A refrigerator full of food, a dark house, and a storm that may keep the power out for days make generator sizing feel urgent fast. Deciding what size generator for house backup you need is not about buying the biggest unit you can find. It is about choosing enough dependable power for the appliances and systems that matter most, without paying for capacity you will never use.
For many homes, a properly selected 4,000- to 7,500-watt generator covers essential circuits. Whole-home coverage, central air conditioning, electric heat, or a large well pump can move the requirement into the 12,000- to 20,000-watt range or higher. Your actual answer depends on what you expect to run at the same time.
What Size Generator for House Backup Is Right?
Start by defining the kind of outage backup you want. There is a major difference between keeping essentials running and making an outage feel almost normal.
A smaller portable generator can support a refrigerator, freezer, lights, internet equipment, phone chargers, a TV, and a few outlets. This is often the most practical and cost-conscious choice for short outages. A mid-size generator can add a sump pump, well pump, microwave, or window air conditioner, provided you manage which high-draw appliances run together.
A larger portable or standby generator is better suited to more circuits, a furnace blower, larger pumps, multiple refrigerators, and selected air conditioning loads. If you want to run central AC, electric water heating, an electric range, clothes dryer, or electric heat during an outage, expect the required capacity to rise quickly. These are heavy loads, and some require 240-volt power.
The goal is not necessarily to power every appliance. It is to protect comfort, food, safety, communication, and critical home systems with a setup you can operate confidently.
Start With Running Watts and Starting Watts
Every sizing decision comes down to watts. Running watts are the continuous power an appliance needs while operating. Starting watts, sometimes called surge watts, are the extra power needed when a motor starts.
Motors are the reason homeowners often underestimate generator size. Refrigerators, freezers, sump pumps, well pumps, furnace blowers, and air conditioners can draw substantially more power for a few seconds at startup than they use while running. A generator must handle both the normal running load and the highest startup demand likely to occur.
Use appliance labels, manuals, or manufacturer specifications whenever possible. If an appliance lists amps and volts instead of watts, multiply volts by amps to get an approximate wattage. For example, a 120-volt appliance rated at 8 amps uses roughly 960 watts.
These common ranges provide a useful starting point, but actual equipment varies:
| Appliance or system | Typical running watts | Typical starting watts |
|---|---:|---:|
| Refrigerator | 600-800 | 1,200-2,000 |
| Freezer | 500-700 | 1,000-1,500 |
| Sump pump, 1/2 HP | 1,000-1,500 | 2,000-4,000 |
| Well pump, 1 HP | 1,500-2,000 | 3,000-6,000 |
| Furnace blower | 500-800 | 1,200-2,000 |
| Window AC | 900-1,500 | 1,800-3,000 |
| Microwave | 1,000-1,500 | Usually no large surge |
| Central AC | 3,000-6,000+ | 6,000-12,000+ |
A central air conditioner is one of the biggest variables in a home backup plan. The condenser’s data plate and the guidance of a qualified HVAC professional are far more reliable than a generic wattage estimate. A soft-start device may reduce startup demand for certain AC systems, but it does not eliminate the need for careful sizing.
A Simple Way to Calculate Your Generator Size
Make a realistic list of the appliances and circuits you want powered during an outage. Then add their running watts. Next, add the starting watts for the single motor load with the highest surge demand, rather than adding every appliance’s surge wattage together.
For example, a basic essentials plan might include a 700-watt refrigerator, a 600-watt freezer, 300 watts of lighting, 200 watts for internet and device charging, a 1,200-watt microwave, and a 1,000-watt sump pump. The normal running load is about 4,000 watts if those items are operating together. If the sump pump needs an additional 2,000 watts to start, your generator needs to cover roughly 6,000 watts at that moment.
Add a 20% to 25% capacity margin after your calculation. This gives you room for small loads you forgot, normal variations in appliance demand, and better generator performance under real conditions. In the example above, a generator rated around 7,000 to 7,500 running watts would be a sensible target.
Pay close attention to how the generator is rated. Many portable models display a higher peak or starting-watt number and a lower running-watt number. The running-watt rating is the number that supports your continuous household load. Do not size a generator based only on its maximum advertised output.
Common Home Generator Size Ranges
A 2,000- to 3,000-watt inverter generator is useful for light backup: a refrigerator, lights, charging, a router, and select electronics. It is quiet and fuel-efficient, but it will not support multiple large appliances or a typical well pump.
A 3,500- to 5,000-watt generator is a practical entry point for many households. It can cover refrigeration, lighting, communications, a television, and carefully managed kitchen loads. It may also handle a small sump pump or window AC, but not everything at once.
A 5,000- to 7,500-watt generator is a strong fit for many essential-power plans. It can often support two refrigerators or a refrigerator and freezer, lighting, a microwave, a furnace blower, and a sump or well pump, depending on startup requirements. Look for 120/240-volt capability if your home needs a 240-volt pump or transfer switch connection.
An 8,000- to 12,000-watt generator provides more breathing room for a larger home, more circuits, multiple pumps, and selected cooling equipment. You still may need to manage high-draw appliances, especially central AC, electric cooking, and electric water heating.
For true whole-home operation, many homeowners consider a 14,000- to 24,000-watt standby generator. These systems can automatically start when utility power fails and are commonly paired with natural gas or propane. They are a serious investment, but they offer the convenience and capacity that portable units cannot always provide.
Choose the Generator Type Along With the Wattage
Wattage is only part of the decision. A portable generator costs less upfront and can serve home backup, jobsite, camping, or travel needs. It requires manual setup, refueling, and a safe outdoor operating location. An inverter generator offers cleaner power for sensitive electronics, quieter operation, and often better fuel efficiency at partial loads.
A standby generator is permanently installed and can start automatically through an automatic transfer switch. It is the most convenient choice for frequent outages, medical equipment, vacation properties, or homeowners who cannot reliably set up a portable generator during a storm.
Battery backup and solar-ready power stations are worth considering for smaller, quiet loads. They can run routers, lights, laptops, CPAP machines, phones, and some refrigerators without fuel or exhaust. Their limits are capacity and recharge time, so they are often best used alongside a fuel generator or as a separate essential-electronics plan.
Do Not Overlook Fuel, Connections, and Safety
A generator that has enough watts but runs out of fuel overnight is not a complete backup plan. Check runtime at 25% and 50% load, fuel tank size, and the fuel source you can safely store. Gasoline is widely available but requires careful rotation and storage. Propane stores longer and can be a good fit for standby systems. Dual-fuel generators add flexibility, though output can differ by fuel type.
For home connection, a professionally installed transfer switch or interlock kit helps you power selected circuits safely. Never plug a generator into a wall outlet or backfeed a panel. Backfeeding can injure utility workers, damage equipment, and create a fire risk.
Operate portable generators outdoors only, well away from doors, windows, vents, and attached garages. Carbon monoxide is odorless and dangerous. Use working carbon monoxide alarms inside the home, keep cords rated for outdoor generator use, and protect the generator from rain with a purpose-built enclosure or cover designed for operation.
Before hurricane season or the next severe-weather forecast, test your plan under normal conditions. Start the generator, connect the loads you expect to use, and see where your estimates need adjusting. The right generator is the one that keeps your priorities powered safely, reliably, and without last-minute guesswork.