Solar powered air conditioning is perhaps the Holy Grail of the home solar energy field, particularly in very hot and humid countries.
A simple and easy-to-build DIY solar powered air conditioner makes a great project, but there’s a little more to it than ‘how many solar panels do I need to run an AC unit?’
We need to make it clear exactly what kind of electrical power we’re talking about and there are several ‘unknowns’ to explore. Here are a few:
- Your AC unit rating in tons
- AC unit rating in kW electrical power
- Relevance of surge current and power factor
- How long is the air conditioner to run?
- Do you need batteries to run your AC?
- What kind of batteries are best for solar storage?
- How to charge solar batteries for an AC unit
- How much power can solar panels produce?
- Solar panel dimensions and sizing
- Why you need an inverter (what it is and what it does)
A portable solar powered air conditioner for home use is of course of great interest to preppers and campers. It’s a very ‘cool’ idea to have such a level of comfort wherever you are in the countryside. In this post I’ll tear the subject apart to see if it’s possible to put together a DIY solar powered ac unit.
Here’s a useful PDF covering the basics of air conditioning for complete beginners.
Can you run air conditioner on solar power?
For a d.c. load, power calculations are very straight forward – power rating is determined by multiplying volts x amps. If a d.c. resistance load has a 12 volt power supply and draws 10 amps of current:
- 12 volts x 10 amps = 120 watts
Home appliances are different. Voltage and current waveforms are constantly changing in a sinusoidal waveform.
If the peaks of V and I coincide, or are ‘in phase’ then volts x amps does equal watts.
However, if not, the same simple equation doesn’t apply. We’ll look at this later on in the post.
How many solar panels do I need for AC?
Solar Panel Basics
Before moving on to the necessary calculations a basic understanding of solar panel principles and the types of solar panels available will help to understand the overall process.
Monocrystalline panels are the most efficient, while polycrystalline are cheaper. More and more in my own projects I’m using flexible mono panels. They are cheaper, lighter and lend themselves readily to portable projects.
Amorphous and thin film are for special applications and are great for DIY solar camping projects or prepper use. However, their efficiency is low and not much good for power projects like solar AC or running a refrigerator with solar panels.
For a home-based application like solar air conditioning it’s probably more suitable to use rigid panels. If you have plenty of space, then the less-efficient poly panels may be more cost-effective than mono – it all depends on your AC power rating and panel mounting space available.
Up to 22%
25 to 30 years
20 to 25 years
How much power does an air conditioner use?
Why rate AC in tons and not watts?
All AC units have different efficiencies – that’s why it isn’t too accurate to use their electrical kilowatt ratings.
Two air conditioners with an electrical power rating of 1 kW may not generate the same amount of cooling power.
A ton in AC ratings is defined as the amount of cooling needed to melt 2000 lbs of ice a day. It’s also known as a short ton.
The higher the tonnage, the bigger volume of air can be cooled per hour. 1 ton is also equivalent to 12,000 BTU/h or 3.5 kW
For info, the cooling capacity of 1 Ton AC in Watts is:
- (12000 x 1055) / 3600 = 3516 Joules / sec = 3516 Watts
What is EER for air conditioning?
EER stands for ‘energy efficiency ratio’ and is a measure of how well an AC unit cools. Divide the air conditioner’s BTU value by it’s electrical consumption in Watts to give a ratio indicating efficiency.
A very efficient AC unit may have an EER of 12. Units with a rating of 10 or more are recommended for hotter climates. EER assumes an outdoor temperature of 95°F and inside temperature of 80°F at humidity of 50%.
It’s more common now to use the SEER rating, which takes into account an AC unit’s cooling power over a range of outside temperatures – the S stands ‘seasonal’. The table below shows the SEER/EER for 6 popular home AC units.
Lennox are making particular gains in supplementing their designs with solar panel support and this will probably become the industry-standard in the not-too distant future.
Brand and Model
Lennox Signature Series XC25
With solar option
Amana AVXC20 & Daikin DX20VC
Lennox Signature Series XC21
Maytag M1200 PSA1BG
Carrier Infinity 20 24VNA0
How much does an air conditioner cost to run per hour?
This can vary widely and depends on:
- the efficiency of the AC setup you have installed
- how often you run it
- how long you run it
- the temperature outside
- your local electricity cost per kWh
As you can imagine, it gets complicated but is it really worth all the fine detail? Let’s take a look at the factors involved.
Long-time air conditioning installers have rule-of-thumb guidelines for most things. One of these is that an AC unit draws about 7 amps for every Ton rating, but this changes as the Tonnage changes. It’s largely accepted that the relationship between amps and tons changes as the cooling power increases:
- 2 ton = 15 amps
- 3 ton = 18 amps
- 4 ton = 21 amps
For installation purposes, for calculating wire size for example, this is fine. For me, particularly when designing a solar installation, I want to know more or less exactly was going on electrically!
There is often some confusion about RLA (Rated Load Amps) on the AC label tag:
It’s wrong to use this as the AC running current – the RLA value is the maximum current value that the compressor is expected to draw while working.
Not to be confused with the LRA number, which stands for Locked Rotor Amps, which is when the compressor is stalled and pulling maximum current and is basically the inrush or surge current when the compressor motor starts from cold i.e. the worst possible conditions.
The actual running current depends on several factors.
Air conditioner inrush current
When any motor starts it draws several times more amps than when it is running continuously. Running current will vary according to the load conditions, but the inrush or surge current is the highest current value the motor draws, unless its rotor is locked and can’t move (a very bad situation.)
The has some implications. Wiring size is generally based on maximum running current + 10%. Inrush currents are generally short-term and the heating effect in the cabling can be largely ignored.
Protection devices such as breakers and fuses need to be sized to accommodate this spike in current draw.
This also has implications for a proposed solar panel installation designed to run an AC unit. If running in the day-time without battery support, the panel array wattage capacity needs to be sufficient to supply this starting current surge.
Also, solar systems designed to operate domestic appliances make use of an solar power inverter to convert d.c. (direct current) voltage generated by the panels into a.c. (alternating current) voltage used in the home.
Power factor for air conditioning units
You really don’t need to know this but it’s interesting, because on a large AC installation using solar panels, it can save money!
AC compressor motors have fixed and moving copper coils whose magnetic fields interact to create the turning force. An obscure electrical rule called Lenz’s Law states that these coils will generate a magnetic field which reacts against the supply current, causing it to ‘lag’ behind the supply voltage. The drawing below explains the basic idea:
Why do I mention this? Because the power in watts for an a.c. circuit doesn’t always equal the volts x amps you might read on a multi-meter. A multi-meter reads the peak values of the waveforms and displays the average height. The real power calculation should take into account the values at any instant in time.
A poor power factor means that an a.c. unit may take more current than necessary and hence the cost increases. It’s measured as a percentage and is expressed as a decimal. A typical power factor for air conditioning is 0.85 to 0.9.
Air conditioner power factor correction
Normally, poor power factor correction is not an issue unless you intend to install big units, but it can become an issue when sizing solar panels for home AC use. If you can save a panel and perhaps battery capacity, that represents quite a saving.
Luckily there are some reasonably-priced options on Amazon (no, I’m not an affiliate!) designed to ‘plug ‘n play’.
They’re called correction units because they re-align the voltage and current waveforms so that the AC unit is operating at maximum electrical efficiency and not wasting power.
Banks of capacitors are packed inside the box and wiring is very simple.
How to reduce starting current of air conditioner
Surge currents, or any spikes in current, need to be looked at carefully when considering solar installation capacity, particularly if no batteries are used.
In this case the panels have to deliver whatever current is demanded at any time and compressor starting current can be several times normal running current. It will reduce the solar installation costs considerably if it can be minimized.
Soft starter for air conditioner compressor
A good soft-starter will reduce surge current by 60% to 70% or more, which represents quite a reduction on the solar system. Instead of connecting the motor directly to the full mains voltage, the unit gradually increases the voltage which has the effect of gradually increasing the current.
The compressor motors starts more slowly, but this has no undue effect on the AC operation and also reduces mechanical strain on all components.
Soft starters also work well for refrigeration units in commercial premises.
How many solar panels to run ac unit?
We’ve pretty well covered what goes on electrically inside an AC unit, now we need to explore the supply side. I’ll focus on 300 watt fixed monocrystalline panels – the bigger sizes are more cost-effective in terms of dollars per watt installed.
How much power does a 300 watt solar panel produce?
Any solar panel’s power output depends on these factors:
- panel efficiency (monocrystalline is 20% to 22%)
- panel area in m2
- insolation (which depends on your location)
- orientation (angle to the horizontal and vertical)
- fixed or auto-tracking installation
Solar panels are rated under accepted Standard Conditions – when the solar irradiance is 1000 watts/m2 at 25 degrees C. This assumes that the panel is perpendicular to the sun and this is rarely the case!
Solar panel output reduces as the angle to the sun changes. Paradoxically, it also reduces when full-on, if the temperature increase above 25 degrees C. The power output reduces about 0.5 watts per degree increase in temperature.
Discover your solar saving potential
If the irradiance is low due to the insolation level in your geometric location, then the output will also be reduced. Nevertheless, we have to make some assumptions to move forward and point the panels as perpendicular to the sun as possible to maximize output.
Solar inverter working principle – The basics
Solar panels generate direct current (d.c.) whereas domestic appliances us alternating current (a.c.), therefore we need a device to convert from d.c. to a.c.
This is called an inverter, a mysterious box filled with electronic circuits and MOSFETS which chop up the direct current into pulses and re-shape them into a smooth sinusoidal waveform.
How many solar panels to run 5 ton ac unit?
Let’s say that the air conditioner is needed in the hottest part of the day, which is the 2 hours before and 2 hours after noon. This is fortuitous as it’s exactly the window of time when irradiance is high and solar panel output is at its maximum.
1 Ton of AC cooling power is approximately equivalent to 1.2kW, so 4 hours running means that a 1 Ton AC unit would consume 4.8kWh. A 5 Ton AC unit would consume 4.8kWh x 5 = 24kWh.
A 300 watt solar panel generates 300 watts x 4 hours for the same time period (assume ideal sunshine conditions) = 1.2kWh. The simple formula for calculating the number of solar panels to run the AC is:
- AC energy consumption (24kWh) / energy generation (1.2kWh) = 20 solar panels rated at 300 watt
That’s a lot of panels!
How many solar panels to run a 2 ton ac unit?
Once again, we’ll assume 4 hours running in the hottest part of the day for 4 hours when the solar panels output is at a maximum. the calculation is as follows, assuming continuous running and ignoring surge current.
- 9.6kWh / 1.2kWh = 8 panels rated at 300 watts
Can you run a 1.5 ton ac unit on solar without batteries?
Clearly, smaller air conditioning units are much more viable using solar panels. The calculation for a 1.5 ton AC unit becomes:
- 7.2kWh / 1.2kWh = 6 solar panels rated at 300 watts
You could run a 1.5 Ton AC unit on 6 solar panels rated at 300 watts as long as the sun shines!
What about the AC surge current?
Surge current is drawn when the compressor motor starts up or when it begins a compression cycle after idling for a while. Starting from cold is the worst situation and the motor may take 3 or 4 times it’s normal running current in amps – we should assume 4 times to be safe.
The 5 ton AC unit in question may need 4 times more power to get going, which equates to 80 solar panels! Obviously, this is not possible, so what is the alternative? And what if you want to run the AC unit when sunshine is low or even at night?
The answer is to use a bank of batteries for solar energy storage. Not only does this strategy ensure a store of extra power when needed but also allows the possibility of running air conditioning when the sun is low, when it’s not there at all or when it’s cloudy.
Best batteries for solar power storage
Some years ago when I was actively installing off-grid solar panel systems it was common to use huge banks of liquid electrolyte cells, basically old lead-acid types. The same kind of cells were commonly used in telephone exchanges.
They were very heavy but it wasn’t an issue as they were never intended to be moved once installed. Nowadays things have changed radically. Modern lead-acid cells in gel format have deep-cycle characteristics and give good performance. However, they are still very heavy and only have a useful life of 500 charge/discharge cycles.
The rapid development of lithium-based batteries have revolutionised solar energy storage. There are several types but lithium phosphate (LiFeP04) are particularly suited to home-base solar installations.
Characteristics of lithium phosphate batteries:
- Good energy density
- Up to 3 times lighter than the equivalent power lead-acid battery
- Safe technology (li-ion type considered to be more volatile)
- Naturally deep cycle (can be discharged to 100% with no damage)
- No memory effect (reduction in capacity due to incomplete discharges)
- Long life (2000 cycles to 100% discharge, 5000 cycles to 80% discharge)
I use powerful LiFeP04 batteries in all applications due to their power and weight. They are perfectly suited for home use.
Charging a battery with a solar panel circuit
What is a solar charge controller?
A solar charge controller is a device that regulates the voltage and current passing from a solar panel to charge a battery. The battery may also be connected to an a.c. load through an inverter to make a complete working solar system with energy storage.
There are two main types, PWM (Pulse Width Modulation) and MPPT (Maximum Point Power Tracking). MPPT type is the most efficient as it tracks the point at which the solar panel generates the most power. Of course, they are more expensive but worth it over the long term, which for a domestic PV system is 25 to 30 years.
Is Solar Powered Air Conditioning Worth It?
The average payback time for professionally installed domestic solar PV systems is about 8 years, but you’re also helping the environment. This payback period can be significantly reduced if you install yourself.
About 30% of a solar installation cost is material, so a DIY solar powered air conditioning could pay back in as little as 2.5 to 3 years, depending on your location and the number of hours you operate it.
Scientific paper on solar air conditioning (Warning: a bit too technical to be of practical DIY use – for info and interest only).
Do solar panels need maintenance?
Regular cleaning should be carried out to maintain optimum efficiency of a solar panel installation. Accumulation of dust and other debris over time can reduce solar panel output by as much as 30%.
How long do solar panels last?
Quality solar panels have a life-time warranty of between 25 to 30 years. After this time the power output deteriorates to about 80% of it’s maximum output when new. Solar panels will continue to generate power at a gradually reducing rate for many years more.