One of the first questions homeowners ask when considering battery storage is:
Can my solar and batteries actually run my air conditioner during a power outage?
The answer is absolutely yes. A properly designed solar and battery system can run an air conditioner during an outage, including many larger central air conditioning systems. But there is an important word in that sentence: Properly designed.
Running a refrigerator, lights, Wi-Fi router and a few outlets is relatively easy for most modern home battery systems. Running a large central air conditioner is different. Air conditioning can be one of the largest electrical loads in your house. It requires a significant amount of power while running and can require an even larger burst of power when the compressor first starts. That means there are really 2 questions we need to answer: Can the battery start the air conditioner? And: How long can the battery run the air conditioner? Those are completely different calculations. Let’s break down both.
A 3 Ton AC Is Very Different From a 5 Ton AC
Central air conditioners come in different sizes. Two common examples are 3 ton and 5-ton systems. A 3-ton air conditioner is commonly used for smaller cooling loads, while a 5-ton system is substantially larger. You may see something like a 30-amp circuit associated with some smaller systems, while larger systems may require circuits around 50 or 60 amps. However, the breaker size does not tell us exactly how much electricity the air conditioner continuously consumes. To properly evaluate an AC unit for battery backup, we want to look at the actual equipment nameplate and electrical specifications.
Some of the important numbers include:
- Voltage
- Minimum Circuit Ampacity
- Maximum Overcurrent Protection
- Rated Load Amps
- Locked Rotor Amps (particularly important when designing battery backup.)
What Is LRA?
LRA stands for Locked Rotor Amps. You can think of this as the amount of current the compressor may demand during startup under locked rotor conditions. An air conditioning compressor is an electric motor. Once that motor is already spinning, keeping it running can require considerably less instantaneous power than getting it started. Think about pushing a heavy vehicle. The hardest part is getting it moving. Once it is rolling, keeping it moving is easier. An air conditioning compressor has a similar issue electrically. When the thermostat tells the AC to turn on, the compressor can create a very large momentary electrical demand.
The utility grid generally handles that surge without you ever thinking about it. During a power outage, however, you are no longer connected to the utility grid. Your battery inverter has to handle it. That is why startup capability matters so much.
Not Every Battery Has the Same Startup Capability
When homeowners compare batteries, they often look at only one specification: How many kWh does the battery store? That number is important, but it only tells us part of the story. A battery system also has power ratings. You need to know how much continuous power it can deliver and how much temporary surge power it can provide.
This is especially important if you want to back up large motor loads such as air conditioning. A battery can have plenty of stored energy and still have difficulty starting a particular AC compressor if its inverter cannot provide the required startup power. That is why we evaluate both sides of the equation. kW tells us how much power the system can deliver. kWh tells us how much energy the system has available over time. You need enough of both.
Can a Tesla Powerwall Run a 5-Ton Air Conditioner?
Potentially, yes. Modern battery systems such as Tesla Powerwall are specifically designed with large household loads in mind. Tesla, for example, publishes motor start capability for Powerwall 3, and this is one of the reasons equipment specifications matters when designing whole home backup. If the battery system has sufficient motor starting capability for the specific compressor, it can start and operate the AC without assistance from the grid.
However, we never recommend assuming that every 5-ton air conditioner will behave exactly the same. Two 5-ton units can have different compressors, different efficiencies, different startup characteristics and different electrical requirements. The correct approach is to look at the AC nameplate and compare the equipment requirements with the battery manufacturer’s specifications.
What If the AC Startup Current Is Too High?
This does not automatically mean you cannot back up the air conditioner. One potential solution is a soft start. A soft start device is designed to reduce the electrical stress associated with starting the compressor. Instead of allowing the compressor to create such a large instantaneous startup demand, the soft start helps manage the starting process. This can make a major difference when operating an air conditioner from battery power.
If the battery system is having difficulty with the compressor startup requirement, installing an appropriately selected and approved soft start may make the system much easier to operate during an outage. This is why we do not automatically tell someone: “Your AC is too big. You cannot back it up.” We look at the equipment first. There are often multiple ways to design the system correctly.
Starting the AC Is Only Half the Equation
Once we know the batteries can successfully start the air conditioner, we have to answer the second question: How long can they run it? This is where battery capacity becomes extremely important. Let’s say an air conditioner is consuming approximately 9,600 watts while operating.
That is: 9.6 kW. If it ran continuously for 1 hour, it would consume approximately: 9.6 kW × 1 hour = 9.6 kWh That is a large amount of energy.
But your air conditioner normally does not run continuously all day. The thermostat turns the system on. The house cools down. The thermostat reaches its set temperature. The compressor turns off. The temperature eventually rises again and then the air conditioner turns back on. This is called cycling, one of the biggest reasons there is no universal answer to how many hours a battery can run an AC.
Your House Determines AC Runtime, Too
Imagine two identical homes with the same 5-ton air conditioning system and the same battery setup. On paper, you might expect them to get roughly the same amount of runtime during a power outage, but that isn’t necessarily the case.
House A is well insulated, with efficient windows, a properly insulated attic, and doors that are well sealed. The thermostat is set to a reasonable 76 degrees, and the home is already cool when the power goes out. Once the AC reaches the desired temperature, the compressor can shut off and stay off for longer periods.
House B is a different story. Older windows, poor insulation, and air leaks allow heat to enter the home much more quickly. If the thermostat is set to 70 degrees and the house is already warm when the outage begins, the AC may need to run almost continuously just to catch up and maintain that temperature.
Even though both homes have the same AC and battery system, their actual battery runtime could be dramatically different. How efficiently a home holds its temperature—and how hard the AC has to work—can make a significant difference in how long stored battery power lasts.
How to Estimate Battery Runtime
There is a simple way to understand the basic calculation. Imagine your battery system has 27 kWh of usable stored energy. For an easy example, assume the AC is drawing 9 kW while running. If it somehow ran continuously and there were no other loads or system losses:
27 kWh ÷ 9 kW = 3 hours. That is the theoretical starting point.
But that does not mean you automatically get exactly 3 hours of air conditioning. Real systems have conversion losses, reserve settings and other household loads. On the other hand, your AC probably will not run continuously. Suppose it averages a 50 percent duty cycle over a period of time.
Instead of consuming 9 kWh every clock hour, its average consumption across that period might be closer to 4.5 kWh per hour. That dramatically extends the amount of time the house can remain comfortable. This is why, depending on the home and battery configuration, you might see several hours of effective cooling from one battery setup and substantially longer from a larger battery system. Rather than promising a specific number of hours, we prefer calculating it based on the actual equipment and home.
What Other Electronics Are You Running During the Outage?
This is another huge factor. Your batteries are not dedicated exclusively to the air conditioner unless the system has specifically been designed that way.
During an outage, you might also be running:
- Refrigerators
- Freezers
- Lights
- Televisions
- WiFi
- Computers
- Pool equipment
- Well pumps
- Microwaves
- Electric ovens
- Water heaters
- EV chargers
- Other household appliances
Every one of those loads consumes energy from the same battery bank. Imagine your AC is running and someone starts an electric oven while another person plugs in an EV. Your battery consumption can increase dramatically. That is why energy management becomes important during an extended outage. If your goal is to keep the home cool overnight, you probably do not want to simultaneously charge your Tesla at maximum power from your home batteries. Prioritize what matters.
Solar Completely Changes the Runtime Calculation
This is where having solar plus batteries becomes much more powerful than having batteries alone. Imagine the power goes out at 2:00 PM on a sunny day. Your air conditioner is consuming 5 kW at that particular moment, but your solar system is producing 8 kW. The solar may be able to operate the AC and other household loads while excess production charges the batteries, subject to the design and operating limits of the system.
Now imagine the batteries were partially discharged overnight. The sun comes up the next morning. Solar production begins again. The solar system can potentially power the home while also replenishing the batteries. That means the question is no longer simply: How many hours will my battery last? It becomes: Can my solar production replenish enough energy during the day to support my loads and prepare the batteries for the next night? That is a much more powerful backup strategy.
Why Solar System Size Matters: Whole Home Backup
If you want serious whole home backup, we do not design around battery capacity alone.
We look at the complete energy system, asking questions including:
- How large is the solar system?
- How much does it produce?
- How many batteries are installed?
- How large is the air conditioner?
- What is the AC startup requirement?
- What does the AC consume while running?
- What other loads need backup?
- How efficient is the house?
- How much electricity does the home consume overnight?
These questions determine whether the system can provide a few hours of emergency backup or potentially sustain critical household loads through a much longer outage when weather and solar production cooperate.
One Solar Battery vs Multiple Batteries
Adding additional battery capacity can dramatically change the experience during an outage. More batteries can provide additional stored kWh and, depending on the equipment architecture, additional available power. That can be particularly valuable in homes with large air conditioners and other heavy electrical loads.
A smaller battery system might be perfectly adequate for:
- Lights
- Refrigeration
- Internet
- Televisions
- Outlets
- Smaller appliances
But if the homeowner says, “I want my house to operate almost normally when the power goes out, including my 5-ton AC,” we need to design around that expectation. That may mean additional battery capacity, adequate inverter output, proper load management, a soft start where appropriate and enough solar production to replenish the energy being consumed.
Whole Home Backup Needs to Be Designed, Not Assumed
This is probably the most important takeaway. Installing a battery does not automatically mean every electrical load in your house can operate indefinitely during a blackout. Whole home backup requires proper design. At Supreme Solar and Electric, we evaluate the actual loads the homeowner wants to operate.
If air conditioning is important, we look at the AC specifications. We determine its startup requirements. We determine its running demand and compare those requirements with the battery system. We look at how much battery capacity is available. And then we look at the solar system to determine how much energy can potentially be replenished during daylight hours. That is how a true backup system should be designed.
Can Solar and Batteries Run Your AC During a Blackout?
Yes, absolutely. A properly designed solar and battery system can operate central air conditioning during a power outage. A smaller 3-ton system will generally be easier to accommodate than a larger 5-ton system, but even larger AC systems can often be backed up when the battery system has sufficient startup and continuous power capability. If compressor startup is an issue, a properly selected soft start can sometimes reduce that startup demand.
From there, runtime depends on the amount of battery storage, the actual power consumption of the AC, how frequently it cycles, the efficiency of the home, the other loads operating during the outage and whether solar is available to replenish the batteries. That last part is critical. A battery by itself contains a finite amount of energy. Solar gives you the ability to generate more. When solar and battery storage are designed together correctly, your home can continue producing electricity, storing electricity and operating major loads even while the utility grid is down.
So, if keeping the air conditioner running during an outage is one of your priorities, tell your solar contractor that before the system is designed. Do not simply ask, “How many batteries do I need?” Ask, “Can this system start my air conditioner, run my air conditioner and still have enough energy to operate the rest of my home?” That is the question that leads to a properly designed whole home backup system.
Learn more about the capacity of your solar energy system and solar batteries, and how they can run your AC during blackouts from our solar professionals at Supreme Solar. Contact our Palm Desert solar energy system and battery experts to schedule an appointment.