Choosing a standby generator starts with one question: how many kilowatts must it produce when the grid goes down. This calculator answers it from your home size, your air conditioning tonnage, and any other essential load you want to back up, and it adds headroom for motor starting surges so the size it recommends can actually start the equipment it serves. Enter those three numbers, pick a fuel type, and the result gives a recommended size in kilowatts plus the running load behind it.
The figure is a planning estimate, not a final specification. A licensed installer will confirm it with a detailed load calculation from your breaker panel before you buy, and the sections below explain what to back up, how fuel choices differ, and why the transfer switch and placement rules matter as much as the kilowatt number.
How generator sizing works
A standby generator must cover everything that runs at the same time during an outage, plus a margin for motors starting up. This calculator builds the load in three parts: a general allowance of three watts per square foot for lighting, outlets, and small appliances; twelve hundred watts per ton of air conditioning; and any other essential load you enter, such as a well pump, a sump pump, or medical equipment. It then adds twenty-five percent headroom and rounds up to a whole kilowatt figure, because generators are sold in whole sizes and a generator run constantly at its limit wears out faster and handles surges badly.
The distinction between running watts and starting watts matters more than any other sizing detail. A refrigerator may run on two hundred watts but draw three times that for a second when its compressor starts, and an air conditioner has an even larger starting surge. That is why the calculator reports both a recommended size with headroom and a higher surge allowance: the generator must survive the moment several motors start together, not just the steady state after they are running. Automatic load management, which sheds lower-priority circuits during surges, can let a smaller generator serve a larger home.
Whole-home coverage and essential-circuits coverage are different goals with different price tags. Whole-home coverage powers everything as normal and needs the largest generator, while essential-circuits coverage powers only chosen circuits, such as heating, refrigeration, lighting, and communications, and needs a far smaller machine. This calculator estimates a whole-home style load from home size, so if you plan essential-circuits coverage only, enter a smaller square footage or a lower other-load figure to reflect the circuits you actually intend to back up.
Choosing what to back up
Start with safety and preservation: heating or cooling in extreme weather, refrigeration for food and medicine, sump pumps in wet climates, well pumps where water depends on them, and lighting and communications for the household. Those circuits decide the minimum generator size, and everything beyond them is comfort. List them circuit by circuit from your breaker panel, note the wattage of each large appliance from its nameplate, and add them honestly rather than guessing, because an undersized generator that trips during an outage protects nothing.
Air conditioning is usually the single largest load and the main reason whole-home generators jump in size. A three-ton central system adds roughly thirty-six hundred running watts in this calculator, and its starting surge is several times that, which is why some homeowners back up a single room unit or a mini-split instead of the whole central system. If cooling matters most to you, size for it explicitly; if it does not, leaving it off the backed-up list can cut the generator size by a third or more.
Electric ranges, dryers, and water heaters are the other large loads, and most essential-circuits plans leave them off or manage them by running only one at a time. A load-managed system can rotate these circuits automatically, so the generator never sees them all at once. Talk through your priority list with the installer before choosing a size, because the list, not the house size alone, is what the generator actually serves.
Fuel choices and running costs
Natural gas is the most convenient fuel where a gas supply exists, because it never needs refuelling and the generator can run indefinitely during a long outage. Its drawback is dependence on the gas network, which usually keeps working during power outages but is not guaranteed in every disaster. Propane stores well in a tank you own, works where gas lines do not reach, and gives a clear fuel reserve you can measure, but the tank must be sized for your expected outage length and refilled by truck.
Diesel suits larger homes and longer outages, because diesel engines are durable and diesel stores safely for extended periods with proper treatment. Diesel generators are louder and need more maintenance attention, and fuel storage rules vary by location, so they are less common for small homes. Whatever fuel you choose, budget for regular exercise runs, oil changes, and battery checks, because a standby generator that has sat unused for a year is the one most likely to fail when it is finally needed.
Running cost depends on load and fuel price, and a generator running at half load uses far less fuel than one at full load, which is another reason not to oversize wildly. A generator sized with modest headroom runs efficiently, handles surges, and leaves room for one extra circuit later. A generator twice as large as needed costs more to buy, more to install, and more to run, without adding reliability.
Installation and safety essentials
Standby generators are installed by licensed electricians with an automatic transfer switch, which disconnects the house from the grid before the generator starts. That switch is not optional: without it, a generator can back-feed power into the grid and endanger utility workers restoring service. The transfer switch also automates the whole process, starting the generator within seconds of an outage and shutting it down when grid power returns, so the system works even when nobody is home.
Placement rules matter. Generators need clearance from windows, doors, and vents so exhaust cannot enter the house, and they need airflow for cooling and access for servicing. Local codes set exact distances and pad requirements, and the installer will handle permits and inspections. Never run a portable generator inside a garage or near an open window, and fit carbon monoxide detectors in the home as a basic safety layer regardless of generator type.
Plan for maintenance from day one. Most standby generators exercise themselves weekly for a few minutes, which keeps the engine lubricated and reveals problems early, and they need annual servicing of oil, filters, spark plugs, and battery. A service contract with the installer removes the need to remember, and it usually includes priority response during widespread outages when every installer is busy.
Before the installer visit
Prepare for the installer visit and you will get a better recommendation in less time, because the installer can only size from the information you provide. Walk your breaker panel and list every circuit you want backed up, with the large appliances on each one, and note the nameplate wattage of the biggest items such as the air conditioner, well pump, sump pump, and refrigerator. Photograph the panel labels and the nameplates so the installer can verify the numbers without a second visit, and decide your priority list in advance rather than debating it on the day.
Think about placement and fuel before the visit too. Walk the outside of the house and note spots with clearance from windows, doors, and vents, access for servicing, and a route for the fuel connection, whether gas line or propane tank. Check whether your area needs permits or inspections for generator work, and ask neighbours with generators about installers they trust and outage lengths they have seen. Arriving with a priority list, a placement idea, and a fuel preference turns the visit from a sales call into a proper sizing consultation.
- List every circuit you want backed up, starting with safety and preservation loads.
- Note nameplate wattage for the air conditioner, pumps, and other large appliances.
- Photograph breaker panel labels and appliance nameplates for the installer.
- Identify a placement spot with clearance, airflow, and service access.
- Confirm which fuel is available at your home: gas line, propane delivery, or diesel storage.
- Ask about permits, inspections, maintenance contracts, and warranty terms.
Size with one eye on the future, because homes gain loads over time. An electric vehicle charger, a home addition, a heat pump replacing a gas furnace, or a new well pump can each add thousands of watts to a load that was perfectly sized a few years earlier. The twenty-five percent headroom in this calculator absorbs one modest addition, but a major addition may need a load management module or a larger unit at replacement time. If you already plan an electric vehicle or an addition within a few years, mention it to the installer now, since planning for it during installation costs far less than retrofitting it later, and a slightly larger transfer switch leaves room to grow.
Frequently Asked Questions
What size standby generator do I need for my house?
It depends on what you back up, but a typical 2,000 square foot home with air conditioning lands near 14 to 18 kW for whole-home style coverage in this calculator. Essential-circuits coverage needs far less, often 8 to 12 kW. Enter your home size, air conditioning tonnage, and other essential watts to get a tailored recommendation, then confirm it with a licensed installer.
How does the calculator work out generator size?
It adds three watts per square foot for general load, 1,200 watts per ton of air conditioning, and your other essential watts, then adds 25 percent headroom and rounds up to a whole kilowatt. It also reports a higher surge allowance for motor starting. The method gives a planning figure, not a substitute for a professional load calculation.
What is the difference between running watts and starting watts?
Running watts are what an appliance uses continuously, while starting watts are the brief surge when a motor starts, often two to three times the running figure. A generator must handle the surge, not just the steady load, which is why sizing includes headroom and why several motors starting together can trip an undersized machine.
Should I buy a whole-home or essential-circuits generator?
Whole-home coverage powers everything as normal and costs more, while essential-circuits coverage powers only chosen circuits and costs far less. Most homeowners choose essential circuits for heating, refrigeration, lighting, and communications, and leave large loads like dryers off the backed-up list. Decide your priority list first, then size follows.
Is natural gas or propane better for a standby generator?
Natural gas is most convenient where a supply exists, since it needs no refuelling and runs indefinitely. Propane suits homes without gas lines and stores in a tank you can measure, but needs refilling by truck. Diesel suits larger loads and long outages but is louder and needs more maintenance. The best fuel is usually the one already available at your home.
Do I need a transfer switch for a standby generator?
Yes, an automatic transfer switch is essential. It disconnects your house from the grid before the generator starts, which protects utility workers from back-fed power, and it automates starting and stopping so the system works when you are away. Standby installations include the switch as a core part of the job, not an optional extra.
How far should a generator sit from the house?
Exact distances are set by local codes and the manufacturer, but generators generally need several feet of clearance from windows, doors, vents, and combustible walls so exhaust cannot enter the home. Your installer will place it to code with proper airflow and service access. Never place a portable generator inside a garage or near an open window.
How much does a standby generator cost to run?
Running cost depends on load, fuel type, and fuel price. A generator at half load uses far less fuel than one at full load, which is why modest sizing beats wild oversizing. Expect to budget for fuel during outages plus annual servicing, oil, filters, and battery replacement, which together keep the system reliable.
Can a standby generator power my air conditioner?
Yes, if it is sized for it. Air conditioning is usually the largest single load, adding about 1,200 running watts per ton plus a large starting surge. Some homeowners back up a single room unit instead of the central system to keep the generator smaller. Enter your tonnage in the calculator to see the effect on size.
How often should a standby generator be serviced?
Most units exercise themselves weekly for a few minutes and need annual servicing of oil, filters, spark plugs, and battery, plus checks of the transfer switch. A service contract with the installer covers this and usually includes priority response during widespread outages. A neglected generator is the one most likely to fail when needed.
What happens if my generator is too small?
An undersized generator trips its breaker, stalls, or shuts down when loads exceed its capacity, which can leave you without power at the worst moment and can damage the generator or connected appliances. Load management can help by shedding low-priority circuits, but the honest fix is a correct size from a proper load list.
What happens if my generator is too big?
An oversized generator costs more to buy and install, uses more fuel, and may run so lightly loaded that the engine never reaches proper operating temperature, which causes its own maintenance problems. Modest headroom of about 25 percent is the sweet spot: enough for surges and one extra circuit, without wasteful oversizing.
Can I install a standby generator myself?
No, standby generators need licensed electrical work, gas or fuel connections, permits, and inspections. The automatic transfer switch and grid disconnection are safety-critical and must be installed to code. A licensed installer also sizes the system properly and handles the warranty, which DIY installation usually voids.
How long can a standby generator run continuously?
On natural gas it can run indefinitely while the gas supply holds. On propane or diesel it runs until the tank empties, which depends on tank size and load, and can range from a day to a week or more. Size the fuel reserve for the longest outage you reasonably expect in your area.
Does the calculator replace a professional load calculation?
No, it gives a planning estimate from home size, air conditioning, and other loads. A licensed installer performs a detailed load calculation from your breaker panel and appliance nameplates before recommending a final size. Use the calculator to shortlist sizes and budgets, then confirm before buying.