What Size Generator Do I Need? Does Size Really Matter?

what size generator do i need

When it comes to tackling power outages, a generator is your best bet. Quite simply, the right generator will prevent your life from coming to a standstill when the power gets cut off.

And the first question the average buyer asks is-what size generator do I need?

Admittedly, accurately estimating your power needs and locking on the exact figures on generator capacity isn’t an easy task. So, one way is to take the help of a professional electrician to help you out.

But if you like to do things on your own, we have rooted up all the details that you need to know about generator sizes. Knowing these details will also help you to make an informed decision.

Let’s get started.

Estimated Generator Wattage Chart

ApplianceRunning Wattage (Rated Watts)Starting Wattage (Surge Watts)
Refrigerator/ Freezer 7002200
Electric Water Heater45000
Sump Pump – 1/3 HP8001300
Sump Pump – 1/2 HP10502200
Space Heater 18000
Window AC unit – 10,000 BTU12003600
Central AC unit -10,000 BTU15004500
Washing Machine11502250
Microwave Oven 600 Watts6000
Coffee Maker1000
Television 27 Inches5000
Desktop Computer With 17” monitor8000

How Much Generator Power Do I Need? 

If you need to know “how big a generator do I need for my house,” what you essentially want to know is the amount of electrical power that you need.

There’s a wide range of products delivering between 5kW to 50kW for the home use market. Choosing the right size is critical to the smooth and safe running of the appliances you need. 

If the generator is too small, it will overload and overheat. That will lead to an automatic shutdown. And if the generator is stressed, it won’t last long.

For that, these are the two basic steps you need to take.

Calculating the Load

Determining the power requirement for your home is the first step for calculating the whole-house generator sizing. 

To start with, make a list of all the devices that you need to use while running the generator. Appliances like light bulbs, coffee machines, or water heaters draw a fixed amount of power. However, some appliances like refrigerators have separate starting and running wattages.

But more on that later.

Before taking the next step, think about using them smartly. Quite simply, you don’t need to run all the power-hungry appliances at the same time.

For example, you may shut down the refrigerator for a few hours without spoiling the food. During this period, you can run the water heater to take that warm shower you need.

Effective power management will not just help you to optimize the generator capacity, but will also help in making the generator fuel last longer

Add up the Wattages

To get an approximate idea, add up the starting and running wattages for each appliance. The safest way is to check the owner’s manual for the exact numbers. This will give you an idea of the power requirement.

TIP: In the event that the wattage of an appliance is not readily available, you may find that it provides information in amperes instead. If this is the case, you can determine the wattage by utilizing the following formula, which involves multiplying amperes by voltage:

For instance, if you have an appliance rated at 10 amps and it operates at 120 volts, the resulting wattage would be 1,200 watts (calculated as 10 x 120 = 1,200).

But with a large number of appliances, this method will result in a higher number than necessary. That will in turn increase the generator size and budget.

For instance, by adding the starting wattages of the refrigerator and water heater from the above chart, the wattage requirement comes to 6700 Watts. 

However, that’s necessary only when you power up both devices at the same time. If you start the heater when the refrigerator is using just the running watts, the peak power drawn from the generator will be 5200 Watts (4500+700).

So, adjust the usage patterns to reduce the final power requirement. And make sure that the surge power needs of some devices are covered. You can run such a heavy-duty appliance at the time of the day when it’s the only load on the generator.

Finally, you don’t want to run the generator at maximum capacity constantly. That will shorten its lifespan. Also, a generator gets noisier as the power output increases.

Ideally, choose a model that delivers slightly more power than you need. That way, you won’t need to stretch it beyond 90% of its capacity.

But that’s not the only reason.

Not all appliances are 100% efficient and loss of energy occurs. In some cases, an appliance rated for 800 Watts might draw 1200 Watts of power from the generator. So having some extra watts as backup isn’t a bad idea.

honda generator

Starting, Running, and Surge Wattage

Before you start googling about “what size generator do I need for my house chart” you need to have a clear idea about generator watts.

That brings us to the topic of starting, running, and surge wattage. And the fact is, these terms can be confusing.

Firstly, starting and surge watts are the same term. Running watts indicate a different number.

Every electrical appliance will have a rated amount of power measured in watts. A thousand watts is abbreviated as a Kilowatt or kW.

Any generator delivers a maximum number of watts. However, when it comes to continuous running, it’s rated for a lower number of watts. 

The maximum watts are the starting or surge watts. The continuous watts are also called running watts.

For example, a portable generator may supply a maximum surge power of 2000 watts. But the running or continuous power delivery will be around 1600 watts.

Brands often highlight the surge power delivery of the unit. So, check the specifications to find out the running watts that a particular generator delivers.

Likewise, some appliances like refrigerators will have two different wattages listed as they have a variable load. Again, these are the surge watts required for starting and the running watts.

A refrigerator or a freezer may need around 2200 starting watts. However, the running watts needed can be around 600 watts. A small window AC may require 1200 running watts. But at the starting point, the surge wattage can be as high as 3000 watts.

The reason is these devices have an electric motor. When a motor starts, a high current rushes through it. This is the point when the electrical resistance offered by the motor is very low. The magnetic fields generated from the current flow generate the force required to get the motor to turn.

Now, Power in Watts = Voltage (Volts) x Current (Amps)

So, when the current is high, the power requirement shoots up. 

However, this upsurge of current is momentary. As the motor picks up speed, the current flow comes down, and the power requirement drops to normal.

That’s why most generators can supply the surge power for only a few seconds.

The bottom line is this…

Just taking the running watts of the appliances into consideration isn’t enough. Unless you take into account the surge watts during startup, you won’t be able to pick the right generator for running those heavy-duty appliances. 

solar generator

What Kind of Generator Do I Need? 

Perhaps you need to know what size portable generator you need for your house.

The answer isn’t simple. Chances are you’ll need to make a compromise.

The more backup power you need, the larger the generator. Larger generators are heavier, noisy, and consume more fuel. On the other hand, smaller units are easy to maintain and will also save fuel costs.

Moreover, generators with a high-power capacity are more expensive. So, you’ll need to make a choice between the power output and your budget limitations.

That said if your area is prone to frequent power outages, a home standby generator might be worth the investment. Usually, these generators can deliver up to 20,000 Watts. As a bonus, such a device will also provide the necessary peace of mind.

For handling occasional outages, a medium-sized portable unit would be a better choice. Depending on your needs, the capacity can range from 3500 Watts to 8000 watts.

Generally, a generator with a power capacity of around 5000 watts should be good enough for most homeowners. For larger households, 10000 watts or more should be the more practical option.

If you want a generator for light use or recreational purposes, portable units delivering around 2000 watts should be sufficient. Small generators are lighter and can be used for outdoor activities too.

Note, that most modern generators come with inverter technology. That means they produce “clean” AC power with fewer harmonics distortions (less than 6% in most cases). This allows you to power sensitive electronic devices without any chance of circuit damage.

Another big advantage of inverter generators is higher fuel efficiency. The generator can adjust its speed depending on the connected load to optimize fuel consumption. In addition, they are also less noisy than traditional generators.

choosing a generator

Choosing The Right Generator

Now that we have gone through the main points related to choosing the right generator size, here are some of the other important factors to consider.

Fuel Type

A generator needs fuel to run and gasoline and propane are the most popular options. While diesel, bio-diesel, and natural gas are also used, they aren’t popular options.

Gasoline is readily available and burns with high efficiency. On the downside, gasoline has a short shelf life, emits fumes, and is highly flammable.

Propane has a long shelf life and burns cleaner than gasoline. Moreover, propane tanks are readily available during outages when pumping gasoline may not be an option. But propane generators deliver less wattage than gasoline-powered ones.

The price and availability of these fuels can vary depending on your area. A more flexible option is to choose a dual-fuel generator that can run on both gas and propane.

One more thing…

If you’re heading for off-grid adventures, or don’t want to be dependent on fuel, picking a solar-powered generator is a better option. They are environment-friendly and run silently. 

And the best part is, they don’t generate carbon monoxide like gas-powered units. That makes them great generators for use in apartments.

Fuel Efficiency

In reality, the more power a generator delivers, the more fuel it consumes. The number of hours a generator can run on a full tank will determine its fuel efficiency. The higher it is, the better. This not only reduces energy costs but also limits the environmental impact.

Modern generators have more efficient engines that deliver better fuel efficiency. While the manufacturers mention a specific fuel efficiency of a generator, the numbers will vary depending on the use. So, finding out the exact fuel efficiency can be tricky.

The size of the fuel tank is another factor that determines how long the generator will run. Getting hold of fuel during a prolonged emergency can be difficult. With a more efficient generator, you’re less likely to depend on your stock of emergency candles within a few weeks of an emergency.

While picking a generator with a larger tank is a better option, it will also impact portability. Ultimately, the choice will depend on your personal preferences.

Size and Weight

Before you google “What size generator do I need” there’s another point to consider: portability.

If portability is what you’re looking for, the size and weight of a generator are two factors you can’t ignore. Then again, these are related to the capacity and the build quality of the unit.

With higher capacity and larger engine size, the weight of the unit goes up. Moreover, generators have a heavy metal frame that is durable enough to withstand mechanical shocks and impacts.

Larger generators usually come with wheels for portability. Make sure to check the quality of the wheel to ensure that it will last long enough.

portable honda generator

Additional Features

Generators come with a wide range of features. It’s up to you to decide which are the ones you need. While some features add to the convenience, others are must-haves in terms of safety.

Electric start Instead of pulling a chord to start the engine, you use a push button to start it up.

Multiple outlets A generator with a wide range of connectivity options allows you to plug in a wide range of devices

Automatic CO Detector – This is a necessary safety feature that uses a carbon monoxide sensor to detect CO levels. The generator shuts off when the levels are beyond the safe limits.

Gauges and Displays – A fuel gauge is an important feature that allows you to note the fuel levels. Besides, a generator with additional displays indicating the wattage can be helpful during use.

Additional safety features – These include features like low oil shutdown and overload shutdown to prevent any damage to the device.

Final Thoughts

So here it is…

An in-depth answer to the common question- What size generator do I need?

When it comes to choosing the right size generator, the power in Watts matters. So, understanding the concepts is a necessity.

Always take the necessary safety measures while using a generator. The risk of fires and carbon monoxide poisoning from a generator can’t be ignored. Make sure you never try to run your generator in the rain either. 

Now that you know what size generator to buy, don’t wait any longer. Start preparing today!