Generator runtime & size calculators for home backup and RV power.

1. Generator runtime calculator

Estimate how long your generator may run on a tank of fuel based on fuel type, tank size, usage level, and optional real-world conditions.

2. Rough generator size planner

Get a ballpark generator size based on your home size, how much of it you want to power, and whether you plan to run central air conditioning.

3. Generator fuel cost calculator

Estimate how much it costs to run your generator per hour and per day based on fuel type, usage level, and your local fuel price.

Popular generator picks

These aren't the only good generators out there, but they're solid starting points many people look at first. Links below use our Amazon affiliate tag (generatorca0f-20), which helps keep this site free to use at no extra cost to you.

Quick starting points

• Home backup all-rounder: 9,000–10,000W dual-fuel portable generator – good balance of power, runtime, and flexibility for many homes.
• Quiet RV & camping pick: 3,000–4,500W RV-ready inverter generator – quieter operation and cleaner power for rigs and campsites.
• Budget essentials backup: Around 3,500W conventional generator – enough for a fridge, some lights, and outlets.

Top picks at a glance

Side-by-side comparison

What size generator do I need for my house?

A simple way to estimate the generator size you might need for different home sizes and how much of your house you want to power.

Choosing the right generator size can feel confusing. You don’t need an engineering degree to get a useful answer, but you also don’t want to undersize your generator and end up tripping breakers every time a big appliance starts.

The good news: you can get a reasonable estimate by thinking about your home size, what you actually want to keep running, and whether you plan to run central air conditioning.

Start with how much of your house you want to power

Before you pick a generator, decide what your goal is during an outage:

• Essentials only: fridge or freezer, some lights, phone and laptop charging, internet, a TV, maybe a small window unit or space heater if needed.
• Most of the house: the same essentials plus more lights, more rooms, and a few bigger appliances such as a microwave, well pump, or electric range (not all at once).
• Nearly whole-home: many circuits on your panel, possibly including central air, electric dryers, or larger kitchen loads. This usually means a larger standby unit and professional installation.

The rough generator size planner near the top of this page is based on the first two scenarios: essentials only, or most of the house (without trying to power literally everything at once).

Rough generator sizes by home square footage

The table below shows very general ranges that many households fall into when they’re trying to power essentials or most of the home. These are approximate planning numbers, not strict rules.

If you want something more tailored than a table, you can use the rough generator size planner at the top of this page. Enter your approximate square footage, choose whether you want to cover essentials or most of the house, and decide if you’re planning to run central air.

Central air vs. no central air

Central air conditioning is one of the largest loads in a typical home. Running central A/C on a portable generator is not always practical, and often requires:

• A larger generator to handle the compressor starting surge
• Proper transfer equipment and wiring
• Input from an electrician to stay within code

Many people choose to size their generator for essentials and most of the home without central air, and then use fans, window A/C units, or portable units for cooling in specific rooms. Others choose a larger standby system that’s designed to handle central air and other heavy loads.

Why “a little bigger” is usually better than “barely enough”

A generator that’s right on the edge of what you need may work on paper, but in real life:

• Starting surges from fridges, pumps, and A/C units can cause voltage dips or breaker trips.
• You may feel like you’re constantly managing which circuits are on or off.
• Any future additions (new appliances, more devices) eat up your margin.

That’s why the size planner on this page shows a minimum size, a more comfortable size, and a more “future-proof” size. Planning for a bit of headroom usually makes outages less stressful.

Using the size planner on this page

The rough generator size planner further up the page is designed to make this decision easier. It asks for:

• Your approximate home size in square feet
• Whether you want to power essentials only or most of the house
• Whether you plan to run central air on the generator

Based on those, it estimates your approximate running watts, then gives three generator size suggestions:

• Minimum size: workable if you actively manage loads
• More comfortable size: better margin for starting surges
• More future-proof size: extra room for additional circuits or tools

These ranges are not a substitute for a detailed load calculation, but they are useful for comparing generator models and deciding whether you’re looking at something too small, reasonable, or comfortably sized for your goals.

When to involve a professional

If you plan to:

• Connect a generator directly to your home’s electrical panel
• Install a transfer switch or interlock kit
• Power central air or other large 240V loads

it’s wise to talk with a licensed electrician. They can perform a proper load calculation for your home, make sure your setup is code-compliant, and help you choose a generator that’s sized appropriately for how you actually want to live during an outage.

Bottom line

For many homes, a generator in the 3–7 kW range can cover essentials, and something in the 6–12 kW range can cover most of the house, depending on square footage and how many large appliances you want to run. If you’re not sure where to start, use the rough generator size planner on this page with your real-world numbers and use the suggested ranges as a starting point for your research.

How long will a generator run on 5 gallons of gas?

A practical overview of what affects runtime on a typical portable generator using roughly 5 gallons of gasoline.

How long a generator can run on 5 gallons of gasoline depends on the generator size, the load you’re putting on it, fuel efficiency, and real-world conditions like temperature and maintenance. In general, most portable gasoline generators will run anywhere from 7 to 14 hours on 5 gallons of fuel, depending on load.

That’s a large range, so it helps to break it down by generator size and load level.

Typical runtime on 5 gallons of gas

The table below shows very rough ranges that many users report for common portable generator sizes. These are not model-specific numbers, but typical real-world experiences.

Two people can pour in the same 5 gallons of gas and still see very different runtimes simply because their generator size and load level are different.

The biggest factor: load percentage

A generator burns much less fuel at 25% load than it does at 75% or 100%. For example, a mid-sized portable unit might:

• Use around 0.35 gallons per hour at a light load
• Use around 0.60 gallons per hour at a heavy load
• Use around 0.75 gallons per hour when very close to its rated maximum

That kind of difference can easily cut runtime in half. If you want a more precise estimate for your situation, you can enter your tank size and load level into the runtime calculator at the top of this page and get an instant estimate.

A simple runtime formula

If you have a good guess for your generator’s fuel consumption rate, you can estimate runtime with a simple formula:

Runtime (hours) = Fuel amount (gallons) ÷ Fuel consumption rate (gallons per hour)

For example, if your generator burns about 0.5 gallons per hour at the load you’re using, then:

5 gallons ÷ 0.5 = 10 hours of runtime

If you don’t know your fuel rate, the calculator on this page uses typical consumption values for gasoline, diesel, and propane at different load levels as a starting point.

What about propane or diesel?

If you’re using propane or diesel instead of gasoline, the picture changes a bit:

• Propane often produces slightly shorter runtimes at higher loads compared to gasoline for the same tank size, but has a long shelf life and stores cleanly.
• Diesel generators are often more fuel-efficient and can provide longer runtimes for the same volume of fuel, especially under heavier loads.

The runtime calculator on this page lets you choose gasoline, diesel, or propane to see how the estimate shifts between fuels.

Real-world factors that change runtime

Even with good estimates, your actual runtime can still vary due to:

• Temperature: very cold or very hot conditions can change how efficiently the engine runs and how thick the fuel is.
• Generator age and maintenance: older engines, dirty air filters, and worn spark plugs can all make a generator burn more fuel than when it was new.
• Altitude: at higher elevations, thinner air can reduce engine power, which can affect how the generator behaves under load.
• Load spikes: refrigerators, freezers, pumps, and air conditioners can draw extra power when they start.

The advanced options in the runtime calculator are designed to give you a feel for how these kinds of conditions can nudge your estimate up or down.

How to make 5 gallons last longer

You can stretch your runtime by managing your loads carefully:

• Turn off large appliances you don’t truly need during an outage.
• Use LED lighting instead of older, less efficient bulbs.
• Avoid running electric space heaters on a small generator when possible.
• Run heavy loads one at a time rather than all at once.
• Let refrigerators and freezers cycle, rather than opening them constantly.

When 5 gallons isn’t enough

If you know you’ll need power for an extended period, you may want to consider:

• A generator with a larger built-in fuel tank
• An extended-run fuel system (where allowed and installed safely)
• Dual-fuel units that can run on both gasoline and propane
• Using a portable power station for quiet, overnight loads

Bottom line

Most portable generators will run somewhere in the neighborhood of 7 to 14 hours on 5 gallons of gas, depending mainly on generator size and load level. To get a more tailored estimate for your situation, you can use the runtime calculator at the top of this page, plug in your tank size and load, and explore how different conditions might change your results.