How to Think About Generator Size for Your Home

Almost every homeowner who considers a generator asks the same question: what size do I need? It sounds like it should have a single answer, and sellers of generators often make it seem that way. In reality, the right capacity depends on what you want to keep running, how those loads behave, and how your house is wired.

This article is not a sizing chart. It is a way of thinking that helps you describe your needs clearly, so that any conversation with an electrician or a supplier starts from facts instead of guesses. It applies to portable units, standby systems and anything in between.

Start with the question of what you want to keep running

Generator size is the answer to a prior question: what is the goal? A household that wants to keep the refrigerator cold and the phones charged has very different needs from one that wants the whole house to behave as if nothing happened.

Most homeowners fall into one of a few broad categories, and it helps to be honest about which one you are in.

• Essentials only. Refrigeration, a handful of lights, a fan, communications and perhaps a sump or well pump.
• Essentials plus comfort. The above plus one or more cooling zones, a television, a microwave and a few small appliances.
• Most of the house. Air conditioning across the home, the water heater, the kitchen and laundry loads that run at the same time.
• Whole-house, everything. A system that picks up the entire panel automatically, including the biggest loads, with little thought from the family.

The larger your goal, the larger the generator, and the more important professional planning becomes. Deciding on the category first saves a great deal of wasted effort.

Running load and starting load are not the same

Electric devices use power in two different ways. Resistive devices, such as lights and heaters, draw about the same amount all the time. Motor-driven devices, such as refrigerators, air conditioners, pumps and some tools, draw much more current for a moment when they start than when they run.

That starting surge matters because a generator has to be able to deliver it without bogging down or tripping. A refrigerator that uses a modest amount of power while running may demand several times that during the second it starts. An air conditioner compressor does the same thing, with bigger numbers.

When a generator is too small for its starting loads, symptoms show up: lights dim when the compressor kicks on, the engine strains, breakers trip, and motors fail to start. Over time repeated stress can shorten the life of both the generator and the appliances.

This is why sizing involves two ideas. One is the total running load that may be on at the same time. The other is the largest starting surge the generator must handle on top of everything else that is already running.

A short example of how the thinking works

Picture a household that wants to keep the refrigerator, a chest freezer, six lights, two fans, a television, a router, a sump pump and phone chargers running. Most of those are small loads. The refrigerator, freezer and sump pump are motors, so each has a starting surge. The sizing question becomes: what is the combined running load, and what is the worst starting surge on top of it?

If the three motors will not start at the same instant, the worst case is the biggest single surge landing on top of the other running loads. That is a manageable number and points to a modest generator. Now add central air conditioning to the same list, and the largest starting surge jumps. The whole answer changes because of one decision.

This is why it is hard to find an honest answer in a one-line chart. Two homes of the same square footage can need very different capacity, depending on what the people living there choose to keep running.

Air conditioning is the biggest swing factor in Florida

In South Florida, cooling is not a luxury during a long outage. It is also the single largest variable in generator sizing. A central air conditioning system, with its compressor and air handler, is often larger than everything else in the house combined.

The type of air conditioning matters. Older single-stage systems have a hard start. Newer systems with variable-speed compressors or soft-start devices draw less current at startup and are far easier on a generator. Ductless mini-split systems are efficient and run in zones, which makes them a natural fit for partial-house backup. Our post on mini-split AC circuit installation shows how those circuits are set up.

• If you only plan to cool one or two rooms, a smaller generator and a ductless setup may be enough.
• If you want the whole central system running, plan for substantial capacity, and ask about soft-start options.
• If you have more than one air conditioning system, ask whether they need to run at once or whether one can be sequenced behind another.

Decide early whether air conditioning is a must-have. That one answer moves your sizing more than any other.

Load shedding is another useful idea for air conditioning. Some systems and transfer equipment can automatically drop a large load, such as a water heater or a second air conditioner, when the generator is already near its limit, and restore it when demand falls. Smart load management lets a smaller generator behave like a larger one, as long as the family is willing to accept that not everything runs at once.

Think too about humidity. In summer, a house without air conditioning builds moisture, and mold becomes a concern after several days. Many Florida households therefore treat at least one cooled and dehumidified space as a core need, rather than a comfort, and size their backup around it.

Other large loads that change the answer

After cooling, a few other categories can swing the decision. They are worth listing, because homeowners tend to remember some and forget others.

• Electric water heater. It draws a steady heavy load while heating. Many households choose to leave it off during an outage or to run it on a schedule.
• Electric range and oven. Large and not essential for most people during short outages, which is why they are often left off a backed-up panel.
• Dryer. Another heavy load that can usually wait.
• Well pump or booster pump. Essential for homes that rely on one, and a motor with a starting surge.
• Pool pump and equipment. Usually optional during an outage.
• EV charger. A very large load that most households will not want a generator to feed.

The honest approach is to decide which of these you will keep and which you will accept going without. It is almost always more sensible to manage the loads than to buy a generator large enough to ignore them.

Notice that several of these items are choices, not requirements. Plenty of households find that a hot shower matters more than a clothes dryer, and that charging an EV can wait until the grid returns. Writing these decisions down turns a vague worry into a clear list, and a clear list is what lets the generator be sized correctly the first time.

Consider voltage as well. Many of the heavy loads in a home, such as the dryer, range and central air, run on 240 volts, while lights and most outlets run on 120 volts. A generator and transfer arrangement must support both, and it should be matched to the loads that you actually plan to serve.

How your electrical service and panel come into it

The size of your electrical service does not determine generator size, but the two are related. A home with a larger service and many circuits tends to have more loads that people hope to keep running. A smaller service often means fewer large appliances and a more modest backup goal.

If you are planning a permanent connection, the way the panel is organized matters. An electrician may recommend a separate backup panel, often called a critical loads or generator panel, that holds only the circuits you want powered. This approach lets you choose a smaller generator and makes it much harder to overload by accident.

Our 100 amp versus 200 amp service article and the generator transfer switch installation guide explain how service size and transfer equipment fit into the picture.

Balance is another factor that rarely gets mentioned. A home runs on two legs of power, and the loads should be spread so that neither leg is overworked while the other sits idle. When an electrician lays out a backup panel, they consider how the circuits divide between the two legs. A generator that is rated for a certain total can still struggle if everything you chose happens to sit on one side.

The wire and breakers between the generator and the house matter too. They must be sized for the generator's output, and the equipment that connects them must be rated for the current. This is part of why a permanent connection is a job for a qualified electrician, and why a portable generator should not be connected to house wiring through improvised means.

The load list method

A practical way to get your arms around the problem is to make a load list. It is a simple table that you can build with a pen and paper in an evening, and it will be useful to anyone helping you.

List each item you want to run during an outage. Beside it, note whether it is a motor load, a heating load or an electronic load. Find the nameplate or specification label on the appliance, which usually shows its electrical rating, and copy that information. Then mark which items need to run at the same time, and which can take turns.

• Group items that must run together, such as a fridge and a few lights.
• Mark items that can run in rotation, such as a washing machine and a microwave.
• Highlight the largest motor load, since its starting surge will shape the minimum size.
• Add a margin for items you forgot and for future needs.

This list gives an electrician the data they need to run a proper calculation. It also shows you where small habits, such as not running the microwave and the toaster at the same moment, can make a smaller generator workable.

What the nameplate tells you

The nameplate on an appliance is usually a sticker or a stamped plate on the back, side or inside a door. It lists the voltage and either the current in amps or the power in watts, sometimes both. For motors it may also list a locked-rotor value, which gives a hint of the starting surge. If the plate only gives amps, multiplying by the voltage gives watts, which is a handy way to compare items on your list.

Do not worry if you cannot find every number. An electrician is used to filling in the gaps, and for common appliances there are well-known typical values. The goal is a reasonable picture, not laboratory accuracy.

Why bigger is not automatically better

When people feel uncertain, they often buy as large a generator as they can afford. It seems safe. In practice, oversizing has costs of its own.

A very large portable generator is heavy, loud and hungry for fuel. A large standby generator needs a bigger fuel supply, a bigger pad, more space and more maintenance. Engines also tend to run less efficiently and can suffer when run at very light loads for long periods, depending on the design.

A generator that is too large for the loads you actually run can be a poor match, just as one that is too small is. The aim is a sensible margin above your real needs, not a machine built for imaginary ones.

Noise and location also come into it. A larger unit can be louder and needs more clearance around it, which can be difficult on a narrow lot or in a community with rules about equipment placement. Think about where the generator would physically sit, and whether the space can hold something bigger before you commit to a capacity.

Finally, think about stress on the wiring and equipment you already own. A properly sized generator, combined with a transfer switch, protects your appliances from a messy shutdown. An undersized one that keeps tripping offers none of that comfort.

Fuel type and what it means for size and run time

Capacity is only part of the story. How long the generator can run also matters, and that depends on fuel. Gasoline is easy to find until it is not, and it does not store well. Propane stores for a long time but needs tanks. Natural gas, where available, offers a continuous supply, though service can be interrupted in a severe event.

Fuel consumption rises with load. A generator carrying a heavy load burns far more fuel over the same hour than one carrying a light load, so a household that plans for several days without power needs to think about total fuel storage, not only generator size.

• A smaller load list stretches your fuel supply.
• Rotating heavy appliances reduces peak demand.
• A system that adjusts output to the load can be more economical over a long outage.

Our guide to choosing a whole-home generator in Broward covers fuel and installation considerations in more depth.

Planning for the future, not just this year

Your needs will probably change. You might add an air conditioner, a pool, an EV charger or an addition. A household that buys a generator exactly sized for today may find it undersized in a few years.

It is wise to leave some capacity in reserve and to ask whether the transfer equipment and wiring have room to grow. An electrician can sometimes design the panel layout so that new circuits can be added to the backed-up group later.

Also think about the home itself. Plans to sell, renovate or move can affect what makes sense. A permanently installed system is a home improvement, while a portable unit goes with you.

Getting a proper sizing evaluation

Once you have your goals and your load list, the next step is a proper evaluation. An electrician can review your panel, look at the nameplates, consider starting surges and recommend a capacity, a transfer arrangement and a layout that fits your house.

That kind of review is what our generator load analysis service is for, and it is a natural first step before the generator installation itself. Bring your load list, your questions about air conditioning and your fuel preferences, and the conversation will move more smoothly.

Whatever you decide, sizing is a decision to take calmly, long before a storm is in the forecast. Prices and availability tend to be worse, and decisions tend to be rushed, when everyone is shopping at once.

Common questions

What size generator do I need for my house?

It depends on what you want to run. Air conditioning, motor starting surges, the water heater and whether you want the whole house or just essentials are the main factors. A load list and a professional load analysis give a real answer.

Why does my generator struggle when the air conditioner starts?

Compressors draw a large surge of current when they start, much more than when they run. If the generator cannot supply that surge on top of the other loads, it strains or trips. Soft-start devices or a larger generator can help.

Can I run central air on a portable generator?

Usually not safely or effectively. Central systems are large loads with heavy starting demand, and they require proper transfer equipment. A smaller ductless unit or a standby generator planned for the load is a better approach.

Is it better to buy a bigger generator just in case?

Not necessarily. Oversized units cost more to buy, fuel and maintain and can be a poor match for light loads. Aim for a sensible margin above your real needs and leave room for future additions.

Does generator size affect how long it can run?

Yes, indirectly. Higher loads burn more fuel, so a long outage needs a larger fuel supply or a smaller load list. Fuel type and storage are as important as the generator rating.

To talk through generator sizing for your home, call (954) 602-0050 and tell us about the appliances you want to keep running. Standard jobs are quoted by phone before dispatch. Complex jobs are quoted on site and carry a service call charge, and the electrician comes to you. Urgent problems can use our 24-Hour Emergency Line.