In most Texas homes, the air conditioner is the reason the generator conversation starts in the first place. Keeping the house cool through a summer outage is the outcome people are paying for. The trouble is that an air conditioner is not hard on a generator while it runs. It is hard on a generator in the first second, when the compressor starts. A soft start kit is a small device that changes that first second, and on the right system it can be the difference between a generator that struggles and one that carries the house without complaint.
Why the compressor start is the hard part
An air conditioning compressor is an electric motor, and electric motors draw far more current when they start than when they run. Kohler's engineering paper on sizing generator sets for motor starting states that starting current is typically six times a motor's rated full-load current. On utility power, the grid absorbs that surge without anyone noticing. On a generator, the surge shows up as a sudden voltage dip, which is why lights flicker, and in a badly matched system, why the AC fails to start or other equipment drops out.
Generac's generator sizing guide puts real numbers on it. It says a 3-ton central air conditioner typically needs about 3.5 to 5 kW of running wattage but can demand up to 15 to 18 kW of starting power to get the compressor moving. That gap between running and starting is what drives so many sizing decisions.
It is also worth having the air conditioner itself in good shape before you rely on it during an outage. ENERGY STAR's heating and cooling maintenance checklist recommends having a contractor do annual pre-season check-ups, and that visit is a good moment to ask your HVAC contractor about a soft start.
What a soft start kit does
A soft start kit is installed at the outdoor condenser unit, wired into the compressor circuit. Instead of letting the compressor slam on at full inrush, it manages the start so the current ramps up over a short moment. The compressor still starts, and it runs exactly as it did before. What changes is the peak current at the instant it starts.
For a home on a standby generator, that typically means:
- A much smaller voltage dip when the AC kicks on, so lights and electronics are less likely to flicker.
- Less strain on the generator each time the compressor cycles.
- In some homes, the ability to start the AC on a generator that otherwise would have been undersized for it.
A soft start does not reduce how much power the AC uses once it is running. If the running load of the house plus the AC is more than the generator can carry, a soft start will not fix that.
Soft start, load management, or both
A soft start is one of several tools for getting air conditioning onto a generator. The other major tool is load management, which controls when large loads are allowed to run rather than how they start.
Both major brands we install offer load management at the transfer switch. Kohler describes its load shed kit as monitoring generator current and frequency to decide when to shed loads, and states that load management allows the use of a smaller generator set. In practice, a well-designed system often uses both. Load management keeps two big loads, such as an AC and a well pump, from starting at the same moment, and a soft start softens the AC start when its turn comes.
When a soft start makes sense
A soft start kit is worth considering when:
- You have one or more central AC systems and want them on the generator.
- The sizing calculation is close, and a soft start keeps you from stepping up to a larger unit.
- You already have a generator and the AC starts roughly, or the lights dip hard every time the compressor cycles.
It is less useful when the running load is the real constraint, or when the system has a variable-speed compressor that already ramps up gently. Before any device is added to an AC system, check with the HVAC equipment manufacturer or your HVAC contractor that it will not affect the equipment's warranty.
How we fit it into the design
We do not pick a generator by square footage. We inventory the loads, work out the worst realistic combination that could start together, and design around it. Our generator sizing page explains that process, and our guide to what size generator a house needs walks through it in more detail. For the principle behind all of this, our earlier post on why sizing is about starting, not running is a good short read.
When a soft start, load management or both will let a smaller generator do the job reliably, we tell you. When they will not, we tell you that too.
Talk to Armored Power Systems
Want your air conditioning on backup power without oversizing the generator? We will look at your AC, your panel and your other big loads and show you the options.
Call (817) 381-6158 or email info@armoredpowersystems.com.