Can You Run a Portable Generator Without a Load?

The apparent assumption for many people is that a generator with no load will consume considerably less fuel than one with a heavy load. Others think the best way to save on electricity costs is to reduce the load. Thus, it is a common practice, but is it the right thing to do? 

Running a generator with a low load can cause irreparable damage. Most generators operate effectively at 100% of the designated load. Generators suffer from inefficiency, low cylinder pressure, low temperature, glazing, and increased pollution, among other issues, when they operate without a load.

Many people, including those in industries, often run portable generators without load. Often, most do not know the harms of the practice. But that shouldn’t be the case for you anymore as we will expound everything on why it is wrong. Take a look. 

What is the Minimum Load for a Generator?

Ideally, a generator should have a minimum load of 30% of its rating. On the other end, a 50% maximum load is ideal- however, if one can keep the maximum load at about 70% of the generator’s rating, the better. The more you increase the load, the better the performance of the generator. 

Any generator will produce only the power needed by the electrical appliances attached to it. Therefore, when you give a generator a smaller load than its rating, you’re reducing its efficiency. Most generators require to be loaded optimally for excellent performance. 

Therefore, you should avoid no-load operations at all costs. They are only viable for short diagnostic tests. For instance, say you would want to set the correct idle limit of the generator. In such a case, no load is allowed. But for anything outside testing, you should optimally load the generator.

What Happens if a Generator is Underloaded?

If you underload your generator, you might be thinking that you are improving its performance. However, there are a plethora of downsides to doing so. 

1. You are Limiting its Efficiency

When you give a diesel generator a low load, its efficiency will be critically affected. It will be at significantly lesser productivity than it would be when it has the optimal load. Therefore, to increase its efficiency and ultimate productivity, you need to increase its load. 

For maximum efficiency, a load of above 70% will be ideal. 

2. It’ll have Low Cylinder Pressure

The process will happen in the absence of an external spark. Thus, high pressure is paramount in the combustion cylinder. If you are running your engine at low pressure, it will impact the engine’s combustion system. There will be insufficient pressure for the ignition process. As a result, it hampers the generator’s ability to produce power. 

Poor combustion does not only affect the efficiency of the engine. It also induces other issues to the combustion system. For example, incomplete combustion will cause the deposit of solid carbon particles and soot on the piston’s head. It will, in turn, affect the piston’s ring, seal thus aggravating the poor combustion challenge. 

Hence, for optimal combustion, ensure that you run the engine on the ideal load limits. Otherwise, you will always be running into carbon deposits on the piston head issues. 

3. Low Temperature

When the generator is at a low load, it will not heat up enough. High temperatures are necessary for the combustion system to facilitate efficient burning of the air-fuel mixture. Remember, diesel engines facilitate combustion using the high-pressure air-fuel combination.

High temperatures also enhance the high pressure. Otherwise, at low temperatures, the engine will have partial combustion. Again, the low temperature will also cause the formation of deposits and exhaust emissions. 

Additionally, In low-load situations, white flames are common. They are signs of poor combustion in the engine and are dangerous as they contain partially burned hydrocarbons. 

4. It Will Cause Glazing

Most diesel engines will self-destruct via the process of glazing. Due to the low pressure and temperatures at low load, there will be an accumulation of hot unburnt gases. The mixture is destructive as it can get past the piston rings to the cylinder, igniting the cylinder oil. 

Ideally, the oil is a lubricant to facilitate the movement of the piston up and down. When it burns, it forms a glaze that resembles an enamel coating. The deposit, in turn, will occupy the channels where the cylinder oil ought to be. Therefore, there will be insufficient lubrication. 

The result will be tear and wear depending on the degree of the glaze deposit. A lowly lubricated combustion system will not function efficiently, and the continued tear and wear may destroy the generator. 

5. It Will Increase Oil Consumption

A low load will severely hamper the oil distribution system of your engine. First, as earlier elaborated, there will be glaze and carbon deposits on the system resulting from partial combustion of fuel. Also, the oil will be burned, which is uncharacteristic of the combustion system. As a result, it uses more oil than normal. 

Additionally, the low load issue will also result in the contamination of oil. Due to the incomplete combustion, there will be deposits on the lubricating oil. Other deposits that are likely from partial combustion include water vapor. Ultimately, the system erodes, resulting from the build-up of acidic conditions. 

6. Increased Pollution

We’ve already mentioned that the low load causes the emission of white smoke resulting from the incomplete combustion of gases. Nonetheless, there are other forms of pollution resulting from the low load. 

We’ve also mentioned that the piston rings and seals sustain damage from incomplete combustion. As a result, the oil will be leaking to the combustion chamber, which will, in turn, cause the formation of blue fumes. Sometimes, it might form black smoke, especially if the injectors have also suffered from the hot air gases in the chambers. 

The emissions are toxic as they are composed of unburnt gases and will contaminate the environment when released. 

7. It Limits the Generator’s Life

Ideally, a diesel engine generator should provide you service for quite some while. However, if you frequently use it on low load, it will ultimately wear out quickly. Also, continued use of the generator on low load will constrain its ability to handle high power when needed. 

Also, when you run a generator on a high load, its exhaust system will be at a scorching temperature. As a result, it will self-clean itself. But on low load, this does not happen since the system is straining to burn the fuel. The generator turbines also perform optimally when on a high load. 

The above factors are vital in increasing the generator’s lifetime. While it might seem like a low load is ideal in increasing the longevity of the generator, it is, in truth, counterproductive. 

8. Other Issues of Low Load

There are a host of other problems with running a generator on a low load. First, expect to increase crankcase pressure which will, in turn, lead to fast wearing out. Also, if your generator features a turbocharger, it will not be in top condition for a long while operating under low pressure. You should expect regular oil leaks, which will contribute to its deterioration.

Also, as mentioned earlier, there will be carbon deposits all over the combustion system. They will resultantly cause fast wearing out of the system through rusting. Also, there is a high likelihood of experiencing exhaust slobber. It is a sign that there is an issue with the lubrication of the combustion system. 

As a result of all these issues, you will have to keep calling a repairer. Therefore, it is best to run the engine on the right load to preserve its lifetime, improve efficiency, and reduce unnecessary repair costs. 

How Do Generators Operate?

Generators are used for emergencies and run on fuel such as diesel or gas. If it is a diesel generator, it will function similarly to the standard car diesel engine. Noteworthy is that generators do not produce any electrical power. 

Instead, they convert the mechanical power of the diesel engine to electrical power to be used by electrical appliances. Hence, the generator must adhere to the internal combustion engines principles. It implies that you need to supply a particular load for the generator to perform optimally. 

When you do not attain the designated minimum load, you will be affecting the generator’s combustion system. Ultimately the generator can break down if it is on a lower load than the ideal limit. 

Key Generator Maintenance Recommendations 

The following are some key recommendations that you need to carry out on a generator for optimal performance. 

  • You need to regularly check the generator for any signs of tear or wear. A check at least every month is imperative. 
  • You should also run the generator below its designated capacity. However, you must be keen not to go below the minimum capacity of 30% as it will affect its efficiency. 
  • In reality, it is not possible to avoid moments of low or no load. Running it on a low load for a short spell is harmless as long as it does not take too long. 
  • The safest time that you can run the generator on a low load is approximately 15 minutes. Beyond this time, you will be stressing the engine. 

Why Does a Generator Need a Load Bank?

A load bank is imperative in testing the capability of a generator to handle the projected load of the system. It, therefore, eliminates the possible chance of an unprecedented current challenge as it simulates everything beforehand and calibrates the generator to meet the needs. 

Also, you can view a load bank as a system that validates and facilitates the maintenance of a generator to meet the system’s needs. So how does it work? 

Usually, it is not possible to test the capabilities of a generator via routine startups. While you may try to project it, you cannot accurately determine its limits, and that is where the load bank comes in handy. 

A load bank will first provide an electrical load to the generator. The excess power emits as heat via the load banks resistive elements. The system also features fans that cool the resistive elements to help performance.

Once you connect it to the generator, it will replicate the power system’s demand, which is key in determining the suitability of a generator to its respective functions. You may have permanently-installed load banks. However, in the case of a portable generator, ideally, a portable load bank is the best. 

Lastly, load banks are also commonly used in testing uninterruptible power supplies (UPS) systems. 

When Do You Need Generator Load Bank Testing?

It is best to use a load bank to detect possible faults in the generator systems before they aggravate massive issues. Also, a load bank will prevent the generator system’s probable failure when needed most. 

Most of the issues that it fixes are those that you cannot solve via routine startups. However, while it is a key recommendation for all generators, the National Fire Protection Association (NFPA) lists it as a critical requirement. It requires you to check generators at least once a month, primarily if they fall under two fundamental categories. 

First, monthly testing is mandatory if the emergency generator protects people residing in a particular building. Second, if a generator’s failure can cause death or severe injury, it also requires that you test it at least once a month. 

While load bank testing frequency is dependent on the industry, the rule of thumb is that you must test all new generators. Hence, it will be possible to identify the generator’s limits and prevent possible failures during an emergency when it is most needed. 

Which are the Different Types of Load Banks? 

There are four primary kinds of load banks. For testing generators, you can apply either of the four types of load banks. Each one has its load of advantages and disadvantages. Therefore, the choice of a load bank will depend on the expected outcome. Let’s explore each one of them in depth. 

Resistive Load Banks

They are the commonest load banks and are capable of testing a generator at full capacity. Measuring load via this technique is relatively straightforward. Typically, the load bank will change electrical energy to thermal energy. The process will be facilitated by resistors hence the name resistive load banks. Next, the heat will be removed via air or, in some cases, water. 

The significance of the resistive load bank is that it will simulate the actual loads that the generator handles in real-life scenarios. Among the different loads include the prime mover capacity and the transient response in Hertz. Nonetheless, it cannot test for alternator capacity and load-sharing capabilities. Also, it is limited in the detection of transient response in voltages and distribution buses. 

Among the various options of resistive load banks that you will come across include the following: 

  • AC Water-cooled load banks
  • DC Portable load banks 
  • AC small Portable load banks

Also, it’s noteworthy that load banks are ideal for testing portable generators and even large generators with less than 200 kVA capacity. 

Reactive Load Banks

The principle of testing using reactive load banks is the same as that of resistive load banks, but we have capacitors or inductors as a load instead of resistors. Another difference is that electrical energy, in this case, is converted to magnetic energy. The commonest reactive load banks feature inductors as load rather than capacitors.  

For the inductive load bank, the testing capacity is up to three-quarters of full power testing. The same is also the limit for capacitive load banks. Nonetheless, in the latter, there is a possibility of creating high power factor loads. 

Combined Resistive/Reactive Load Banks

There are also load banks that combine the resistive and reactive properties. They are capable of testing a generator’s lagging at the full nameplate kVA rating. In principle, such load banks will have resistors and inductors in the same system. As a result, you can modify them to allow resistive, reactive, or different lagging power factors depending on the need in question. 

All such load banks will have a rating in kilovolt-amperes (kVA), and you can also find a resistive, inductive, and capacitive (RLC) combination. If your generator features a capacity larger than 200kVA, these are the ideal load banks to use. They are also ideal for loads of 1 MVA and above.

Electronic Load Banks 

The other important load bank is the electronic type. It is ideal for average and peak loads and works in the same fashion as the others. It will first apply an electrical signal to the generator, and from that, it will determine the device’s capacity.

The significance of the method is that it will give more precise data of the generator than the load banks mentioned above. Nonetheless, it is pretty expensive than the others, and thus you should be ready to spend. 

It is an accurate technique, and another upside is that it is ideal for any type of generator. 

Do Portable Generators Need to Rest?

Hence the question of the period when a generator can run continuously. A portable generator is a machine, and thus it is prone to tear and wear, especially if it is overused. Thus, it also needs to rest for some time before resuming operation. 

Generators are usually emergency devices that you employ while you get the main electricity fixed. However, you might have a scenario when you have to run it longer than usual due to various circumstances. It could be that you cannot promptly fix the main power and thus you’ll have to keep the generator running. 

The period of rest will depend on the type of portable generator that you are using. Mainly, there are two main types, and they include a gas generator and a diesel generator. Each will run for a different period before you must rest it. 

Gas-Powered Generator

If your portable generator runs on gas, you have a relatively short period when you can run it continuously without rest. The rule of thumb is that you must switch off a gas generator when refueling it. It might seem innocuous, but it is potentially dangerous and can result in a severe accident. 

Note that the generator will heat up after running for some time. Thus, if you add fuel while running, the gas fumes can ignite the gas in the hot tank. Consequently, the tank can burst in flames. Considering that all the engine parts are heated up, such a fire is dangerous and hard to put off. 

Therefore, you must switch off a portable gas-powered generator, at least when adding fuel. Fuel tanks are of various sizes. Hence, depending on the size of your tank, your generator can run for a few hours to days. After this period, you must give your generator some rest as you refill the tank. 

Diesel-Powered Generators

For diesel-powered generators, we have two types. First, there is a portable generator that is relatively small, and the other is a standby generator that is bigger and immobile. In most scenarios, a portable generator cannot withstand running for many days. Hence, if you intend to draw power for an extended period, a standby generator will serve you best. 

For most standby generators, the recommended period of constant running is 500 hours, approximately three weeks. You can opt to run it for a more extended period than this, but you’d be risking destroying its parts. 

On the other hand, since portable generators are small, they also have relatively small fuel tanks. They can thus run for several hours, but few will run continuously for days. Nonetheless, at some point, you’ll be required to change the engine oil. 

For most portable generators, the recommended maximum period of using engine oil is 200 hours. After the period, you have to shut down the generator and change the oil. Also, this is the ideal time to perform routine maintenance practices on the generator. 

If you have a standby generator operating on an unlimited fuel supply, you must give it some rest after 500 hours of continuous running. Also, remember to keep a close eye on a generator if you are running it continuously for extended periods. 

Lastly, it is not advisable to run a generator for an extended period without giving it rest. Doing so will cause the deterioration of its parts and reduce its operation life. 


Running a generator at a low or no load is not advisable because it affects its efficiency. As we have discussed above, generators are designed to convert mechanical energy to electrical power. Hence, they must burn fuel in the device’s combustion system. 

The process requires a minimum load of at least 30% of the generator’s capacity to operate effectively. Ideally, the best load should be 70% of the generator’s capacity. Also, it is imperative to run a road bank on the generator. The process facilitates identifying the capability of the generator to deliver. 

Lastly, a generator also requires some rest. For gas engines, you should switch off the generator when refueling. You often do not need to switch off a diesel generator regularly. However, for a portable diesel-powered engine, you need to rest it after 200 hours. For a standby generator, the ideal rest time is 500 hours. 


Hubert Miles

I've been conducting home inspections since 2002. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and an FHA 203k Consultant. I started to help people better understand whole-house and portable generators.

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