DIY Solar Powered Air Conditioner | Solar Powered Portable AC Unit

A DIY solar powered air conditioner is an ideal project for the avid solar energy enthusiasts. In this post I’m going to attempt to put one together and test its performance to see if it does the job and for how long!

In researching DIY solar cooling system ideas and designs on the internet I came across a lot of theory, facts and figures and dubious calculations. As an ex-installer of solar PV systems, I know that most professionals use rule-of-thumb guidelines for designing many aspects of a solar panel installation.

Can you run an air conditioner on solar power?

For this particular DIY solar air conditioning I take a very practical approach, measuring when I need to and using basic calculations as sparingly as possible – it’s easy to get bogged down in the detail!

The project is broken down broadly into these 5 Steps:

1. My portable air conditioner – measuring how much energy it consumes
2. Testing solar panel output – flexible and folding solar panels
3. How many solar panels to run portable air conditioner? The calculation
4. Battery needed for a rechargeable portable air conditioner – battery type and sizing
5. Solar system control equipment required – solar charge controller and inverter
6. Performance testing of a rechargeable portable solar powered air conditioner – does it work and for how long?

Material used:

• Multimeter
• Alternating current energy meter (to measure portable AC unit electricity consumption)
• Portable air conditioner (560 watt nominal electrical power rating)
• 100 watt flexible monocrystalline solar panels x 2
• MPPT solar charger
• 1000 watt solar inverter (converts DC volts to AC volts)
• Dokio 80 watt folding solar panel with PWM charger

By the end of this instructional post you will know how to assess your AC power consumption, how many solar panels would be needed to run a portable air conditioning unit and what kind of battery storage is advisable.

Let’s go!

Designing A DIY Solar Powered Air Conditioner

• Step 1: How many watts does a portable ac use per Hour?

Air conditioning units are measured in two ways – electrical power input in watts and cooling power in watts. Already the subject gets a little confusing. What is the difference between electrical watts and cooling watts?

A watt is a way of expressing transfer of energy. It equals one joule per second (I know, more technical information than we need right now.) In short, AC electrical watts drives a certain amount of heat in Watts out of your house, and electrical watts is always lower.

It saves a lot of confusion if we express the amount of heat in BTU (British Thermal Units) instead. That way there’s no confusion. The portable air conditioning unit I’m using for this DIY project is rated at 5000BTU.

Solar Powered Portable AC Unit Project – Power Consumption

In the USA big AC units are rated in Tons. 1 Ton = 12000BTU. Converted to Tons my portable AC unit is rated at 0.42 Tons – pretty small but it does the job and will cool a small room of around 180 square feet (14m2). I’m not a big fan of these rule-of-thumb calculations as there are other variables to consider, such as room height and direct sunlight.

Try the calculator below to see how big an area your AC should cool in theory.

My portable AC unit label declares electrical input as 561 watts but what does that really mean? Does it consume that all the time? I know that air conditioners, like fridges and freezers, go on and off during the day so real consumption is obviously lower than this number.

Both systems have motor-driven compressors that go through load/unload cycles. An AC unit compressor can be running for anything between 40% to 70% of its time and is dependent on many factors

It’s important to know if we want to figure out running costs but peak current in amps is also important when sizing solar panels for air conditioning. Rather than use installer’s rule of thumb measurements, I would rather connect measuring instruments and get true realistic values for electrical consumption.

The main instrument I’m going to use is an a.c. energy meter.

Using An A.C. Wattmeter To Measure Air Conditioner Input Power

What Does A Wattmeter Do?

An a.c. energy meter measures watts, volts, amps, kWh and power factor

The photo on the left shows the display of the exact wattmeter I’m going to use for this project. As you can see, it measures more than just voltage and current.

We could use a multi-meter to measure volts and current, but in an a.c. circuit there’s a lot more going on than meets the eye! In a d.c. circuit watts = volts x amps, but in an a.c. circuit sometimes it does and often it doesn’t.

This is because the voltage and current waveforms are constantly changing in a sinusoidal waveform pattern, sweeping through minimum and maximum values 60 times a second (Hz) – if you live in the U.S.

The frequency is 50Hz if you live in Europe, but the principle and effect is just the same.

Portable AC Unit Energy Consumption Test Results

Before looking at the figures, I’d just like to say some of the values are of more interest than others. Not only is the overall energy consumed in watt-hours important to calculate the battery storage needed, but instantaneous values of current also tells us how much maximum power we need.

Compressor Running and Peak Currents

An air conditioning unit has three states:

• it is stopped and no current flows
• it is running and current flows
• it is just starting – in this case the current may be 3 or 4 times the running current

The first thing to take on board is that there is no steady state. An AC unit will run until it reaches the temperature set by the thermostat, stop while the temperature is below a certain limit and then start again when the thermostat calls the motor into action.

There are 3 different levels of current. In particular, the surge or peak current needs to accounted for. If we don’t have a big enough supply to meet the peak current, then the compressor won’t start and will probably burn out.

Voltage and Frequency

When operating on mains electricity neither voltage nor frequency will move, or hardly at all. The voltage may dip a little, but this normally only happens when big loads start up.

However, when running on an inverter which converts DC to AC, the output voltage will drop as the battery discharges. When it reaches a certain level, a beeping sound normally tells you that something isn’t quite right. The inverter will trip out when the volts becomes too low, perhaps beeping only when the volts drop as the motor starts.

For this test I’m going to monitor these values:

• running watts and current
• standing watts and current (fan running only)
• compressor surge current (motor start)
• the Power Factor (PF)

Note: In an AC circuit with motor loads, the current drawn is out of phase with the circuit voltage. It lags behind it. In this case watts does not equal volts x amps as in a DC circuit, but is less.

The current is bigger because the voltage and current waveforms are not matched. The Power Factor (PF) can be anything from 0.8 to 0.99. A low PF can make a considerable difference to the maximum current needed and should be taken into account when sizing solar panels.

Portable AC Test Results:

As it isn’t summer here at the time of writing this post, I decided to run the unit at the current ambient temperature of 21°C (69.8°F) and set the target temperature to 17°C (62.6°F). Normally, I wouldn’t bother using AC unless the temperature inside my designated ‘cool room’ was approaching 30°C (86ºF).

One pleasant surprise was the motor surge current when the compressor kicks in. These units run the fan for a period of time and then start the compressor. The compressor motor start current wasn’t that much more than the running current, so the unit must have soft-start circuitry.

Here are the test results and other stats:

There are no real surprises – as expected from a new appliance, the Power Factor is very good at 0.99 with the compressor running and the surge current isn’t much more than compressor running current.

Why am I so interested in surge current? Because a low power factor causes the current draw to increase and this maximum values has to be matched by the solar panel/battery combination if we want to have a reasonable cooling time with the unit.

How many watt-hours does this portable AC unit use?

The unit takes 45 minutes to lower the room temperature by 5°C (9°F), so for this period of time the unit is drawing 450 watts. Let’s work out how much energy in watt-hours this air conditioner uses during operation:

Energy consumed – startup period = 450 watts x .75 hours = 337.5 watt-hours (W/hrs)

When cooling temperature is achieved, the unit runs fan only for 35% of the time and then the compressor runs for 65% of the time in order to maintain temperature.

Energy – at temp (per hour) = (450 watts x 0.65 hours) + 109 watts x 0.35 hours)

= 292.5 W/h + 38.15 W/h = 330.65 watt-hours

The startup energy used for the first 45 minutes is almost the same as running the AC for 1 hour, so I’ll split the difference and say that this unit uses 335 W/hrs or 0.335 kW/hrs.

This is our target consumption of energy – we need a solar system capable of running the unit for, say, 4 hours per day – on to the solar panels …

• Step 2: How Much Energy Does A 100 Watt Solar Panel Produce?

This process is much less complicated than you might think. You don’t need multi-meters or other complicated test instruments. Professional solar installers use ready-made tables and data from third-party sites to estimate solar panel output.

What affects solar panel output?

Several factors affect solar panel output, such as temperature, cloud cover, regular cleaning and the type of solar charge controller used, but mostly we’re concerned with just 3:

1. Irradiance for your location (how much sunshine the solar panel gets)
2. Orientation (the direction the panels points to)
3. Title angle

Orientation and tilt angle are minor points, because we can adjust them to optimize solar panel output. Point the panel to the South (if in the Northern hemisphere) at about 45° and it will be fine. Orientation is more important than title angle.

Once those parameters are taken care of we’re left withe one thing we can’t control – irradiance.

How to find out out how much irradiance in your location.

There’s an incredibly useful site for finding solar data for any region (screenshot below). It’s called Global Solar Atlas and once to figure out how to use it, it can save a lot of time.

The image below shows the irradiance for Houston, Tx and this value can be used to work out how much energy overt time your solar system will deliver – I explain further underneath the image.

Just enter your location at the top of the page and you can find out how much any size of solar installation will generate for your exact location.

In Houston, Texas the hottest months are June, July and August. The table below shows how much sunshine can be expected from historical records:

The value of kW/hrs/m2/day is also known as Peak-Sun-Hours. Assessing the output of solar panels becomes a piece of cake when I tell you that it can be found by multiplying this figure by the kw rating of your PV panels.

If I had a 100 watt solar panel in Houston, Texas, I could expect this amount of energy to be generated each day in August:

0.1 kW x 5.44 Peak-sun-hours = 0.544 kW/hrs

(My location in Portugal has about the same amount of sunshine, incidentally!)

• Step 3:How many solar panels to run portable air conditioner?

Remember that the AC unit needs 0.335 kW/hr each hour, and a 100 watt solar panel generates o.554 kW/hrs of energy per day, so we need one 100 watt panel for each hour the AC will be running.

Let’s say I want to run the AC for 4 hours, so I need 400 watts of solar – I could make it 500 watts to be safe and account for a cloudy day or two, but I’ll see how it goes.

What happens if a cloud passes over and the sun is hidden for extended periods? What if it’s steaming hot, but cloudy (it can happen) and solar output is down 50%?

I suffer from the heat and really depend on some cooling during the hottest months – if you’re the same,  I would suggest the following:

1. Use a deep cycle solar storage battery capable of running the AC for a period without any solar power at all. The combination of the two measures will ensure you have cooling when you need it.

Battery back-up is the only way to go for complete security, but you’ll also need a solar charge controller and an inverter.

• Step 4: How many 12 volt batteries to run an air conditioner?

As a general rule for a full size air conditioner, too many batteries would be required to run it. For a portable AC unit with solar backup, a 12 volt 240Ah deep-cycle battery would run a 560 watt rated unit for 4 hours.

My portable AC unit uses 1.5 amps (on average) of AC power at 240 volts (I’m in the EU). Accounting for some losses, this translates in 12 volt  battery current to:

1.5 amps x 240/12 = 30 amps at 12 volts

I would need a deep-cycle battery with a capacity of 60 Ah (ampere-hours) for each hour it’s intended to run.

Although deep-cycle batteries are advertised capable of deep discharge of up to 80%, this isn’t really recommended as it really reduces the overall life of the battery.

At 50% discharge, a deep-cycle lead acid battery is generally good for around 500 charge/discharge cycles or twice as long than if discharged to 80%.

To run my 560 watt rated portable AC unit for 4 hours a day in August I would need 400 watts of solar feeding into 240 Ah of deep-cycle batteries.

How long does a lithium iron phosphate battery last?

IMO lithium phosphate batteries are far superior to lead acid, although I use both. I originally turned to lithium because I was getting to old to lug around lead acid cells, which are 3 to 4 times heavier.

In fact, they have many more advantages, including a long life. They can be discharged safely to 80% to 90% with charge/discharge of 5000 times, so for many applications a lithium phosphate battery is a life-time purchase.

In my portable AC application, I would need only 150 Ah capacity if using lithium phosphate. Of course, they are twice as expensive up-front, but cheaper over the long term.

How much does it cost to run a portable ac?

The price of electricity in my location is equivalent to $0.27 US or 27 cents. At 0.335 kW/hr I would pay just $0.0536 or 9 cents per hour.

Over 24 hours I would pay 24 x 0.09 = $2.17 per day ($65/month, $780/year.)

The table below gives you some idea of the cost of running my 560 watt portable air conditioning unit for a day, a month and over the year.

Note: Keep in mind AC isn’t used all year – I added this table for interest only!

What else do I need to run a portable AC unit with solar panels?

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We discussed how batteries would be need to run portable AC effectively – you would also need a solar charge controller (SCC) to charge up the batteries and an inverter to convert the solar panel/battery voltage from DC to AC.

Four 100 watts solar panels can either be connected in parallel, in series or a combination of the two. For this project I’m going to use a 350 watt rated MPPT charge controller which has a maximum PV input voltage of 50 volts.

Note: MPPT solar controllers extract more power from solar panels than the cheaper PWM and can accept higher input voltages. If you use a PWM regulator, the panels must be connected in parallel.

For the charge controller I’m using, I can connect the 4 solar panels both ways – I’ll connect them in series in pairs, and then connect the pairs in parallel. In this way the combines open circuit voltage (Voc) for the arrangement will be 42 volts, which is within the recommended maximum voltage of the SCC (50 volts) – see the drawing below:

What size inverter do I need?

A good rule of thumb is to allow double or triple the power requirements of the appliance. In this case we would need 335 watts, so I’ll use a 1000 watt continuous rated modified sine wave inverter.

Modified sine wave inverters are the cheaper versions – pure wave inverters are more expensive but don’t interfere with electronic devices, which can happen with these cheaper models.

In this case, my usage will be intermittent and I don’t have sensitive electronic equipment in the vicinity.

Here’s the final circuit:

People Also Ask

How many solar panels to run ac unit?

As a general rule allow 1200 watts of solar power (4 x 300 watt solar panels) for every 1000 watts of rated AC power. Energy storage in the form of deep cycle batteries and a suitably sized inverter would also be required.

Do portable air conditioners use more electricity than window air conditioners?

A small portable unit of the same size as a window ac will consume more power because it is harder to exchange heat. A portable ac unit requires venting hot air outside with duct piping, which actually warms up and heats the very air that you are trying to cool!

In contrast, if your room has many people in need of cooling or warm heating inside, then having a large stationary AC would be better suited for this situation while being just as efficient at getting rid of energy like the smaller one does but without using all those extra watts on sucking out heat from your home’s atmosphere.

Resources:

Global solar atlas – find peak sun hours per location

Solar irradiance per location – 12 month spread