Why Backup Generator Sizing Matters More Than You Think

When a homeowner in The Woodlands starts thinking about a backup standby generator, the first question is almost always "how many watts do I need?" The second question is usually "what brand should I buy?" Neither one is the right place to start. The right place to start is a load calculation: what, specifically, is this generator going to power when the grid goes down?

What Sizing Actually Means

A generator's rated capacity is measured in kilowatts (kW). A 22kW whole-home generator can deliver 22,000 watts continuously. That sounds like a lot until you start adding up the loads. A central AC system pulls between 3,000 and 5,000 watts running, but at startup it surges to two or three times that for a few seconds while the compressor motor gets up to speed. An electric water heater draws 4,500 watts. A kitchen range can pull 8,000 to 12,000 watts with all burners and the oven on. An electric dryer takes another 5,000.

Sizing is not about adding up every appliance in the house. It is about deciding which loads you must keep, which you can shed, and whether those critical loads can all start at the same time without exceeding the generator's surge rating.

The Surge Problem

Running watts and starting watts are different numbers. Motors surge. Compressors surge. A well pump surges. If two or three motor loads start simultaneously, even a correctly sized generator can trip its internal breaker. This is why proper generator installations include a load management system or a sequencing delay. The transfer switch brings circuits online in stages so the generator never sees every motor start at once.

A generator that is slightly undersized for total running load but paired with good load management will outperform an oversized generator with everything dumped on it at once. Sizing is not just a number on a spec sheet. It is a system design.

Undersized: What Goes Wrong

An undersized generator in a Texas summer means one of two outcomes. Either the AC will not start at all because the surge exceeds the generator's capacity, or it starts and the generator runs at full load continuously. Generators are rated for continuous operation, but running at 100% load in 100-degree ambient heat pushes oil temperatures up, shortens engine life, and burns through fuel faster. The engine was designed to cruise at 50 to 75 percent of its rating. Sustained full load is an emergency condition, not a design point.

The worst version of this: the homeowner does not realize the AC is overloading the generator until the power goes out during a storm, they switch to backup, and the generator shuts itself down on overload within minutes. Now they have no grid power and no backup power.

Oversized: Not a Free Pass

Going bigger seems like the safe play, but it has real costs. A 48kW generator for a house that never pulls more than 18kW runs at under 40% load almost all the time. Diesel and natural gas engines at light load do not burn fuel efficiently. They also accumulate carbon in the cylinders and exhaust because the combustion temperature never gets high enough to clear it. The industry calls this "wet stacking" on diesel units, and it leads to premature engine failures. On natural gas units the problem is less severe but still measurable in higher fuel bills and shorter maintenance intervals.

Oversizing also costs more up front. A 48kW unit costs roughly double what a 22kW unit costs, and the concrete pad, gas line sizing, and transfer switch all scale with it. That money is better spent on the load management system that lets a correctly sized generator handle your actual loads.

How a Proper Load Calculation Works

A load calculation starts at the electrical panel. Every circuit is listed with its breaker size and what it feeds. The installer identifies which circuits are critical (AC, refrigerator, well pump, medical equipment, security, lighting, internet) and which are optional (pool heater, hot tub, electric vehicle charger, second oven). Critical loads get added up for both running and starting watts. Optional loads are either excluded or put behind a load-shed module that disconnects them automatically if the generator approaches its limit.

• Central AC (3.5 ton): approximately 3,500W running, 10,500W surge
• Refrigerator: approximately 150W running, 500W surge
• Well pump (1 HP): approximately 1,000W running, 3,000W surge
• Sump pump (1/2 HP): approximately 500W running, 1,500W surge
• LED lighting (whole house): approximately 500W, no meaningful surge
• Gas furnace blower: approximately 800W running, 2,400W surge

These are rough numbers. Every unit is different, and the nameplate data on the equipment itself is the only reliable source. A load calculation done from a catalog is a guess. A load calculation done from the actual nameplates and a clamp meter reading is an answer.

The Texas Context

Grid outages in southeast Texas come from two directions: summer storms that take out distribution lines, and the rare winter event that overloads generation capacity. In both cases, the outage can last days. A generator that is correctly sized and properly installed with an automatic transfer switch starts within seconds of a grid drop and runs unattended for as long as the fuel holds. Natural gas piped from the utility main does not run out the way a propane tank or diesel belly tank does, which is why most standby installations in The Woodlands connect to the gas main.

AOT Mechanical installs backup standby generators for homes, businesses, and industrial applications across The Woodlands and has since 2009. If you are considering one, the first step is the load calculation, not the brand catalog. Call (281) 883-9972 and get it sized right the first time.