There is no universal answer to 'which generator should I buy?' That is not a dodge; it is the result of reviewing equipment submittals for a living. I am a quality/compliance manager at a generator distributor. I review roughly 200+ spec sheets and installation handoffs every year. In Q1 2024, I rejected 3% of first deliveries because the rating matched the brochure but not the load study. The difference between those deliveries was rarely dollars. It was the wrong question.
The Scenario Split: Three Different Problems
Before you compare kW numbers, identify which scenario you are in. The choice between a 100 kW Cummins emergency generator and a 120 kW Cummins backup generator is not always the same choice.
- Scenario A: You need permanent, automatic backup for a facility. This is where the 100 kW vs 120 kW decision lives.
- Scenario B: You need temporary or portable power. An electric portable generator may be the right call, at least for now.
- Scenario C: The generator you already own is misbehaving. That is where the fuel filter and the blinking red charger light come in.
How do you know which one you are in? I will get to that at the end. First, let's be honest about each branch.
Scenario A: 100 kW vs 120 kW Cummins Emergency Generator
Let's start with the question I get most: Should we install a 100 kW Cummins emergency generator or a 120 kW Cummins backup generator? My answer: It depends on how the load starts, not just how much load you think you have.
The old 'always add 20%' rule feels safe, but it ignores how a genset behaves at light load. This was true 20 years ago when few people had real load data and starting inrush was a bigger mystery. Today, you can get a load bank test and review logged data. A 120 kW unit running at 35% load for days can be worse than a 100 kW unit running at 70% load, especially on an older diesel engine. The 120 kW unit is not automatically better. It costs more, uses more fuel, and may need heavier conductors and a larger pad.
The question isn't 'How many kW?' The question is 'Can I stage the loads so the generator survives the first three seconds?'
If you have motor loads like chillers, pumps, or elevators, look at starting kVA, not nameplate kW. A 100 kW unit with load-shed contacts can handle a large motor that starts in sequence. A 120 kW unit with no sequencing will trip on the second motor if all loads hit at once. The larger number does not fix a system architecture problem.
According to NFPA 110 (nfpa.org), a Level 1 emergency power system must be able to pick up its connected load within 10 seconds after a normal power source fails. That code requirement is about a tested, reliable transfer scheme, not just the size of the engine. A 100 kW set with a proper automatic transfer switch and load-shed controls can meet that requirement. A 120 kW set with a weak fuel system or no commissioning will fail it. The sticker kW number is the last thing that matters on the night of a real outage.
In March 2024, we had a client with a hard event date. A 120 kW unit was available 'probably' in three weeks. A 100 kW unit could be delivered and commissioned in five days, but it needed a small load-shed panel and a $400 rush delivery. We paid for the 100 kW solution. The extra $400 bought certainty, not speed. The alternative was missing a date that would have cost far more. Uncertain cheap is often more expensive than certain premium.
Even after we placed that order, I second-guessed myself: What if the load-shed panel has a hiccup and we lose a critical circuit? That worry stayed with me until the load bank test showed 99 kW stable for four hours. The certainty I paid for showed up exactly when it mattered.
Another common mistake: I say 'full building backup' and the vendor hears 'all loads at the same time.' Those are two different projects. We once used those exact words, discovered the mismatch when the load schedule arrived, and had to add breaker interlocks and load sequencing. It cost time, not a bigger generator.
Does that mean you should never buy a 120 kW Cummins backup generator? No. If your facility has a large motor load that cannot be sequenced, the extra starting kVA of a 120 kW unit is a valid technical answer. The problem is selecting it for the wrong reason. Extra room in a standby set is not safety; it is capacity, and capacity has a price.
Also check the rating definition. Cummins publishes generator ratings under ISO 8528. Standby rating is for limited running time during an interruption; prime rating is for continuous load. A 100 kW standby unit might only be rated for 90 kW continuous prime. If you expect multi-day outages, the prime rating is the number to compare, not the standby number.
Scenario B: When an Electric Portable Generator Is the Smarter Start
Not every backup power problem is a 100 kW or 120 kW emergency generator problem. For a temporary event, a small tool circuit, or a construction trailer, an electric portable generator might be the right start. A 5,000-watt unit can keep a refrigerator and a boiler control running for a day. A 10,000-watt unit can support temporary lighting or a lift station in a pinch.
But if your facility needs power within 10 seconds, or needs to run unattended, an electric portable generator is not a replacement. It is a bridge while you wait for the permanent Cummins unit. One facility I reviewed used a portable unit with a manual transfer switch and an on-call operator. That worked, but it only worked because someone was in the building at the time.
If you are running a portable generator on a temporary fuel tank, put a good fuel filter between the tank and the generator. A WIX 3/8 inline fuel filter is a common choice for small engines, but verify it matches the fuel pump flow and pressure. A $12 filter that stops water and sediment can save an engine, but a filter with the wrong flow rating can also starve it. Check the service manual before trusting a fuel system part.
Scenario C: Why Is My Lithium Battery Charger Blinking Red?
Scenario C starts with a small alarm: the red LED on your lithium battery charger blinks instead of showing a steady charging state. In most cases, that is a fault code, not a waiting state. It usually means one of four things: the battery voltage is outside the charger's range, the cells are too unbalanced, the temperature sensor detected a fault, or the charger is not designed for lithium chemistry.
So why is my lithium battery charger blinking red? On many chargers, a red blinking light indicates a protection event. The charger has stopped delivering current because it sees something abnormal. A solid red light sometimes means it is in bulk charge; blinking red almost always means it has stopped and you need to read the manual.
If that charger is on your generator start battery, this is the cheapest warning you will ever get. Replace the battery or charger before you need the generator, not after. In an outage, a dead starting battery turns a 100 kW Cummins emergency generator into a very expensive paperweight.
On the fuel side, the same logic applies. A WIX 3/8 inline fuel filter can be the right part for a small portable generator, but it is not a substitute for the main fuel filters on a 100 kW Cummins set. On larger diesel gensets, the primary filter is usually a spin-on filter with a water separator, not a 3/8-inch inline unit. I am not saying you should ignore the WIX product; I am saying you should put the right filter on the right machine. A 3/8 inline filter used as a polishing filter before a small pump is fine. Used as a substitute for the engine's fuel filter, it will cause more harm than good.
A facility once saved $30 buying a generic inline filter that looked similar to the factory part. The filter collapsed under suction, the fuel pump cavitated, and the genset shut down during a business day. The diagnostic and rework bill was over $2,100. The original premium filter would have been cheaper.
How to Know Which Scenario You Are In
If you are still unsure, use this test. Ask: Does the load require automatic restoration within seconds? That puts you in Scenario A. Is the power need temporary, manual, and small? That is Scenario B. Is the unit already installed and the problem is a warning light, fuel supply, or starting battery? That is Scenario C.
If you are in Scenario A and still deciding between 100 kW and 120 kW, get a load study. It costs less than one day of rental at the wrong size. Ask for the standby and prime ratings, review the starting kVA of the largest motor, and ask what happens when the second motor starts before the first one has settled.
At the end of the day, a generator is not a status symbol. It is a system that has to perform when the grid disappears. Check the fuel path, understand the charger warning, and pay for delivery certainty when the deadline matters. Those details determine whether your Cummins generator is a paper spec or a machine you can trust.
