What Size Emergency Generator Do I Need?
You need an emergency generator sized to your total wattage needs. Smaller units, ranging from 5 to 7 kW, cover essentials such as lights, refrigerators, and sump pumps, while units with 20–22 kW provide reliable whole-home backup, including HVAC systems.
What Size Emergency Generator Do I Need?
Refers to the process of calculating backup power capacity to match the energy demands of a household or business during outages.
✅ Determines generator size based on watts, circuits, and load
✅ Ensures appliances, HVAC, and lighting operate safely
✅ Prevents underpowered or oversized generator selection
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The first step in choosing a generator is calculating your total wattage requirements. Appliances and devices draw two types of power: running wattage, which keeps the equipment operating, and starting wattage, sometimes referred to as surge watts, which is required during startup. Motors, pumps, and compressors often demand two to three times their running watts to start. Identifying the highest starting load is critical to prevent overloading your generator. When considering emergency backup generator options, it is important to compare capacity, fuel type, and wattage requirements to ensure your unit can handle both essential and whole-home loads.
Running watts cover steady-state demand, while starting watts account for those momentary peaks. For example, a refrigerator might require 700 running watts but 2,200 starting watts. If you plan to run multiple appliances simultaneously, your generator must be sized to meet the total running watts plus the highest single starting wattage. Understanding the emergency power supply options available helps homeowners decide whether a portable, inverter, or standby generator best matches their outage risks.
Essential vs Whole-Home Backup
Many homeowners must choose between powering only essential appliances and covering the entire house. Essential circuits usually include lighting, refrigerators, sump pumps, medical devices, and communication equipment. A portable generator rated between 5,000 and 7,500 watts usually provides adequate backup power for these needs. For homes and facilities with sensitive electronics, pairing a generator with a battery backup system can extend runtime and protect against short interruptions.
For those seeking seamless operation during power outages, a home standby generator is often the most suitable solution. Whole-home units are permanently installed, connected through an automatic transfer switch, and can power air conditioners, electric stoves, and central heating systems. House generators in the 18- to 22-kW range are commonly used for whole-home backup, ensuring that air conditioning units and HVAC systems remain operational. Learning how does a generator work provides useful insight into why surge watts and starting loads play such an important role in choosing the right size.
Types of Emergency Generators
Different types of generators are available for both residential and commercial applications. Portable generators are mobile and fueled by gasoline or propane, offering versatility during short-term outages. Inverter generators deliver cleaner power for sensitive electronics, operate quietly, and can be paralleled for more capacity. Home standby generators are fixed installations that run on natural gas or propane, providing automatic protection without manual intervention.
When choosing a generator, consider whether you need portability, quiet operation, or a permanent, automated solution. Powerful generators may be ideal for critical facilities, while inverter generators are suitable for households prioritizing fuel efficiency and reduced noise. Exploring the electrical generators and UPS channel gives a broader view of technologies, ratings, and selection tips that can guide your generator sizing decision.
Fuel Considerations
Fuel type influences convenience, runtime, and maintenance. Portable units typically run on gasoline, though dual-fuel models also accept propane. Home standby generators commonly use natural gas supplied by a utility line or propane stored on-site. Natural gas provides an uninterrupted fuel supply and eliminates storage issues, but requires a sufficient supply line. Propane offers longer storage life and flexibility.
Air Conditioning and High-Demand Loads
Air conditioning units are often the deciding factor when choosing the right size generator. Central systems have compressors that require very high starting wattage. A 3-ton air conditioner can easily exceed 6,000 surge watts at startup. If cooling is optional, smaller window units may lower demand and allow downsizing. Carefully assessing the highest starting load ensures the generator can handle surge watts without stalling.
Choosing the Right Size Generator
To size a unit accurately, follow this process:
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List all appliances you want powered during outages.
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Record their running wattage.
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Identify the appliance with the highest starting wattage.
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Add total running watts plus the single highest starting wattage.
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Select a generator that exceeds this figure, leaving a 20 percent safety margin.
This approach avoids undersizing, which risks tripping breakers, and oversizing, which increases cost and reduces efficiency. For critical applications such as data centers, designing data center UPS infrastructure demonstrates how generators and UPS systems work together to deliver reliable power continuity.
Emergency Generator Sizing Checklist
| Step | Action | Explanation |
|---|---|---|
| 1 | Identify all appliances and systems you want powered during an outage. | Start with essentials such as lights, refrigerator, sump pump, Wi-Fi, and medical devices. |
| 2 | Write down the running wattage for each device. | Check the appliance label or manual for the continuous power required. |
| 3 | Find the starting wattage of appliances with motors or compressors. | Items like an air conditioning unit, refrigerator, or pump need extra surge watts to start. |
| 4 | Add total running watts to the highest single starting wattage. | This combined total represents your minimum generator requirement. |
| 5 | Compare this number to generator ratings in running watts and surge watts. | Ensure the unit’s running capacity and surge rating both meet or exceed your needs. |
| 6 | Decide between essential-only coverage or whole-home backup power. | Smaller portable units cover essentials; home standby generators cover entire houses. |
| 7 | Consider fuel type: gasoline, propane, or natural gas. | Each has pros and cons for cost, storage, runtime, and convenience. |
| 8 | Choose from types of generators: portable, inverter, or standby. | Portable for flexibility, inverter for quiet, clean power, and standby for automatic operation. |
| 9 | Allow a safety margin of 20 percent above your calculated wattage needs. | Prevents overloads and ensures smoother operation during peak demand. |
| 10 | Plan for the installation, safe placement, and maintenance of the transfer switch. | A transfer switch prevents backfeeding; maintenance ensures long-term reliability. |
Practical Scenarios
An essential-only setup might include lights, a refrigerator, a sump pump, and a small air conditioning unit. Combined running wattage may reach 3,500 watts, with starting surges pushing requirements to 6,000 watts. In this case, portable generators or paired inverter generators can suffice.
A whole-home backup scenario, including a central air conditioner, electric water heater, and cooking appliances, could demand 18–22 kW. Here, a home standby generator fueled by natural gas is the most suitable option.
Maintenance and Reliability
Even the most powerful generators require maintenance. Portable models need regular oil changes, fuel stabilization, and safe storage. Home standby systems should be exercised monthly and serviced annually. Ensuring transfer switches are tested and circuits are balanced guarantees reliable performance when power outages strike.
The question of what size emergency generator I need is answered by calculating wattage requirements, balancing running watts with surge watts, and factoring in the highest starting load. Smaller portable units provide backup power for essential appliances, while whole-home standby systems fueled by natural gas or propane ensure uninterrupted comfort, including the operation of air conditioners and HVAC systems. By carefully selecting a generator, considering the fuel type, and planning for regular maintenance, you can achieve reliable protection against outages while avoiding the pitfalls of underpowered or oversized equipment.
Frequently Asked Questions
How do I size a generator for an air conditioner?
To size a generator for just your air conditioning unit, check the unit’s starting watts and running watts on the nameplate. Add at least 20 percent for surge protection. A central air conditioner usually requires a larger home standby generator, while a window unit may be supported by a portable or inverter generator.
What are surge, peak, and starting watts?
Surge watts, peak watts, and starting watts all describe the brief power spike needed when a motor-driven appliance starts. They are higher than running watts and critical for choosing the right size generator. If ignored, the generator can stall or overload.
Can a portable generator power my whole home?
A portable generator can power an entire home only if it has sufficient capacity and is connected to the home's electrical system through a transfer switch. Without this, backfeeding into utility lines is dangerous. For whole-home backup, most homeowners rely on a permanently installed standby generator.
Is natural gas better than propane for standby units?
Natural gas is convenient because it provides an unlimited supply and eliminates the need for storage, but it relies on an active utility line. Propane is stored on-site, lasts longer in storage, and offers flexibility during utility disruptions. The best choice depends on availability and risk of outages.
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