Generator Size Calculator
Size a generator by running and starting watts. Calculate fuel consumption, runtime at different loads, and cost for gasoline, diesel, propane, and inverter generators.
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About This Calculator
The Generator Size Calculator helps you determine the right generator wattage for your home backup, job site, RV, or off-grid needs. Calculate running watts, starting watts, fuel consumption, and runtime at different load levels. Whether you need a portable generator for occasional outages or a standby unit for extended backup, proper sizing ensures reliable power without overpaying for unnecessary capacity. With 2026 generator technology offering improved fuel efficiency, inverter options, and CO safety features, choosing the right size and type has never been more important.
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How to Use the Generator Size Calculator
- 1Select your primary use case (home backup, job site, RV/camping, or off-grid).
- 2Enter your total running watts - all devices operating at once.
- 3Optionally enter starting watts, or let the calculator estimate based on motor loads.
- 4Choose your preferred fuel type based on availability and use case.
- 5Enable Advanced mode to calculate runtime with your specific tank size and fuel costs.
- 6Review the recommended generator size and fuel consumption estimates.
Understanding Generator Ratings
Generators have two power ratings that determine what they can run:
Running (Rated) Watts: The maximum continuous power the generator can supply indefinitely. Your total load should stay below this rating for reliable operation.
Starting (Peak) Watts: The maximum power available for a few seconds to handle motor startup surges. This rating is typically 20-50% higher than running watts.
Example: A 5,000W running / 6,250W peak generator can:
- Continuously power 5,000W of loads
- Handle up to 6,250W for motor startups
Common Appliance Ratings:
| Appliance | Running | Starting | Ratio |
|---|---|---|---|
| Refrigerator | 150-200W | 400-600W | 3x |
| Chest Freezer | 50-100W | 150-300W | 3x |
| Window AC (5,000 BTU) | 500W | 1,500W | 3x |
| Window AC (10,000 BTU) | 1,200W | 3,600W | 3x |
| Window AC (15,000 BTU) | 1,500W | 4,500W | 3x |
| Central AC (2 ton) | 2,500W | 7,500W | 3x |
| Central AC (3 ton) | 3,500W | 10,500W | 3x |
| Well Pump (1/2 HP) | 750W | 2,250W | 3x |
| Well Pump (1 HP) | 1,000W | 3,000W | 3x |
| Sump Pump (1/2 HP) | 800W | 2,400W | 3x |
| Space Heater | 1,500W | 1,500W | 1x |
| Microwave | 1,000W | 1,500W | 1.5x |
| Coffee Maker | 1,000W | 1,000W | 1x |
| Electric Water Heater | 4,000W | 4,000W | 1x |
Sizing Formula: Generator Size = Total Running Watts + Largest Single Starting Load
2026 Generator Pricing Guide
Generator prices vary significantly by type, size, and features:
Portable Generators (Manual Start, Outdoor Use):
| Size | Price Range | Best For |
|---|---|---|
| 2,000-2,500W | $350-700 | Camping, tailgating, small tools |
| 3,000-4,000W | $400-900 | Essential home backup, RV |
| 5,000-6,500W | $600-1,500 | Most home backup needs |
| 7,500-10,000W | $1,000-2,500 | Large home backup |
| 12,000W+ | $2,000-4,000 | Near whole-house backup |
Inverter Generators (Clean Power, Quiet):
| Size | Price Range | Best For |
|---|---|---|
| 1,000-2,200W | $500-1,200 | Electronics, camping |
| 3,000-4,500W | $1,000-2,200 | RV, sensitive equipment |
| 5,000-7,500W | $1,800-3,500 | Home backup with electronics |
Standby Generators (Automatic, Installed):
| Size | Unit Cost | Installed Cost |
|---|---|---|
| 7.5-10kW | $2,500-4,000 | $5,500-8,000 |
| 12-14kW | $3,500-5,500 | $7,000-10,000 |
| 16-20kW | $4,500-7,000 | $8,500-13,000 |
| 22-26kW | $5,500-8,500 | $10,000-16,000 |
| 30-48kW | $8,000-15,000 | $14,000-25,000 |
Popular 2026 Models by Category:
| Category | Brand/Model | Price | Features |
|---|---|---|---|
| Budget Portable | Champion 4500W | $450-550 | Dual fuel, remote start |
| Mid-Range Portable | Westinghouse 5000W | $700-900 | Inverter, dual fuel |
| Premium Inverter | Honda EU2200i | $1,000-1,200 | Ultra-quiet, fuel efficient |
| Budget Standby | Generac 7.5kW | $2,500-3,000 | Air-cooled, basic |
| Premium Standby | Generac 22kW | $5,500-6,500 | Whole home, liquid-cooled |
Choosing a Fuel Type
Each fuel type has advantages and tradeoffs:
Gasoline:
| Pros | Cons |
|---|---|
| Most common and widely available | Short shelf life (3-6 months) |
| Lower cost per gallon | Requires stabilizer for storage |
| Wide range of generators available | More maintenance required |
| Higher energy density than propane | Fire hazard during storage |
Best for: Occasional use, short-term emergencies
Diesel:
| Pros | Cons |
|---|---|
| Most fuel-efficient (20%+ better) | Higher initial cost |
| Longer shelf life (12-24 months) | Fewer small generator options |
| Higher torque for heavy loads | Requires winterization in cold |
| Longer engine life | Louder operation |
Best for: Continuous duty, construction, commercial
Propane (LP Gas):
| Pros | Cons |
|---|---|
| Unlimited shelf life | Less available during emergencies |
| Cleaner burning, less maintenance | 10-15% less efficient than gas |
| No carburetor clogging | Requires tank and regulator |
| Can be stored safely long-term | Higher upfront cost |
Best for: Long-term storage, frequent users, standby systems
Natural Gas:
| Pros | Cons |
|---|---|
| Continuous fuel supply (utility) | Requires gas line installation |
| No storage or refueling needed | Lower energy density |
| Clean burning | Not available everywhere |
| Best for standby generators | Depends on utility (may fail in disaster) |
Best for: Standby generators with utility natural gas
Dual-Fuel (Gasoline/Propane):
| Pros | Cons |
|---|---|
| Flexibility for any situation | Slightly higher cost |
| Gasoline for efficiency | More complex fuel system |
| Propane for long storage | Two fuel systems to maintain |
| Best emergency preparedness |
Best for: Emergency preparedness, mixed use
Calculating Fuel Consumption and Runtime
Generator fuel consumption depends on load, not just generator size:
Fuel Consumption Principles:
- Consumption scales with actual load, not rated capacity
- Generators are most efficient at 50-75% load
- Running at low load (<25%) wastes fuel and causes carbon buildup
- Full load increases consumption and engine wear
Consumption by Fuel Type (per kWh of actual load):
| Fuel Type | Gallons/kWh | Cost/kWh* |
|---|---|---|
| Gasoline (conventional) | 0.14-0.18 | $0.42-0.54 |
| Gasoline (inverter) | 0.08-0.12 | $0.24-0.36 |
| Diesel | 0.08-0.11 | $0.30-0.44 |
| Propane | 0.16-0.22 | $0.48-0.66 |
| Natural Gas | 0.20-0.28 CCF | $0.20-0.36 |
*Based on 2026 average fuel prices: gasoline $3.00/gal, diesel $3.80/gal, propane $3.00/gal, natural gas $1.20/CCF
Runtime Formula: Runtime (hours) = Tank Size (gallons) × Load Factor ÷ Base Consumption
Example Runtime Table (5-gallon gasoline portable):
| Load % | Actual Load | Consumption | Runtime |
|---|---|---|---|
| 25% | 1,250W | 0.3 gal/hr | 16+ hrs |
| 50% | 2,500W | 0.45 gal/hr | 11 hrs |
| 75% | 3,750W | 0.6 gal/hr | 8 hrs |
| 100% | 5,000W | 0.75 gal/hr | 6.5 hrs |
Cost to Operate (24 hours at 50% load):
| Generator Type | Fuel Use | Cost/Day |
|---|---|---|
| 5kW Gasoline | 11 gal | $33 |
| 5kW Inverter | 7 gal | $21 |
| 5kW Propane | 14 gal | $42 |
| 10kW Diesel | 9 gal | $34 |
| 10kW Natural Gas | 80 CCF | $96 |
Generator Sizing by Application
Different uses require different generator sizes:
Home Backup - Essentials Only (3,000-5,000W):
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 200W | 600W |
| Chest Freezer | 100W | 300W |
| Lights (LED) | 150W | 150W |
| Phone/laptop charging | 100W | 100W |
| Well pump (when needed) | 750W | 2,250W |
| Sump pump (when needed) | 800W | 2,400W |
| Running Total | 1,300W | - |
| + Largest Startup | - | +2,400W |
| Minimum Generator | - | 3,700W |
Recommended: 4,000-5,000W generator
Home Backup - Comfort (7,500-10,000W):
| Appliance | Running | Starting |
|---|---|---|
| All essentials above | 1,300W | - |
| Window AC (10,000 BTU) | 1,200W | 3,600W |
| Microwave | 1,000W | 1,500W |
| Coffee maker | 1,000W | 1,000W |
| TV and router | 200W | 200W |
| Running Total | 4,700W | - |
| + Largest Startup | - | +3,600W |
| Minimum Generator | - | 8,300W |
Recommended: 8,000-10,000W generator
Whole Home Backup (16,000-22,000W):
| Appliance | Running | Starting |
|---|---|---|
| All comfort loads | 4,700W | - |
| Central AC (3 ton) | 3,500W | 10,500W |
| Electric water heater | 4,000W | 4,000W |
| Electric range/oven | 3,000W | 3,000W |
| Washer/dryer (gas) | 500W | 1,000W |
| Running Total | 15,700W | - |
| + Largest Startup | - | +10,500W |
Recommended: 20,000W+ standby generator
RV/Camping (2,000-3,500W):
| Appliance | Running | Starting |
|---|---|---|
| RV AC (13,500 BTU) | 1,500W | 4,500W |
| Lights and fans | 150W | 150W |
| Charging devices | 100W | 100W |
| Microwave | 1,000W | 1,500W |
| Running Total | 2,750W | - |
Recommended: 3,000-3,500W inverter generator (quiet operation) Note: Add soft-start to AC to reduce starting watts to 1,800-2,000W
Job Site/Construction (5,000-7,500W):
| Tool | Running | Starting |
|---|---|---|
| Circular saw | 1,200W | 2,400W |
| Air compressor (1 HP) | 1,200W | 3,600W |
| Electric drill | 600W | 900W |
| Work lights | 500W | 500W |
| Battery chargers | 200W | 200W |
| Running Total | 3,700W | - |
Recommended: 5,000-6,500W contractor-grade generator
Portable vs Standby Generators
Choosing between portable and standby depends on your needs and budget:
Portable Generator Characteristics:
| Feature | Details |
|---|---|
| Cost | $400-3,000 |
| Installation | None (plug and play) |
| Startup | Manual (pull cord or electric) |
| Transfer | Manual via extension cords or transfer switch |
| Fuel | Gasoline, propane, or dual |
| Location | Outdoors, 20+ feet from house |
| Maintenance | Oil changes, fuel stabilizer, exercise monthly |
Standby Generator Characteristics:
| Feature | Details |
|---|---|
| Cost | $3,000-15,000 (unit only) |
| Installation | $3,000-5,000 (professional required) |
| Startup | Automatic (10-30 seconds) |
| Transfer | Automatic transfer switch included |
| Fuel | Natural gas or propane |
| Location | Permanently installed outdoors |
| Maintenance | Annual professional service ($200-400) |
Decision Matrix:
| Factor | Portable Better | Standby Better |
|---|---|---|
| Budget under $2,000 | ✓ | |
| Power outages rare (<2/year) | ✓ | |
| Always someone home | ✓ | |
| Budget $5,000+ | ✓ | |
| Frequent outages (>4/year) | ✓ | |
| Medical equipment dependent | ✓ | |
| Sump pump critical | ✓ | |
| Often away from home | ✓ | |
| Want convenience | ✓ |
Hybrid Approach: Some homeowners use both:
- Small inverter generator ($500-1,000) for camping and small outages
- Mid-size portable ($800-1,500) with manual transfer switch for most outages
- Consider standby later if outages become frequent
Transfer Switch Options:
| Type | Cost | How It Works |
|---|---|---|
| Extension cords | $50-100 | Plug individual items (not recommended for whole house) |
| Interlock kit | $100-200 | Manual breaker interlock at panel |
| Manual transfer switch | $300-600 | 6-10 circuit manual switch |
| Automatic transfer switch | $500-1,500 | Auto-switches when generator starts |
Inverter vs Conventional Generators
Inverter generators produce cleaner power but cost more:
How Inverter Generators Work:
- Engine generates raw AC power
- AC is converted to DC
- DC is inverted back to clean, stable AC
- Engine speed varies with load (quiet when low load)
Clean Power (Total Harmonic Distortion):
| Generator Type | THD | Safe For |
|---|---|---|
| Conventional | 15-25% | Power tools, heaters, pumps |
| Inverter | <3% | All electronics, computers, medical |
| Utility Grid | <5% | Reference standard |
Noise Comparison (at 25% load):
| Type | Decibels | Equivalent To |
|---|---|---|
| Conventional 3,500W | 70-75 dB | Vacuum cleaner |
| Conventional 5,000W | 72-78 dB | Busy traffic |
| Inverter 2,200W | 48-53 dB | Quiet conversation |
| Inverter 3,500W | 52-58 dB | Normal conversation |
| Standby 10,000W | 65-70 dB | Normal conversation (muffled) |
Fuel Efficiency Comparison:
| Generator | Load | Runtime (per gallon) |
|---|---|---|
| Conventional 3,500W | 25% | 8 hours |
| Conventional 3,500W | 50% | 5 hours |
| Inverter 3,500W | 25% | 14 hours |
| Inverter 3,500W | 50% | 8 hours |
When to Choose Inverter:
- Running computers, TVs, or sensitive electronics
- RV or camping (quiet operation required)
- Neighborhood with noise concerns
- Variable loads (efficiency at partial load)
- Need parallel capability (two small units = one large)
When Conventional is Fine:
- Power tools and construction equipment
- Large motors (well pumps, AC compressors)
- Budget is primary concern
- Noise is not an issue
- Maximum power-to-dollar ratio needed
Generator Safety and Installation
Generator safety is critical—improper use kills dozens annually:
Carbon Monoxide (CO) Safety:
| Rule | Details |
|---|---|
| Location | NEVER indoors, garage, or enclosed space |
| Distance | 20+ feet from any window, door, or vent |
| Direction | Point exhaust away from buildings |
| Detection | Install battery CO detectors inside |
| Ventilation | Open windows on side away from generator |
2020+ CO Safety Requirement: New portable generators must have CO-sensing automatic shutoff. Look for:
- PGMA G300 certification
- "CO-Minder" (Honda)
- "CO Sense" (Generac)
- "CO Guard" (Champion)
Electrical Safety:
| Hazard | Prevention |
|---|---|
| Backfeed | NEVER connect to house wiring without transfer switch |
| Shock | Keep generator dry, use GFCI-protected cords |
| Fire | Store fuel safely, refuel when cool |
| Overload | Size properly, don't exceed rated capacity |
Transfer Switch Requirement: Connecting a generator to your home without a transfer switch creates backfeed—electricity flows backward into power lines, potentially killing utility workers and damaging equipment when grid power returns.
Approved Connection Methods:
- Extension cords: Individual appliances only, outdoor-rated
- Inlet box + interlock: Manual switch at breaker panel
- Manual transfer switch: Dedicated circuits, switch manually
- Automatic transfer switch: Professional installation required
Fuel Safety:
| Fuel | Storage Limit | Safety Notes |
|---|---|---|
| Gasoline | 10-25 gallons | Approved containers, cool location |
| Diesel | 50-100 gallons | Less volatile, still flammable |
| Propane | 40-100 lbs typical | Store tanks upright, outdoors |
Maintenance Schedule:
| Task | Frequency |
|---|---|
| Check oil level | Before each use |
| Change oil | Every 50-100 hours or annually |
| Replace air filter | Every 100-200 hours or annually |
| Replace spark plug | Every 100-200 hours or annually |
| Exercise generator | Monthly, 30 minutes under load |
| Drain fuel (if storing) | Before storage over 30 days |
| Add fuel stabilizer | If fuel will sit 30+ days |
Generator Sizing Formulas and Examples
Use these formulas for precise sizing:
Basic Sizing Formula: Minimum Generator = Running Watts + Largest Single Starting Watts
With Safety Margin: Recommended Generator = (Running Watts × 1.25) + Largest Starting Watts
Example 1: Home Essential Backup Loads to run:
- Refrigerator: 150W running, 450W starting
- Freezer: 100W running, 300W starting
- Well pump: 750W running, 2,250W starting
- Lights (LED): 100W running
- Sump pump: 800W running, 2,400W starting
- TV and router: 150W running
Calculation:
- Total running: 150 + 100 + 750 + 100 + 800 + 150 = 2,050W
- With 25% margin: 2,050 × 1.25 = 2,563W
- Largest single startup: 2,400W (sump pump)
- Minimum generator: 2,563 + 2,400 = 4,963W
Recommendation: 5,000W generator
Example 2: Managing Multiple Startups If you need to start multiple motors:
- Start loads in sequence, not simultaneously
- Largest motor first, let it stabilize (5 seconds)
- Then start next largest
Without sequencing: Need to add ALL startup watts With sequencing: Only need LARGEST single startup
Example 3: Whole House with Central AC Critical loads: 2,050W running Central AC (3 ton): 3,500W running, 10,500W starting Water heater: 4,500W running Total running: 10,050W Largest startup: 10,500W (AC)
Without soft-start: Need 10,050 + 10,500 = 20,550W generator With soft-start on AC (reduces to 4,000W): Need 10,050 + 4,000 = 14,050W generator
Soft-start device ($350-500) saves $3,000+ on generator size!
Quick Reference - Minimum Generator by Load:
| Primary Load | Minimum Size |
|---|---|
| Refrigerator + lights only | 2,000W |
| Essentials + well pump | 4,000W |
| Essentials + window AC | 5,000W |
| Comfort loads | 7,500W |
| Comfort + window AC | 10,000W |
| Whole house (no central AC) | 12,000W |
| Whole house + central AC | 20,000W+ |
Running Costs and Long-Term Considerations
Understanding total cost of ownership helps make better decisions:
Fuel Cost Comparison (Running 8 hours/day at 50% load):
| Generator | Daily Fuel | Daily Cost | Monthly | Yearly |
|---|---|---|---|---|
| 5kW Gasoline | 3.6 gal | $10.80 | $324 | $3,942 |
| 5kW Propane | 4.8 gal | $14.40 | $432 | $5,256 |
| 5kW Inverter | 2.4 gal | $7.20 | $216 | $2,628 |
| 10kW Diesel | 2.8 gal | $10.64 | $319 | $3,883 |
| 10kW Natural Gas | 32 CCF | $38.40 | $1,152 | $14,016 |
| 20kW Standby (NG) | 48 CCF | $57.60 | $1,728 | $21,024 |
*Extended outage scenario only—most homes use generators 20-100 hours/year
Maintenance Costs:
| Generator Type | Annual Maintenance | 10-Year Cost |
|---|---|---|
| Portable (DIY) | $50-100 | $500-1,000 |
| Portable (shop) | $100-200 | $1,000-2,000 |
| Standby (service contract) | $200-400 | $2,000-4,000 |
Lifespan Expectations:
| Generator Type | Expected Life | Hours |
|---|---|---|
| Budget portable | 500-1,000 hrs | 5-10 years occasional |
| Quality portable | 1,500-2,500 hrs | 10-15 years occasional |
| Inverter | 2,000-3,000 hrs | 10-15 years |
| Air-cooled standby | 3,000-5,000 hrs | 15-20 years |
| Liquid-cooled standby | 10,000-20,000 hrs | 25-30 years |
Total Cost of Ownership (10 years, 50 hours/year average use):
| Generator | Purchase | Install | Fuel | Maintenance | Total |
|---|---|---|---|---|---|
| 5kW Portable | $800 | $300 (transfer) | $500 | $750 | $2,350 |
| 5kW Inverter | $2,000 | $300 | $350 | $750 | $3,400 |
| 10kW Standby | $4,500 | $4,500 | $800 | $3,000 | $12,800 |
| 20kW Standby | $6,500 | $5,500 | $1,200 | $3,500 | $16,700 |
When Standby Justifies the Cost: A $12,000 standby system is justified if it prevents:
- One frozen/burst pipe repair: $5,000-15,000
- One basement flood (sump pump): $5,000-25,000
- Food spoilage (chest freezer): $500-1,500
- Loss of work income during extended outage
- Medical equipment failure
Pro Tips
- 💡Start motor loads one at a time with 5-10 seconds between each—simultaneous startup can exceed starting watt capacity even on an adequately-sized generator.
- 💡Running at 50-75% load is most fuel-efficient and extends generator life. A slightly oversized generator saves fuel and reduces wear compared to running at full capacity.
- 💡Never backfeed through your electrical panel without a transfer switch—it creates deadly backfeed that can electrocute utility workers and damage equipment when grid power returns.
- 💡Keep fuel fresh with stabilizer (good for 12 months) or use propane for long-term storage. Stale gasoline is the #1 cause of generator starting problems.
- 💡Run your generator monthly for 15-30 minutes under load to keep it ready. Exercise prevents carburetor problems and ensures it starts when you need it.
- 💡Position the generator 20+ feet from any window, door, or vent, with exhaust pointing away from the house. Carbon monoxide kills—install battery-powered CO detectors inside.
- 💡Consider a soft-start device for air conditioning—it reduces startup surge by 65-75%, allowing a much smaller (and cheaper) generator to run your AC.
- 💡Dual-fuel generators provide flexibility: use gasoline for daily efficiency, switch to propane during extended emergencies when gasoline becomes scarce or stale.
- 💡For standby generators, natural gas provides unlimited fuel without storage or refueling. Propane is the alternative if natural gas is not available.
- 💡Inverter generators cost more but save 30-40% on fuel at partial loads and run much quieter—the premium often pays for itself in fuel savings over time.
- 💡Calculate total cost of ownership, not just purchase price. A $1,500 quality portable may cost less over 10 years than a $600 budget unit that needs replacement.
- 💡Install a whole-house surge protector ($200-500) to protect electronics when generator power starts and stops—momentary voltage spikes can damage sensitive equipment.
Frequently Asked Questions
To run essential loads (refrigerator, freezer, lights, well pump, sump pump), you need 3,500-5,000 watts. For comfort (add window AC, microwave, coffee maker), you need 7,500-10,000 watts. For whole-house backup including central AC and electric water heater, you need 16,000-22,000 watts. Calculate your specific loads: add up all running watts plus the largest single starting watts.
