What Size AC and Generator Do You Actually Need?

Both of these get oversized on instinct. One wastes fuel, the other leaves you with a cold, damp room and a compressor that never runs long enough to do its job.

Oversizing is the expensive mistake

With both air conditioners and generators, the instinct is to buy bigger than you need for safety. With a generator that mostly wastes fuel. With an air conditioner it actively makes the room worse.

An oversized AC cools the air to the thermostat setpoint quickly and shuts off before it has run long enough to pull moisture out. The room ends up cold and clammy, the compressor short-cycles, and both efficiency and equipment life suffer. ENERGY STAR is blunt about this: bigger is not better.

The room air conditioner sizing table

ENERGY STAR publishes capacity brackets by floor area rather than a flat rate per square foot.

A ton of cooling is 12,000 BTU per hour, a unit inherited from the days when cooling was measured against the ice it replaced.

Then adjust for the room

The bracket assumes an ordinary insulated room with ordinary glazing and an 8 ft ceiling. Four adjustments apply:

• Very sunny room: add 10%.
• Heavily shaded room: subtract 10%.
• More than two regular occupants: add 600 BTU/h per extra person.
• Kitchen: add 4,000 BTU/h.

Ceilings above 8 ft scale the base roughly in proportion, because you are cooling volume rather than floor area. Worked through for a sunny 300 sq ft kitchen with 9 ft ceilings and three occupants:

That is nearly double the unadjusted bracket, which is the whole argument against a flat per-square-foot rule. Sunrooms, converted garages and rooms directly under an uninsulated roof break it worse still. Run your own room through the BTU calculator.

Generators: two numbers, not one

Anything with a motor draws far more current for the first fraction of a second than it does once spinning. Sizing on running watts alone is the classic error — the generator handles the load fine until the well pump kicks in and the whole thing trips.

Figures are typical rather than universal — the nameplate on your own equipment beats any table. Resistive loads such as heaters, kettles and incandescent lighting have no surge at all.

Adding it up

Only the largest surge counts, because two motors starting in the same instant is unlikely and can be designed out by staggering what you switch on. For a fridge (700), furnace blower (800), sump pump (1,050) and household electronics (300), the running load is 2,850 W. The biggest surge is the sump pump’s extra 1,100 W, giving a peak near 3,950 W — so a 5,000 W generator, not a 3,500 W one.

Two more deductions from the headline rating: generators lose roughly 3% of output per 1,000 ft of altitude, and the surge rating is a momentary figure that cannot be treated as continuous capacity. The generator size calculator applies the headroom for you.

Once you know the running watts, what it costs to run an appliance turns that into a number on the bill, and how many solar panels power a house covers the same load maths for generation rather than backup.