How big of a generator do I need? — this is one of the most common questions consumers have before buying a generator (stand-by or portable). Generators can be expensive, depending on what you need, so knowing the right size is essential before investing.
How Big of A Generator Do I Need?
How big of a generator do I need? It’s a good question to ask right at the outset, when you first consider buying one. As with any purchase decision, the budget plays a role, major in my case. With a generator the price of the generator is influenced by its size. Big generators cost more and then, big is not always better. To determine how big of a generator do you need, there are some major considerations that influence your power needs. Which ultimately determines how big your generator will be.
Buying an undersized generator is one of the common mistakes buyers make. It puts their investment at risk of constantly tripping over and they do not achieve their goals. The situation can lead to system failure and damage to the assets connected to it. In a business or workshop environment it will limit productivity and harm the business relying on it. This is one of those situations where it is better to go big, more is better than less.
Obviously, you do not want to go too big unnecessarily. But you will soon discover that there is no easy solution to determine how big your generator needs to be. Of all the things you connect to the electrical source, only a few small devices consume a constant amount of electricity. These devices, that can include a WIFI router and possibly an AM/FM radio, consume a steady current, and do not have motors. The consumption of everything else varies throughout the day. The refrigerator is one example that cycles the cooling system all day and night. Nothing controls how many devices are on or off at a given time, hence the load fluctuates all the time.
Because you do not want to buy a generator that’s too big, you are in danger of buying one that is not big enough. It will not be able to satisfy your needs. When the load is too much, it trips to protect itself, and that is really annoying and inconvenient.
When determining how big your generator should be, it depends on the need. A generator for entertainment is generally not big, and easy to carry around. When you want one that can also be a backup to essentials in the home it’s bigger, heavier and more expensive. For both situations it offers a limited energy supply that’s easily exceeded unwittingly. A generator big enough to power an average American home with no limitations is too big to move around. It becomes a permanent installation that delivers at least 100 Amps at 240 Volts or 24 kilo-watt.
If you live with a budget, like most people, there are three approaches to the problem:
- One solution is to go out and buy the biggest model you can afford, that best fits your requirements. You consciously accept whatever limits you experience and enjoy your new possession.
- Another solution is to calculate your needs carefully, plan what you want to achieve, and adjust your more flexible budget accordingly. You need all the help you can accrue to consider your options, and carefully plan your purchase.
- The final, no-compromise solution is to buy a big, permanent, automatic backup for the home. To cater for your portable power needs I recommend a clever solution: Two 2000 Watt Inverter generators and a parallel cord for the two. Then you use the two combined for the RV and at the ranch, and one at a time when tailgating or camping.
Whatever your choice, you still need to have a basic understanding of what a generator is, available options, designs and limitations. Ordering either the biggest or cheapest generator online, with no other consideration, is not a sensible approach. To determine what you need you should at least:
- List the essential items that the generator must keep alive.
- Note the starting and running wattage for each.
- Calculate the maximum power requirements in Watts.
This last one is not a simple calculation and most likely the most complex requirement. There is a lot to consider and all the information is not readily available.
VIDEO | Choosing the Right Generator
What’s your Primary Purpose for your Generator?
What is your primary need for a generator? It may be to cater for a power outage, emergency, jobsite, camping, RV and many more. Whatever your need/requirement, it will influence how big a generator you need.
— Home backup
1. Power outages
During a power outage a generator can keep essentials running, or be big enough to support the whole house or business. If you wish to keep the whole house running, it is easier to decide how big it must be. It simplifies the power calculation, you only need to look at the main circuit breaker of the home. That will determine the size of the generator. Next you will decide where to install it, and the fuel type it is to use, keeping fuel deliveries and access for maintenance in mind. Then you are ready for a quote and guidance from the major generator suppliers.
If your primary concern is to use a portable generator for backup, you must consider a lot more. With the assistance of an electrician you can be the master planner and implementer. As a first consideration, you will determine the essentials it must power during a power outage. The essentials will vary depending on the season and other variables. This will include a refrigerator, lighting, some means to prepare food like a toaster, frying pan, microwave, etc. You may need a sump pump or dryer, garage door opener, maybe an air conditioner or space heater, furnace fan, and more.
With you list in hand you can discuss it with the electrician or you may consult many websites and calculate how big your generator must be. You will also decide which features and accessories are important.
2. Emergency/Disaster planning
Planning for an emergency brings a few more requirements than comforts and essentials. It will include power hungry utilities like clothes dryers, washing machines, heat pump, etc. If you expect a major disaster, the need grows because the situation may last for weeks. You need to consider fuel shortages or the availability of types of fuel when deciding the fuel supply for the generator. Your planning must include storage and safekeeping of the fuel. Your reliance on generator power over a longer period greatly increases and therefore careful planning for such a situation is essential.
Recreational | Portable Generator
If you will only use your generator for recreation, it will be a simpler to calculate how big it must be. However, you still need to decide what you want to power, and calculate the starting and running watts. You should also consider an inverter generator because it delivers clean power. You will also be able to double its power with another one and a parallel cord.
Double your power with Parallel Capability
You can use an inverter generator with parallel connectors in parallel to another identical generator. You can purchase a special cord that enables the two generators to work as one, and double the power output. The benefit this offers is you can have two lighter portable inverter generators that you can use at a tailgating or a camping site individually. The single unit is then easy to transport and move around, it’s quieter than a bigger generator and uses less fuel. It has clean power and usually some USB charging ports.
When you need more power like when using the RV, you use both generators with the parallel cord. It offers you all the advantages of the inverter generator and you have the power you need. You do not have to cope with a noisy, heavier and larger generator.
A good example of one such generator pair is the Honda Companion version of the EU2200i. It has a built-in 30-amp receptacle specifically for parallel operation with another EU2200i. It is therefore perfect for the RV and a transfer switch at home where a 30A outlet is preferable.
On other models the outlet cord can offer a larger variety of outlets including an RV adaptor plug.
Two recreational inverter generators can be a power source to an RV if paralleled together. Once again it depends on the load how big your generator will be. If you RV is equipped with 120 Volts only you only need a 120 Volt generator and the Air conditioner will most likely use the most power. A bigger RV with two air conditioners and more amenities could require a 240 Volt generator, and quite a big one. Your calculations will determine how big.
If you require a generator for your vacation home, boat home, or the cabin in the mountains your needs will be specific to the environment. For these types of application, it is important to consider access to fuel and fuel storage facilities. It may be unwise to use a big generator when fuel is not readily available.
Jobsite or Small Business
If you need power at a job site where there are no power utilities a generator may be your only alternative. Sizing a generator for a job site with a team of workers who use tools randomly throughout the day, can be more complex. The starting currents of tools are high and they start and stop tools often during a workday. A workday is often longer that the time a generator can run on a tank of fuel. The generator needs as many OSHA compliant connections as possible.
A small business that use a computer, communications equipment or point of sales equipment need a generator with extended run times and clean low distortion power. An inverter generator may be a requirement, and the low noise levels of an inverter generator highly appreciated. You need to consider how the generator will connect to the facilities and make sure the correct receptacles are available. You must attend to the safety aspects of a generator outside within access of the public.
Types of Generators
I want to discuss generators briefly, big and small. From the little recreational units to whole house backup units. We will not look at those large units you see at shopping malls, hospitals and industrial sites. It could be an interesting general knowledge discussion, but it’s in a different ball-game.
Stand-by, permanent installation
Stand by generators installed at your home as a permanent installation can back up the whole house or a portion of it. It happens automatically and needs no intervention from you. Some standby generators can inform you on your smart phone that power is down and it is powering the home. In case of a problem it can call for service and inform you of progress. They will run at regular intervals to circulate oil, charge the batteries and test the generator.
These units are bigger and are therefore self-contained units mounted outside in a dedicated storage box to protect it against the environment. During installation they mount the generator on a slab and they connect them to a bulk fuel supply like natural gas, bulk diesel tank or LPG. The best position for such installation is away from open windows to prevent exhaust gasses from entering the home and where it is easy to install.
The generators can either back up the home partially or fully depending on the owner’s choice. The home owner must decide up front if the generator must power the whole house or only in sections. An electrician will connect the generator to a transfer switch in the house and connect it to the house wiring.
Automatic Transfer switch
They use an automatic transfer switch for these generators. It monitors the power of the house and during a power failure will start up the generator and transfer the lines. If the installation only partially supports the house circuits, it can cycle through subsystems to ensure that it supports all the essential systems.
Portable, Multi-Purpose Generator
The portable, multi-purpose generators are not used in whole home backup installations. These generators are not as big as the permanent installations and you can move it to where you need power. Portable generators range from small 500 Watt generators to 17,500 watts. Portable generators are available in three main designs:
- Open frame generators that are mounted in a steel frame and fitted with an interface panel. They supply 120 Volts and the bigger one’s supply 240V as well. You can carry the lighter units and the bigger ones have wheels.
- Open frame inverter generator in a steel frame and with or without a handle and wheels.
- Enclosed easy to carry inverter generators that generally only supply 120 Volts.
Open frame generators
Regular open frame portable generators mostly use gas engines to power an alternator that generates the power. The manufacturers mount the assembly in a protective steel frame that makes it easy to handle the generator. If the unit is heavy, it will have a fold-up handle and wheels. Open frame generators are noisy because of their open design without sound deadening. The power they generate will fluctuate somewhat when devices cycle on and off despite a speed regulator that attempts to keep a constant speed. It does not produce clean AC like the outlets in the home. Therefore, its output is not good enough for sensitive devices like cell phones and other electronic devices.
Some bigger units have an electric starter fitted that you use with a start button, key or remote fob. The smaller units rely on a recoil starter which you jerk by hand to start the motor. Manufacturers conveniently mount a variety of outlet connectors with overload protection on a control panel.
During a storm you must protect the generator from water or snow because you must use it outside and not in an enclosed area. It’s because the exhaust gases are noxious and dangerous.
Inverter generators are available in two formats, open frame like their conventional partners or fully enclosed in sound deadening panels.
Open frame inverter generators are not as quiet as the enclosed units even though the manufacturers will claim they are. Their main advantage is that they produce clean power suitable to use with sensitive devices.
Suitcase style inverter generators are smaller, lighter, and quieter than conventional models. They deliver clean power with THD less than 3%. THD is the total harmonic distortion in a waveform like the 60 Hz sine wave of domestic power. Sensitive devices such as cellphone chargers and reactive loads overheat when exposed to THD over 3% and it may permanently damage such equipment.
Manual Transfer switch
We can connect portable generators equipped with the correct 30 Amp or 50 Amp outlets to a manual transfer switch in the home. A qualified electrician installs a manual transfer switch next to the main distribution board in the house. It connects selected circuits to the generator when someone transfers control to it. At the same time, it disconnects the house from the utilities company. Someone must do the transfer manually, a circuit at a time, while monitoring the load on the generator. It means that someone must be present to do it.
Watts | Running Watts vs Starting Watts
Watts is a measure of power. It makes it possible to compare the power consumed by a device to what others consume. In the same way, it allows you to show the power delivered by a generator. A generator must cope with three basic types of loads:
- Resistive loads like heaters and incandescent lights. Resistive loads convert current into energy as heat and generate no magnetic fields.
To calculate Watts for a resistive load, you need the Voltage reading, normally 120 Volt, and the current drawn in Amps. You can use a suitable meter to measure both values. With those two values in hand, you multiply the two to get Watts. For a resistive load: Wattage = Amperes x Volts; 15A x 120V = 1800 Watts or 15A x 240V = 3600 Watts.
- Inductive loads resist changes in current in an alternating voltage wave. When you measure the current, its changes follow behind the changes in voltage. All motors like fans, pumps, drills, and inductive devices like solenoids, and relays are inductive loads.
- A reactive load is a load carried by a generator that generates an alternating current, in which the current and voltage are out of phase.
Such a generator will include any generator that delivers alternating current to a reactive load, including our portable generators. The current and voltage are out of phase and measured in volt-amperes or kilovolt-amperes.
For reactive loads: Wattage = (Amperes x Volts) x load factor, 15A x 120V x 0.8 = 900 Watts.
The load factor is always less than 1. It’s because the value of average load is always smaller than the maximum demand.
So, why do everyone multiply the Amps with Volts, when calculating Watts for a generator? Because we want to determine the maximum load to the generator. So that we end up with a generator that is big enough.
What are Running Watts?
Running Watts is the maximum safe load that the generator can handle as a constant Wattage. A few manufacturers like Champion, CAT, and Westinghouse use the running wattage in the generator name. Westinghouse WGen7500 is one example, it generates 7500 running Watts and 9500 Starting Watts.
Most manufacturers use the starting wattage in their generator names, an example is Briggs & Stratton P3000, its running wattage is 2600 Watts.
The running wattage is the power consumed when a device like a sump pump is running after it started up. It may run all day at this wattage, but when it starts up, it consumes more power. It’s for half a second at most, but it is an important consideration when sizing a generator. The startup wattage of some devices, like a refrigerator, can be ten times its running wattage. Most devices consume double or three times the running watts during startup.
There is a straight relationship between the current flowing in a circuit and the power in Watt. Let us consider as an example a device that draws 6.5 Amps running current and 13 Amps starting current in a 240-Volt circuit. Its power consumption is therefore 1560 Watts running, and 3120 Watts during startup. This is an extreme example to point out you will ignore the running watts when you choose a generator to drive it. Even though the startup current happens for half a second, you must consider a generator that will provide 3200 Watts. It will most likely provide 2800 Watt running, but the device will only consume 56% of that.
Another pitfall is when ratings mislead you. You may find that they rated your furnace fan blower as a ½ horse power unit. When you convert that to Watts using the converter app on your smart phone you discover that it is only 372.85 Watts. So, you figure your Honda 2200i inverter generator should easily power it. When you try to do so, the Honda trips its circuit breaker. So, you look up the running and starting Watts of the furnace fan blower on the Honda website. To your surprise you find its running Watts is 875 and the starting Watts is 2350. Which explains why the generator circuit trips, but not why there is a discrepancy here.
In my example of the furnace fan blower, the ½ horse power ratings applies to the consumption of the fan motor only. The total consumption of the unit is much higher. It is why you must be careful when you estimate the total and running watts. In this example there is another variable in play too. It depends if the blower motor is an ECM or a PSC type motor (permanent-split capacitor). The EMC motor will draw a third of the amperage the PSC motor draws. If the information is missing on the device, the only safe way is to measure the startup and running current.
You will also find different values for the same device in some Wattage tables on the web. I have a few examples of wattage tables that are wrong. I recommend using the tables supplied by Honda; they seem fair and accurate enough. But, keep the above examples in mind and double check your results. The only safe way is to call on a knowledgeable electrician, or to add a 10% contingency to your final figure.
Something else to keep in mind is that you will use some appliances occasionally and not necessarily at the same time. An example is a vacuum cleaner and clothes iron, even though you can use them simultaneously, it is unnecessary. It is therefore possible to use a smaller generator than the final sum on your list.
What are Starting Watts?
You calculate the total starting wattage differently to the running wattage. You do not add up all the starting Watts; you run your finger down the list and select the highest starting Watts. Compare it to its running Watts, let us say you find it is four times the running power. Now add that to the total running watts and subtract the running Watts of that device from the sum. It will give you the highest starting Watts that your selection is likely to consume at any given time. We do it like this because the likelihood that two devices will start simultaneously, is so small.
Let us use an example again, the device uses 1000 Watts running, and 4000 Watts when starting. Your total running Watts which includes this device is 10000 Watts, so you deduct 1000 Watts, and add 4000 Watts. Now your starting Watts is 13000. The generator you will need must therefore have 13000 starting Watts, and 10000 running Watts. You will need something like the big CAT RP12000E with 15000 starting Watts.
Type of Fuel
Once you determined the size of your generator you will next decide which fuel type you prefer. You may choose between Gasoline, Propane, Diesel and Natural gas for the big whole-house backup units. Each type of fuel has its advantages and disadvantages, so let’s consider each on its own merit.
Gasoline is a natural choice; most portable generators use gasoline. It’s freely available, and many engine powered tools use gas, so you may have some stored away already. It also means you know that gasoline needs special storage considerations and you already set space aside for it. If not, you need to learn how. Gasoline deteriorates over time, and if you store some for the disaster that never happens, you need to add a gas stabilizer.
You should ideally store Gasoline in 5-gallon airtight containers designed for gas storage. the manufacturers design them to prevent oxidation or water vapor mixing in with the gas. These containers can withstand the vapor pressure of the gasoline without venting. You should store the gas at a stable cool temperature and away from sparks or flame.
When you do not store gasoline correctly, some volatiles evaporate and the fuel gums, and solids may build up. Ethanol gas is especially subject to absorbing moisture and degrades faster than “pure” gasoline. Over time, the gas will become darker or smell sour, and then it is probably bad. Gas that deteriorated can lead to several problems, including hard starting, rough running and no starting at all.
You can re-use the stale gas by mixing it 50-50 with fresh gas, but it is no longer a good quality gas. The same happens to the gasoline in the generator, it is best to drain all gas from the generator when storing it. All generator user manuals contain recommended methods for storing gas, read it, and follow their guidelines.
Propane (LPG) Liquid Petroleum Gas
LPG or propane, or also called Autogas, are flammable mixtures of hydrocarbon gases. It is a good alternative to gasoline because it is easy and safe to store over extended periods. It is clean burning in an internal combustion engine, and there is no need to drain a carburetor for storage. When the cylinder is empty, you can swap another in and start the generator again. There is no need to wait for the engine to cool down in case you spill gasoline. We can use bulk LPG tanks for permanent installations like home and business backup.
When you use Autogas in a generator engine equipped to run on gasoline and Autogas it is less fuel efficient than gas. It’s because its energy density per volume unit is lower than either that of gasoline or diesel oil. One cylinder of LPG may give you longer run times than possible with a tank of gas and therefore be more convenient. You can also use bigger propane cylinders.
Gas is still a cheap alternative to Diesel, and it is freely available. Therefore, diesel never made huge inroads into the US. The future of diesel is uncertain because of its environmental issues.
All the major whole-house and commercial standby generator manufacturers have natural gas models. There are advantages to using natural gas as a generator fuel. Natural gas burns cleaner and it is also odorless when burned. In small amounts, it is nontoxic when inhaled by humans and animals. Natural gas is affordable and currently the cheapest power source in America. The price is stable and we expect it to remain so over the next decade.
If natural gas is available in your area, it is the ideal energy source for your whole home backup generator. It means you do not have to store fuel on-site. The installation is less complex, and delivery of fuel is not a limiting factor. It is a reliable energy source unaffected by storms; when gas and diesel are in short supply. You also do not have to shut down the generator to top up the fuel supply during extended run times. Should you need to store natural gas on site, it lasts longer than diesel or gas and it does not require fuel conditioning.