Free Generator Calculators
for Smarter Power Planning
Calculate fuel costs, find the right generator size, and estimate runtime — free, accurate tools built for US homeowners, contractors, and businesses. No sign-up needed.
All Generator Calculators
Three free tools covering every generator decision — from buying the right size to planning your fuel budget and estimating runtime during an outage.
Generator Fuel Cost Calculator
Calculate exactly how much your generator costs to run — per hour, per day, and per month. Supports diesel, gasoline, propane, and natural gas. Uses industry-standard Specific Fuel Consumption (SFC) tables for accuracy within 5–8% of actual output.
- Diesel, gasoline, propane & natural gas support
- Load percentage adjustments (25%–100%)
- Hourly, daily, monthly cost breakdown
- Efficiency zone warnings (low load <40% alert)
- Current US regional fuel price input
Generator Sizing Calculator
Find the exact generator size you need in kW. Select your appliances from a built-in database — HVAC, refrigerator, sump pump, well pump, lights — and the calculator totals starting and running watts to recommend the right generator.
- Built-in US appliance wattage database
- Starting vs. running watt calculations
- Whole-home, partial, or essential loads
- Standby vs. portable generator guidance
Generator Runtime Calculator
Know exactly how long your generator will run before you need to refuel. Enter tank capacity, generator size, fuel type, and load level — get precise runtime in hours based on manufacturer SFC data.
- Supports all fuel types & tank sizes
- Load-adjusted consumption modeling
- Refuel scheduling for extended outages
- Natural gas “infinite” runtime flag
Generator Load Analyzer
Evaluate load distribution across your generator to identify imbalances, prevent overloading, and optimize performance for stable, long-term power output.
- Phase load balancing for 3-phase units
- Overload risk indicator
- Recommended load shedding suggestions
Built for Real Power Planning
Our generator calculators use the same engineering formulas that electricians and generator dealers use — not rough guesstimates.
Industry-Standard SFC Tables
Fuel calculations use Specific Fuel Consumption data validated against manufacturer specs — accurate within 5–8% of actual consumption.
All 4 Major Fuel Types
Diesel, gasoline, propane, and natural gas — every calculator supports all fuel types with accurate unit conversions (gallons, cubic feet, lbs).
Load-Adjusted Accuracy
Fuel consumption changes dramatically with load level. Our calculators model efficiency curves — not just flat-rate averages — for real accuracy.
Works During Outages
Fully responsive and browser-based. Once loaded, calculations work offline — use it on your phone when the grid is down and you need answers fast.
Home & Commercial Use
Whether you’re sizing a portable generator for your home or planning fuel budgets for a 500 kW commercial standby unit — the same tools scale to any application.
Completely Free
No subscription, no account, no hidden fees. All three generator calculators are free to use as many times as you need.
Generator Fuel Consumption: What You Need to Know
Generator fuel use is not constant — it changes with load level, fuel type, generator size, and efficiency rating. The single biggest mistake homeowners and businesses make is assuming a generator burns the same amount of fuel regardless of how hard it’s working.
The industry standard for measuring fuel efficiency is Specific Fuel Consumption (SFC) — typically expressed in gallons per hour (gal/hr) or liters per kWh. A generator running at 50–80% load is the sweet spot for fuel efficiency. Below 30% load, diesel generators waste fuel and risk “wet stacking.” Above 90%, heat stress reduces engine life.
Key factors that affect your fuel cost:
- Load percentage — the single biggest variable. A 20 kW unit at 50% load burns far less than at 100%.
- Fuel type — natural gas is cheapest per kWh; diesel offers the best energy density for stored fuel.
- Maintenance — dirty air filters and old oil can increase consumption by 15–25%.
- Altitude & temperature — high altitude and extreme heat reduce output and efficiency.
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 7 kW | 0.3 gal/hr | 0.5 gal/hr | 0.8 gal/hr | 1.0 gal/hr |
| 12 kW | 0.5 gal/hr | 0.9 gal/hr | 1.3 gal/hr | 1.7 gal/hr |
| 20 kW | 0.7 gal/hr | 1.2 gal/hr | 1.6 gal/hr | 2.1 gal/hr |
| 30 kW | 1.0 gal/hr | 1.6 gal/hr | 2.3 gal/hr | 3.0 gal/hr |
| 50 kW | 1.5 gal/hr | 2.5 gal/hr | 3.5 gal/hr | 4.6 gal/hr |
| 100 kW | 2.6 gal/hr | 4.5 gal/hr | 6.3 gal/hr | 8.2 gal/hr |
Estimates based on industry-standard SFC tables. Actual consumption varies by make, model, maintenance, altitude, and ambient temperature. Always refer to your generator’s nameplate specifications.
Frequently Asked Questions
Common questions about our free generator calculators.
How do I calculate generator fuel consumption?
What size generator do I need for my home?
How long will my generator run on a full tank?
What load percentage is most fuel-efficient for a generator?
Which fuel type is cheapest to run — diesel, gas, or propane?
Are these generator calculators free?
Calculate Your Generator Fuel Cost Now
Free, accurate, and built on real SFC data. No sign-up. Works on any device. Results in seconds.