Buying a Backup Generator: A 7-Point Cost Manager's Checklist

The checklist you need before buying a generator

I manage procurement for a 120-person facilities company. I have managed our backup power budget (roughly $90,000 a year) for six years, and I have tracked every generator invoice, test, and repair in our cost system. This is the checklist I use when we spec, buy, and maintain backup power. It is not a textbook list. It comes from mistakes that cost real money.

The trigger event that changed how I think about backup planning was in March 2023. We lost a utility feed during a scheduled maintenance window, the generator did not start, and a deadline went missed. That one event taught me that a spec sheet is not a plan.

This checklist has seven steps. If you are comparing a Briggs & Stratton 18kW generator or a Cummins portable diesel generator, start with step one before you look at price. If you already own a generator, skip to step 5; that's where most of our after-sale failures actually showed up.

Step 1: Calculate your actual load, not the sales sheet load

The 12kW vs 18kW question is not about the brand. It is about the load.

I request a one-line diagram and a completed load calc from every site before I issue a PO. For a typical home, the biggest motor is usually the AC, well pump, or furnace. Add up the running watts of everything you want to run at the same time, then add a margin for motor starting current.

Load calc also has to include future additions. If you plan to add a heat pump or EV charger, put that load in now, not after you have spent the capital. In our 2024 audit, one site needed 13.4 kW for a continuous load. A Briggs & Stratton 12kW generator would have tripped under their AC's inrush. We went with a Briggs & Stratton 18kW generator because the margin felt right. Did we 'need' 18kW? Technically no. But an overload trip during a storm is a worse budget line item.

Step 2: Choose fuel and portability honestly

Do you need a portable unit or a standby unit? This is a cost decision, not a preference.

A Cummins portable diesel generator is a strong option for job sites and temporary backup. It can be moved, fueled from a tank, and hooked to a transfer switch. But it does not automatically start when the grid goes down unless you add an ATS and monitoring. A permanently installed standby unit, like a Briggs & Stratton 18kW or 12kW natural gas/propane model, will start automatically.

Fuel matters too. Diesel stores well but requires more maintenance. Natural gas is convenient but not available everywhere. We run dual fuel on job sites because propane gives us flexibility. Noise and fuel availability are real costs. Diesel is louder and needs a fuel polishing schedule. Natural gas is quiet but only worth it if the pipe size and utility capacity can handle the generator.

Step 3: Get an all-in quote and ask what's not included

Here is where my attitude got a reset. I used to focus on the unit price. Now I focus on the total cost.

In January 2025, I asked four vendors to quote the same 18kW installation. One vendor's quote was $3,200 below the second-lowest. Then I read the line items. It did not include the transfer switch, battery, concrete pad, permits, or startup labor. Once I added those, it was $1,100 more than the quote that listed everything.

I've learned to ask 'what's NOT included' before 'what's the price.' The assumption is that a higher quote means a vendor is padding margins. The reality is often the lower quote is hiding scope. A vendor who lists fees upfront, even when the total looks higher, usually costs less in the end. I do not trust a quote that requires a follow-up phone call to understand.

Step 4: Test under load before you need it

A generator that starts is not a generator that works. Load testing is the part most people skip.

We run a loaded test at 70% of nameplate for 45 minutes on every standby unit. This finds loose connections, undersized breakers, and fuel supply issues. NFPA 110 is the standard we reference for emergency power system maintenance. You do not need to quote the code number if you do not maintain emergency systems; you do need to run the unit under load.

We do a no-load run on the first Tuesday of every month, but a no-load run alone does not find a loose lug or an undersized feed. The first real load test is where the breaker trip usually happens.

Step 5: Stop treating the battery charger like an afterthought

Most of my 'sudden generator failure' calls are not the engine. They are the battery and the charger. The standby generator sits for weeks, and the battery must stay full.

We use a 6 and 12 volt battery charger with auto float and overcharge protection for our fleet. It sounds like a small item, but when a 12V battery is at 11.8V, the generator will not crank. A 6 and 12 volt battery charger often has a voltage selector. If it is set to 6V on a 12V battery, it will slowly undercharge until the battery sulfates.

In 2023, one of our sites failed a storm test because the charger had been unplugged during a cleaning. That cost us a truck roll, a service call, and a very embarrassed facilities manager. (Note to self: add charger checks to the regular inspection list.)

Step 6: If your circuit breaker keeps tripping, check these four things

The phrase 'why is my circuit breaker tripping' shows up a lot in our maintenance log. Usually, it is one of four causes:

1. Overload: The generator's output breaker is doing its job. The connected load exceeds the circuit's rating.
2. Short circuit: A damaged cord or a failed load device is causing a hard short.
3. Ground fault: Moisture in a receptacle or a generator end can leak current to ground and trip a GFCI breaker.
4. Transfer switch issue: The breaker on the load side is not the problem, but the transfer switch contacts or wiring are.

Per the National Electrical Code, circuit breakers are not supposed to run continuously at 100% of their rating. Most generator breakers are designed for continuous loads at 80% of nameplate. If a 12kW generator's main breaker is 50A, the safe continuous load is 40A. Seeing the breaker trip at 42A may be normal operation, not a defect.

When the breaker trips instantly, think short circuit or ground fault. When it trips after a few minutes under load, the more likely issue is a continuous load over 80% of the breaker rating.

Step 7: Keep a cost log and review it every quarter

This is the step that makes everything else actionable. I keep a spreadsheet with columns for date, unit serial, service type, cost, and vendor. Every invoice gets logged, even the $40 fuel ones.

In Q3 2024, the log showed that 7 of 11 generator service calls were battery or charger related. We switched the fleet to a better float charger and the calls dropped by more than half. That decision would not have been obvious without the data. We review this log before the annual budget cycle. It is easier to justify a maintenance plan with a service call history than with a vague estimate.

The part people forget

The most expensive generator decision I made was not buying the wrong brand. It was buying the right generator with the wrong installation scope.

I am not going to tell you that a Briggs & Stratton 18kW generator is always better than a 12kW, or that a Cummins portable diesel generator belongs in every garage. Both have jobs. Sizing is about your load. Fuel is about your site. Total cost is about the full quote.

Before you sign a purchase order, ask about hidden fees, test the unit under load, and put the battery charger on your maintenance list. That checklist would have saved us a missed deadline in 2023. I should have written it sooner.