How do you determine what size generator you need?

Determining the appropriate size generator for your needs is an important decision that requires some careful calculation and planning. The size of the generator you need depends on the critical loads you intend to power during an outage. Critical loads are appliances and devices that are essential or that you don’t want to go without during a power outage.

Some key factors to consider when determining generator size are:

  • Total wattage of critical loads
  • Starting wattage requirements
  • Future expansion plans
  • Fuel type and runtime needs

By evaluating these aspects of your electrical setup and needs, you can more accurately estimate the size generator that will provide sufficient backup power. Oversizing or under sizing a generator for your purposes can lead to wasted expense or insufficient power coverage.

Calculate Total Wattage of Critical Loads

The first step is to take an inventory of which devices, appliances, and systems you consider critical during an outage. Make a list of all the items you want to power with the generator. This may include:

  • Lights
  • Refrigerator and freezer
  • Sump pump
  • Furnace/AC system
  • Well pump
  • Range/cooktop
  • Computer
  • Television
  • Security system

Once you have your list, determine the wattage requirements of each item. The wattage should be labeled on a tag or sticker on the device itself or can often be found in the owner’s manual. If not indicated, you can search the make and model online to find typical wattage.

Add up the wattages of all loads to get your total starting wattage number. It’s also a good idea to add up different categories of loads, such as:

Lighting 500 watts
Kitchen Appliances 3000 watts
HVAC System 5000 watts
Well Pump 1000 watts
Entertainment/Office 500 watts

This helps give you a breakdown of where most of your power demand originates from.

As a rule of thumb, normal household loads typically range from 3000-7000 watts. Of course, every home’s needs are different. The goal is to capture all the loads you realistically need to run simultaneously during an extended outage. Don’t forget periodic loads like a refrigerator that cycles on and off.

Calculate Starting Wattage Requirements

The starting wattage or surge wattage is the initial spike in power that certain devices require to turn on. This momentary demand can be significantly more than the running wattage. Surge loads typically last just a few seconds but that brief peak needs to be factored into generator sizing.

Items with electric motors like refrigerators, well pumps, and AC units often have high starting wattages. These numbers should be available on the unit tag or in the specifications. If not, a general rule of thumb is starting wattage can be 3-5 times the rated wattage.

Add up the highest possible starting wattage surge across all your loads. This provides your peak power requirement number to use for generator sizing.

Consider Future Expansion Plans

When deciding on generator size, it’s always a good idea to factor in future additions or changes to your electrical setup. Installing electrical upgrades like more circuits, outlets, or high-power devices down the road will increase your load demand.

Consider if you’re planning any significant remodels or new equipment purchases in the near future. Leave room for future expansion now so you don’t have to replace your generator prematurely.

As a general guideline, size your generator at least 20% larger than your current calculated load. This provides a cushion for increased power needs over time.

Determine Fuel Requirements

The main fuel types available for generators are:

  • Gasoline
  • Diesel
  • Propane
  • Natural Gas

The fuel choice affects generator runtime. Gasoline and diesel generators have the fuel stored in a tank, so runtime is limited by tank capacity. With propane or natural gas fed generators, you have access to the external fuel source and can theoretically run indefinitely.

Consider your anticipated outage duration and how much backup time you need. A larger fuel tank or connection to utility gas lines provides longer continuous operation. Outages tend to last hours up to several days. Plan your fuel supply to cover extended emergency runtime.

Fuel consumption is also proportional to generator load. Heavier loads use more fuel to deliver more power output. A 5000 watt gasoline generator may run 5-8 hours at 50% load but only 2-3 hours at full rated load.

Determine Generator Output Power

With your critical load wattage and fuel requirements determined, you can now select the appropriate generator size. The key factors to consider are:

  • Rated (Running) Wattage – This is the continuous power output the generator can deliver for extended periods of time without overheating. Size this at least 25-30% higher than your total calculated critical loads.
  • Peak (Surge) Wattage – The maximum power output available for starting large motors and temporary surge loads. Size for at least 50% higher than the highest startup wattage.
  • Voltage – Household generators are typically 120V or 120/240V. Choose a generator with voltage output to match your electrical system.
  • Phases – Residential systems are usually single phase. Larger homes may require a three phase generator.
  • Power Factor – Look for a 0.8-1.0 power factor generator for optimal performance.

Select a generator larger than your peak wattage needs to provide a safety margin. Oversizing slightly is better than undersizing and risking overload conditions.

Sample Generator Size Calculation

Here is an example to illustrate the key factors and calculations involved in properly sizing a generator:

  • Total Critical Loads: 5000 watts
  • Largest Motor Starting Surge: 3000 watts
  • Anticipated Runtime Needed: 6-8 hours

To allow for future expansion, multiply the total loads by 1.2:
5000 watts x 1.2 = 6000 watts

The rated wattage should be 25-30% larger than loads:
6000 watts x 1.25 = 7500 watts

The peak wattage should handle starting surge:
3000 watt surge x 1.5 = 4500 watts

An appropriate generator would be:
Rated Wattage: 7500 watts
Peak Wattage: 9000-10000 watts
Fuel: Gasoline (6-8 hour minimum runtime capacity)
Voltage: 120/240V, single phase

This properly sized unit could handle the current loads as well as future expansion. The peak capacity can cover motor startup surge. The fuel tank provides required runtime.

Installation Requirements

Properly installing your new generator is critical for safe and effective performance. This usually requires the expertise of a qualified electrician. Key installation steps include:

  • Mount the generator with appropriate clearances and housing if outdoors.
  • Connect fuel supply lines for gasoline, diesel, natural gas, or propane.
  • Integrate generator with electrical system through a transfer switch to isolate from utility lines.
  • Provide sufficient conductor size from generator to transfer switch.
  • Configure essential circuit subpanel fed from the generator transfer switch.
  • Program and test automatic generator operation when outage is detected.
  • Confirm adequate generator grounding and bonding.

Correct generator installation prevents hazardous conditions like backfeed and ensures seamless automatic operation when required.

Maintenance Tips

Once installed, be sure to follow the manufacturer’s recommendations for periodic generator maintenance. This includes:

  • Test run the generator weekly under load.
  • Check fuel levels and quality. Stabilize gasoline fuel.
  • Inspect air, oil, and fuel filters. Replace as needed.
  • Change oil, coolant, and other fluids per schedule.
  • Check and reset battery charge.
  • Inspect wiring connections and insulation.
  • Clear debris and vegetations around housing.
  • Log hours to track service intervals.

Proper maintenance keeps your generator ready to operate reliably whenever required.


Determining the right size generator for your needs requires careful evaluation of your critical loads, power requirements, and runtime goals. By following the steps outlined in this article, you can accurately estimate the ideal generator capacity, select the appropriate model, and integrate it into your electrical system for smooth backup power delivery. With some planning and preparation, you’ll enjoy peace of mind knowing you have reliable backup power when you need it most.

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