When a compressor motor starts it pulls a lot more current than if running normally.

The running current in amperes depends on how big the load is, but the surge current is normally the biggest current the motor can pull.

The wiring size is calculated by using running current plus 10%. Surge currents last only for a few seconds and so any heating effect is very small.

Circuit breakers or other protection devices have to be the right size, so that surge current can flow without tripping these devices.

This inrush or surge current also has implications when sizing a solar panel array for solar AC.

When running the AC during daylight hours without any battery backup, the number of solar panels has to be adjusted upwards to make sure the extra motor inrush current can be comfortably supplied.

A solar power system supplying home appliances incorporates an inverter which changes the d.c. voltage produced by solar to the a.c. used in our homes.

Home appliancse like AC need an inverter

Sola AC uses an inverter to change power from DC to AC

How many solar panels to power an ac unit?

For large applications like AC, fixed solar panels with 250 to 300 watt rating are better, as larger wattage panels are more cost effective in terms of dollars per watt.

How much power does a 300 watt solar panel produce?

The output power of a solar panel depends on these things:

  • How efficient is the solar panel? (20% and 22% is normal for a high quality panel)
  • What is the solar panel area in m2?
  • What is the irradiance (kWh/m2/day) for your location?
  • What is the solar panel array orientation? (which direction does it point)
  • Is the installation fixed or is solar tracking to be used?

A 300 watt rated solar panel can produce around 1.2kWh per day using the US average peak-sun-hours value of 4.

How do solar inverters work? The basic operation

Solar panels produce DC (direct current) but home appliances mostly use AC (alternating current). An inverter is required to convert the DC to AC.

An inverter modifies DC power using electronic devices called MOSFETS into a smooth sine wave, which is just the same as the power the utility company delivers to your home.

How many solar panels to run 5 ton ac unit? Calculation

AC is needed when temperatures are high, which is mostly during the four hours around mid-day for most locations.

This is lucky, because this the time when the sun is at its height and solar panel power output watts also at its highest.

1 Ton of AC cooling rating translates to 1.2kW of electrical watts, so running 1 Ton of ac would need 4.8 kilowatt-hours of energy.

A 5 Ton AC unit will use:

1.2 kilowatt x 4 x 5 = 24 kilowatt-hour

A 300 watt solar panel can produce 1.2kWh per day (in full sunlight). A simple way to calculate how many solar panels you would need to power a 5 ton air conditioner would be:

Energy consumed (24kWh)/energy produced by a 300W solar panel (1.2kWh) = 20 panels

20 solar panels with 300 watts rating = 6000 watts

A 5 Ton AC unit is a decent size and will cool a substantial area, if it has enough solar panel and battery backup to support the cooling cycle.

For instance, if a 5 Ton AC unit runs during the night-time hours, you would need to install about 500Ah of batteries for every hour that it runs, which isn’t really practical.

For daytime running a 200Ah battery would be enough to supply the inrush current and provide a power reservoir for the times when clouds pass over the panels and production is reduced.


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