Generator Sizing & Load Calculations

Why sizing matters more than brand

An undersized generator stalls or trips when the AC kicks in — the exact moment you need it. An oversized one costs thousands more, burns more fuel per hour, and can suffer from light-loading issues over a long life. The right size comes from adding up your home’s actual electrical loads, accounting for motor starting surges, and adding headroom. Any quote that sized your unit from square footage alone is a guess.

Running watts vs. starting watts

Every appliance has two numbers: running watts (what it draws continuously) and starting watts (the brief surge when a motor starts). For resistive loads like lights and electronics the two are nearly equal. For motors — refrigerator compressors, well pumps, and especially your central AC — starting watts can be 3–5x the running watts. The generator has to survive that surge, which is why a 3.5-ton AC with a ~4,000W running draw can demand 12,000W+ at startup.

Typical Tampa-home running loads for reference: central AC (3–5 tons) 3,500–5,000W running; refrigerator 600–800W; freezer 500–700W; sump pump 800–1,050W; microwave 1,000–1,500W; lighting circuits a few hundred watts; well pump 1,000–2,000W. Your installer measures or looks up your specific equipment — these are planning figures, not your home’s numbers.

How a real load calculation works

1. Inventory the loads. Every circuit the generator should carry, with running watts from nameplates or measurements.
2. Add the starting surges. The largest motor starts are added to the running total — worst-case sequencing matters.
3. Apply code methodology. Optional standby systems fall under NEC Article 702; the calculation approach follows established load-calc methods (Article 220 style) adapted to the generator, not a rule of thumb.
4. Decide the coverage strategy. Whole-home, essentials-only, or managed loads — this choice changes the target kW dramatically.
5. Add headroom. ~10–20% above the calculated peak covers growth and measurement error without gross oversizing.

Common Tampa sizing outcomes

• 7–12kW: essentials only — fridge, lights, fans, outlets, one small window unit. ($5,500–$8,500 installed)
• 14–18kW: typical 1,800–2,500 sq ft home including one central AC. ($8,000–$10,500–$9,500–$12,000)
• 22kW: the most common whole-home size — handles central AC plus the rest of the house. ($10,000–$14,000)
• 26kW+: large homes, dual AC systems, pool equipment, or electric-heavy households. ($13,000–$25,000)

Load management: the cheaper path to whole-home

Smart load management uses priority relays or an intelligent panel to sequence your big loads: the generator carries essentials continuously and brings the second AC or the pool pump online one at a time, or sheds them for the seconds of a compressor start. The result is a smaller unit doing a bigger unit’s job — often saving thousands on equipment and burning less fuel for the life of the system. If your quote doesn’t mention load management, ask why.

Fuel and electrical sizing are one job

The kW number drives everything downstream: the gas line size (an undersized gas line starves the unit under load — a common cause of “mystery” generator failures), the transfer switch rating, and the propane tank size for your target runtime. Sizing isn’t one number, it’s a matched system.