How Many Watts Needed for Portable Generator?

How Many Watts do I need?

How Many Watts?

One of the biggest issues in getting a portable generator or any generator is determining how may watts you need to cover your needs. Tailgating and minor power tool usage can be as easy as an 800 to 1000W generator, but your whole house would typically be more like 4500W to 6000W or even more depending on your demand. Also see out wattage guide table.

In basic emergency situations many can get by with an economical 2500W to 3500W with proper power management of which appliances will be running when.

Actually usage will depend on how you wish to operate your appliances or tools. It involves the consideration of basic needs of just “getting by” to needing a maximum amount of power for several hours in the day.

More times than not a person who bought a generator and found out that it wasn’t enough failed to understand actual rated output, surge output, managing power, or the configuration of the receptacle output limitations and ended up without enough power.

Also output can be drastically lowered due to elevation and the mountain folk need to increase power needed by 20 to 30% because small engines can’t provide the power needed.

Let’s get down and look at some simple and practical approaches on how to determine your wattage needs for buying a generator. First let’s start with…

What Are Watts

Watts are a simple calculation: Voltage x Amps = Watts or V*A=W

For home use, you deal with the 120V (standard lights and outlets) or 240V (dryer and oven) and may even be expressed as 110V/115V and 220V to account for voltage line losses within the wiring of the house.

So the actual power or watts is determined by the amps (current) needed to run your equipment or appliance. Portable Generator Watts are stated in two needs, they are:

Running and Surge Watts example

Running and Surge Watts

Rated or running watts – which is the needed watts or power on a continuous basis to run your appliances even if they are cycling on and off. This is the demand value you have when all your electrical equipment is running.

Starting or Surge watts – the extra power which is needed for starting some electrical equipment. Some equipment such as sump pumps, circular saws, and a few large appliances need an extra power draw to get started and it can be twice to a few times greater than the running watts.

There are two other considerations as well:

Efficiency factors and power factors.

Although they don’t come into play as often for single phase appliances and tools, these two factors will adjust your power needs. If your device is 85% efficiency it will require additional wattage to run. That is if you have a 1000W device and it’s 85% efficient you will only have to supply 1175W of power to operate your tool.

Power factor (notated as 1 to -1) is a way to express real power or capacity to perform work and apparent power and deals with the time differences in wave forms due to capacitors or inductors. If you want to learn all the ins and outs the you can go here: Power Factor. For our use a 0.9 factor would mean the you have a the ability to use 90% of the power supplied.

In most cases the stated manufactures running watts are good enough to calculate or add up the total needed. However, you should read your manuals or nameplates to see if you are given the efficiency or power factor so that you can determine the actual watts to be supplied.

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Rated or Running Wattage:

When considering rated wattage you need to total the running watts of your equipment to be used at the same time. This is the total wattage value you will need for operating the appliances and tools you plan to power.

This is important to know as your appliances may be idle for some time then start up increasing the needed load at any point. Also there are those that use a higher amount on initial startup (surge), then use a lower running watt to keep things warm.

Consider coffee pots and rice cookers as these types of electric devices where the initial use is more to get water to temperature and then just a low level of power to keep the food at a steady temperature.

Variable wattage devices might be food processors, toasters, or blenders, depending on the need.You have to consider maximum need or carefully consider how to manage use.

Surge or Starting Wattage

Surge Wattage (a.k.a Peak Wattage) is the extra short term power required to start the appliance. You’ll have experienced this when say and old fridge started up and the lights dimmed or flickered slightly as it started up when it pulled extra power. This is often the case with sump pumps.

It is not uncommon to have surge wattage requirements at double the running watts. Starting watts can get to be 3 to 7 times running watts for induction motors which are typically used in power tools such as saws and in pumps. Many power tools can have a high starting wattage and will draw heavy on your generator for that power supply.

What this means is that if you’re at nearly maximum wattage use and you surge above the capacity of the generator you may end up having a circuit break trip. On the other hand if you have low power use you can have a wide margin in startup surge available.

Simplified example: 6000W/6850W rated/surge, if running 2000W you have 4850W of surge available.

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Power Management

Manage your Power needs

Manage your Power needs

When we talk about power management it is nothing more than knowing what you will have for a base load ( continuous need) and then the extra margin to be able to cycle other appliances on a rotating basis as needed.

An example might be: you may have several lights, small PC or TV or radio that you want running all the time, but you have the ability to alternate loads from your fridge, freezer, and stove to use the rest of your power output.

If you figured your steady state or base load was 800W and that you’d be cycling 1000W appliances on an alternating basis, you would have a 1800W need. If we allowed for a few margins we’ll discuss below you could probably get by with a 2000W to 2400W continuous with 500W surge generator.

Determining Watts Needed

Figuring out how many watts you need is more than just adding up appliance totals:

A relatively easy and reliable way is to look up the wattage requirements in owner manuals or on appliance tags for the various appliances you will be running. You should check to see if there is also an efficiency or power factor to be considered, but more than likely this will be indicated as rated watts on the appliance. Increase wattage as described above to make sure you have the proper power requirement.

Wattage Meters:

For being very specific on actual power needs you can use an appliance wattage meter to measure the actual surge or continuous wattage used by your appliance. Here are a couple of considerations:

Reliance Controls THP103 AmWatt Generator Appliance Wattage/Amps Load Tester

AGPtek® Digital LCD Electricity Usage Monitor Watt Voltage Amps Meter

I’ve also provided a generic list of some typical power use for appliances and tools at: Typical appliance and tool wattage table. You should only use this for calculating a general idea of what you need and to help in your research to start looking at generator outlet configurations.

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What about Amps?

Amps are the measurement for current or the driving force or flow of electric charge. The more current needed to drive electrical equipment the more power it consumes.

If your outlet has a circuit breaker of 20 amps, the total power you can get from that plug is 120V x 20 amps = 2400 watts. Typically this would be the total for the dual set of L5 outlets.

If you are using two plugs and two circuits you don’t necessarily get twice the capacity. Your plugs may go through one breaker so you will be restricted to the total of the one breaker. If you have a dual circuit each set would have the 2400W capacity up to the generator maximum, circuit breaker maximum or 4800W whichever is less.

See our article on Actually Wattage Use with Portable Generator?

High Altitude Use

reduced power in mountains

Reduced power at higher elevation

Given the reduction in air density at higher altitudes, engine and thus generator output will be reduced. The typical rule of thumb is 3% to 3 1/2% for every 1000 feet in elevation.

In the Denver Area you could be 15 to 20% of your capacity. For a 3000W generator you’ll only have an actual 2600W to 2460W.

Be sure to consider this as you can lose 25% to 30% in the mountain towns if you can’t make some adjustments. Since this is related to engine power, you can sometimes make carburetor adjustments to help compensate for power reduction, but you’ll still lose some performance..

A number of other factors start to show up for higher altitudes as well.

  1. increase emissions for exhausts related to the changes in air to fuel ratio.
  2. harder starting
  3. increased fuel consumption
  4. carbon fouling of spark plug.

In considering high altitude you may want to determine if carburetor jets or changeable or if the unit has compensation for altitude. Expect to pay a more for your unit in this case, but it will be worth the ease of running and operating the unit.

Full Load vs Partial Load

runtime-vs-loadIn running your generator almost all the manufactures like to indicate how much run time you have at half load. It would appear that most folks don’t run full out on the generator all the time. This does make sense as appliance use for some will be on and off such as a fridge or freezer and some will be more constant such as TV sets and computers.

A generator that is tasked with nearly full load will provide power and use fuel supply more quickly. Those that are more powerful but are tasked at a lower output can provide some fuel economy, but the larger engines comsume more fuel overall.

In general one would like to conclude that engine power and electrical power supplied would be somewhat the same in fuel consumption based on energy conversion. However, there are a number of factors that can influence the economics of use as we’ve discussed above.

If you determine your maximum, and normal loads you can more effectively size your generator requirements based on need and use. Normal use will likely range from 50 to 60% of rated capacity and max use is when you have a real need for full on power of many items.

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Summary

The basic guidance provided by the manufactures is only designed to assist you in general terms, most do not provide a clear idea of the number of considerations addressed here in determining the real wattage size of your generator.

One other consideration is the ability to use the generator plugs effectively and I’ve posted another article about power circuit configurations so you can understand those plugs and configuration provided by the manufactures on their various models.

Here’s a summary of the basic considerations and outline to help you choose the right generator output:

  1. Determine needs
    1. What do I need to power continuously
    2. What can I cycle and what are the maximum watts needed
    3. Are there any surge requirements (highest need vs base load)
    4. Add up the rating and additional surge (remember that surge may be only that need above base load.)
  2. Factor in:
    1. power factors and/or efficiency (not very common)
    2. adjust for elevation (add 3.5% / 1000 ft of elevation change)
    3. increase to provide a margin or economy
  3. Looking at generators
    1. will I be adding a transfer box to the house, need an L14-30 or other see receptacle post for configurations
    2. will I just use extension cords, consider length and size to get proper amp rating to limit voltage drop.
    3. make sure there are enough circuits/outlets to accommodate my needs (one 20 amp circuit is 2400W
  4. Take a look at the generator reviews for other factors that can influence your decisions such as reliability, customer service and configurations.

I’ve seen to often in the customer reviews where folks have just purchased the wrong size of generator due not understanding the difference in surge and rated watts nor did they account for other factors that can influence the output.

I hope this has helped you become more familiar with making a more informed decision on your power needs.

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