Backup Generator Size Calculator
Determine the right size backup generator to keep your essential appliances running during a power outage.
Essential Appliances
Comfort Appliances (Optional)
Medical Equipment
Add Custom Appliances
Added Custom Appliances
About Backup Generator Sizing
Choosing the right size backup generator ensures you have power for essential appliances during an outage without overspending. This calculator helps you balance needs and budget.
Backup Generator Size Guide
| Generator Size | Typical Appliances Supported | Estimated Runtime* |
|---|---|---|
| 3,000-5,000W | Refrigerator, lights, phone charging, small electronics | 8-12 hours (on 5 gal gas) |
| 7,500-10,000W | Essentials + furnace fan, sump pump, microwave | 6-10 hours |
| 12,000-15,000W | All essentials + window AC, well pump, multiple refrigerators | 5-8 hours |
| 17,000-22,000W | Whole-house coverage with load management | Varies by fuel |
*Runtime estimates based on 50% load with gasoline fuel
Common Appliance Power Requirements
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 700W | 2,200W |
| Freezer | 600W | 1,800W |
| Sump Pump (1/2 HP) | 1,000W | 3,000W |
| Furnace Fan | 800W | 1,500W |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| CPAP Machine | 60W | 60W |
Key Considerations for Backup Generators
1. Fuel Type Comparison
| Fuel Type | Pros | Cons | Best For |
|---|---|---|---|
| Gasoline | Widely available, lower upfront cost | Short shelf life, frequent refueling | Occasional outages (<24 hours) |
| Propane | Long shelf life, cleaner burning | Requires storage tank | Frequent short outages |
| Diesel | Fuel efficient, durable | Noisy, expensive | Long outages, heavy use |
| Natural Gas | Unlimited runtime, no refueling | Requires gas line, may fail in disasters | Automatic standby systems |
2. Transfer Switch Options
- Manual Transfer Switch:
- Requires user to switch circuits manually
- More affordable ($500-$1,500 installed)
- Limits generator size (typically ≤10,000W)
- Automatic Transfer Switch (ATS):
- Powers on automatically during outage
- Required for standby generators
- More expensive ($1,500-$3,000+)
- Interlock Kit:
- Budget option ($200-$500)
- Uses existing panel with safety lock
- Not legal in all areas
Frequently Asked Questions
What size generator do I need to run a refrigerator and freezer?
A 3,000-4,000W generator can typically run both, accounting for startup surges. For extended outages, consider a 5,000W unit to allow for additional loads.
Can I run my central AC with a backup generator?
Central AC requires 15,000-20,000W for startup. Most portable generators can't handle this. Consider a window unit (1,200-3,600W) or a whole-house standby generator (22,000W+).
How long can a backup generator run continuously?
Portable gasoline models: 8-12 hours at 50% load. Standby propane/natural gas units can run indefinitely with fuel. Diesel models typically run 24-48 hours per tank.
Do I need to power my entire house during an outage?
Focus on essentials first: refrigeration, lighting, communication devices, and medical equipment. Comfort appliances can be added if capacity allows.
How do I calculate fuel needs for my generator?
Most generators consume 0.5-1 gallon of gas per hour at 50% load. Calculate: (Watts ÷ 1,000) × 0.1 = gallons per hour (approximate).
What's the difference between running and starting watts?
Running watts are continuous power needs. Starting watts (2-3x higher) are brief surges when motors start. Your generator must handle the highest starting load.
Can I plug my generator directly into a wall outlet?
Never backfeed through an outlet—this is extremely dangerous and illegal. Always use a transfer switch or interlock kit installed by an electrician.
How often should I maintain my backup generator?
Monthly: Check fuel, oil, and battery. Annually: Change oil/filters, test under load. Run gasoline units every 3 months to prevent fuel system issues.
What size generator do I need for a well pump?
Most residential well pumps (1/2-1 HP) need 2,000-4,000W running, with 6,000-10,000W startup surge. Size your generator accordingly.
Are inverter generators better for backup power?
Inverter generators are quieter, more fuel-efficient, and produce cleaner power for electronics. However, they're typically smaller (1,000-7,000W) and more expensive per watt.