AC & HVAC Generator FAQs

If you’re new to generator sizing, you may want to start with the Home Generator Sizing Guide or the General Generator FAQs before diving into AC-specific details.

What size generator do I need to run a house with two AC units?

Two central AC systems are almost always the limiting factor in a residential backup setup. The exact generator size depends on:

• The tonnage of each system (2 ton vs. 3–5 ton).
• How efficient they are (older vs. newer high-SEER units).
• Whether soft-start or hard-start kits are installed.
• What else you want running at the same time (pumps, range, dryer, etc.).

As a rough range for typical U.S. homes:

• Two 2–3 ton systems with gas heat and mostly gas appliances often land in the 24–26 kW standby range for “most of the house” coverage.
• Two larger 3–4 ton systems plus higher plug loads or some electric heat may require 26–30 kW or more to run both systems and major household loads comfortably.

Installers commonly use load management modules so that only one compressor starts at a time. Even if both AC systems run together, staggering start-up reduces the instantaneous surge the generator has to cover.

The safest way to size for two systems is to:

1. Collect the nameplate data (amps, volts, “LRA” or locked-rotor amps) for both condensers and air handlers.
2. Estimate running kW and starting kW for each system.
3. Run those numbers through a load calculator and then have an electrician confirm.

Will a 10,000 watt generator run central air?

Sometimes — but only under the right conditions, and usually for a single smaller system with careful load management.

A typical 2–3 ton single-stage central AC system often has:

• Running demand in the ballpark of 2,500–4,500 W (compressor + outdoor fan + indoor blower).
• A short starting surge that can be 2–3× the running watts without a soft-start kit.

For a 10 kW portable to work with central air, you usually need:

• A system on the smaller/more efficient side (for example, 2–2.5 ton, modern unit).
• A generator with a decent surge rating (many 10 kW portables surge to 12–12.5 kW).
• Ideally a soft-start or hard-start kit on the AC to reduce the inrush current.
• Very careful load management: start the AC first, then add other loads like the fridge, lights, and well pump.

If your system is larger (3–4 ton), older, or you have other heavy loads that need to run at the same time, a 10 kW portable is often marginal for central air. In that case, many homeowners either:

• Use the generator for essentials and window or mini-split units instead of central air, or
• Move up to a larger portable or a properly sized standby generator.

How many watts does it take to start a 4-ton AC unit?

Exact numbers depend on the specific equipment, but a 4-ton (≈48,000 BTU) single-stage central AC system commonly looks like this:

• Running demand: roughly 4,000–6,500 W for the compressor, condenser fan, and indoor blower combined on a typical system.
• Starting (surge) demand: often 2–3× running for a fraction of a second if no soft-start kit is installed.

Many 4-ton systems therefore need 10,000–18,000 W of short-term starting capacity from the generator, even though the running load is far lower. The nameplate “LRA” (locked-rotor amps) on the condenser is the best indicator — multiply that by 240 V to estimate peak starting VA.

Soft-start modules can significantly reduce that spike, sometimes bringing a system that would otherwise be out of reach for a mid-size generator into a workable range. It’s still important to confirm with a qualified HVAC and electrical pro before counting on a marginal setup.

What size generator do I need to run a 5-ton AC unit?

A 5-ton (≈60,000 BTU) AC system is a large residential or light commercial load. Without soft-start equipment, many 5-ton condensers have:

• Running demand in the neighborhood of 5,000–7,500 W.
• Short-term starting demand that can exceed 15,000 W and sometimes much more, depending on LRA.

For most homes that truly need a 5-ton system, you’re generally in standby generator territory, not small portables. Common outcomes:

• A mid-20s kW standby unit may be adequate if you’re willing to limit what else runs during compressor start and use load management.
• Larger homes with significant additional loads (multiple refrigerators, pumps, office gear, electric appliances) often end up closer to 24–30 kW or more for comfortable operation.

Again, the only way to know for sure is to pull the actual nameplate data, use a load calculator, and have an electrician verify the design.

How many watts does a typical whole-house AC system use?

For a conventional split-system central AC, a rough rule of thumb is:

• 2 ton (≈24,000 BTU): around 2,000–3,500 W running.
• 3 ton (≈36,000 BTU): around 3,000–4,500 W running.
• 4 ton (≈48,000 BTU): around 4,000–6,500 W running.
• 5 ton (≈60,000 BTU): around 5,000–7,500 W running.

Actual values vary based on efficiency (SEER/EER), design (single-stage, two-stage, variable speed), ductwork, and blower size. The starting surge can be 2–3× those running values if the system does not use inverter technology or a soft-start module.

Newer variable-speed or inverter-driven systems tend to have lower running watts and softer starts, which can make them easier to run on a generator than older, less efficient equipment of the same tonnage.

What size generator do I need to run a window AC unit?

Window units are much easier to back up with a small portable. Typical ranges:

• 5,000–8,000 BTU (small bedroom): about 400–800 W running, 1,200–2,000 W starting.
• 10,000–12,000 BTU (larger room): about 800–1,400 W running, up to ~3,000 W starting.
• 15,000 BTU and up: can approach 1,800+ W running with higher starting surge.

In practice:

• A 2,000–3,000 W inverter generator can usually handle one small-to-medium window unit plus some lights and phone charging.
• A 3,000–4,000 W unit can often handle a larger window unit and a refrigerator, as long as big loads don’t start at the exact same moment.

Always check the unit’s nameplate or manual; some older or less efficient models draw considerably more power than newer, high-efficiency ones.

Do I need soft-start kits if I plan to run AC on a generator?

You don’t always need them, but they are often the difference between “works on paper” and “works reliably in real life,” especially with smaller generators.

A properly sized soft-start or hard-start kit can:

• Reduce the instantaneous inrush current when the compressor starts.
• Help prevent nuisance tripping or voltage sag.
• Make it possible to use a generator that would otherwise be marginal.

These kits must be matched to the equipment and installed correctly, so this is a good place to involve both your HVAC contractor and electrician.

Want to see how your AC loads affect generator size? Open the generator sizing calculator and include your AC tonnage when you run the numbers.

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