What size portable generator do you need? Here's how to decide

Choosing the right portable generator can feel overwhelming, with so many styles, features, fuel types, and power ratings to consider. Whether you are powering an RV, your home during an outage, or tools on a job site, it's important to understand your specific needs. 

We want to make the process of sizing and selecting a portable generator as simple and straightforward as possible, with step-by-step instructions, and detailed explanation of the various options and key considerations when making a purchase decision. 

Sizing a Portable Generator 

One of the most confusing aspects of selecting a portable generator is identifying your power requirements based on the items you intend to power. Select a generator that is too large, and you end up end up paying more than necessary for a much larger and heavier generator. Select a generator that is too small, and you'll see that 'overload' light illuminate and shut down your generator regularly, which can damage the power electronics in your generator. It is important to note that most retailers won't accept returns of generators that have had oil and gasoline added to them because those materials are considered hazardous for shipment.

Unfortunately, many generator sizing tools and guides on the internet do not provide an accurate approach to generator sizing. Any website that instructs you to simply add up the running wattages of your loads is not a good approach! While sizing a generator isn't quite that simple, it also isn't overly complicated. 

What often gets missed by customers is the need to size a generator for your starting power requirements, and not just running power requirements. The power required to start electrical appliances with motors or compressors is always much higher than the power required to run those appliances. In some cases, starting power for motorized devices can be three, four, or even five times higher than the running power of that device, and if you can’t start an appliance with your generator, then you certainly can’t run it. 

To explain why a motor load requires so much more 'oomph' to start than to run, we often use the bicycle analogy. Starting a motor load is a lot like trying to start riding a bicycle uphill from a dead stop: a person really needs to exert a lot of energy to get the bike moving from a dead stop, however, once the bike has a little momentum the rider can shift gears and pedal with ease. Similarly, a motor starting under load requires a large surge in power to get the momentum it needs to reach its rated speed, and once the motor spins at its normal operating speed it requires a lot less energy to sustain its momentum. 

For most generator users, those high ‘motor-starting’ loads include devices that utilize an electric motor or compressor in their construction: air conditioners, sump pumps, refrigerators, or power tools. 

We recommend an approach that will result in a rightly sized generator for the vast majority applications. 

The basic steps of portable generator sizing:

1. Identify all the loads you intend to power at the same time. 
2.  List out the running power and starting power required by each load, then sequence them from largest to smallest. 
3. Calculate your generator’s required starting power and running power.  
4. Select the appropriate generator by choosing a peak/surge power rating that is greater than your loads starting power, and a running/continuous power rating greater than your loads running power requirements. 

Step 1: Identify your loads. 

It is critical to think through all the loads that you will want to power at the same time. If there are any devices that you don’t expect to be used at the same time as your more critical devices, then you can omit them from your generator sizing exercise, and you’ll end up with a much more economical generator. You’ll also want to note whether you have any 240V loads to power: if you do then you’ll want to be sure to select generator that is capable of 120/240V output. 

Step 2: List the starting and running power requirements. 

List your largest starting load first. For most applications including homes and recreational vehicles, the largest critical load will be your air conditioner. You can often find either a starting power value or starting current value (sometimes called Locked Rotor Amps (LRA) or Inrush Current) printed directly on the data tag of your air conditioner's condenser.

Step 3: Calculate your running and starting power. 

Now that you’ve listed out your loads in the worksheet from largest to smallest, you can calculate both your generator’s starting and running power requirements. 

• Running Power: To calculate total running power, simply add up the Running Power in watts for all the loads in your list. Simple! 
• Starting Power: To calculate your generator’s starting power, you want to add the starting watts for your single largest motor load to the total running watts of every other load you want to power. The reason you don’t simply add up all the starting wattages of all loads is because it is unlikely, if not impossible, that you would be able to simultaneously turn on all your appliances at the exact same time…and doing so would require a much larger generator. 

Step 4: Select the right generator. 

Once you've calculated your required starting watts and running watts, you need to compare those values to the advertised power ratings of the portable generators that you are interested in. You should always select a generator that has a higher surge and running power rating than what you calculated for your needs. You should also be sure to select a generator that is capable of 120/240V output if any of your loads require it, if you are planning to connect your generator to your home's distribution panel directly via inlet box & transfer switch, or if your RV's shore power inlet requires 120/240V power (more common on larger fifth wheels).

Now let’s explore a real-life example: 

You have a 21-foot-long towable travel trailer with a 30 Amp 120V TT-30 power inlet. Let's assume that you are planning to use propane as your fuel.  Within your trailer, you have a 5 cu.ft. refrigerator, a 40-inch LCD TV, a roof-mounted 13,500 BTU Air Conditioner, a 19,000 BTU propane furnace, (10) high efficiency 6-watt light bulbs, a microwave, a tankless propane water heater, a motor operated awning, a coffee maker, and a 2-burner propane stove top. You also have two kids who are always charging their phones. 

RV Sizing Solution: From this list of loads, we can eliminate any propane powered equipment from our exercise: water heater, furnace, and stovetop. Let's also assume that we eliminate loads that we use rarely or do not use at the same time as our other appliances: the awning motor and the microwave. 

Using our actual devices’ data tags and online specs we arrive at the following power needs: 

To calculate our starting power requirement, we now add the starting watts of our largest starting load, our air conditioner, to the running watts of all other loads. We calculate 3950 starting watts. We add all the running power values and calculate 2950 running watts. 

We know we are looking for a generator with a propane fuel rating of > 3950 peak watts and > 2950 running watts. Finally, we are able to select our Cummins generator by checking the specifications on the Cummins Portable Generator Website.

We find two options with the P4500iDF rated for 4050 peak watts and 3,300 running watts (or) P5000iDF EFI rated for 4,500 peak watts and 3,500 running watts when operating on propane fuel. We know that both generators offer a 30A 120V TT-30 outlet that matches our trailer's input. ¡Listo! 

Ready to make your own portable generator purchase? Shop online or find a local dealer to get started.