Before You Size a Generator, Heat Pump, or AC System

“How big of a system do I need?” is actually five different questions depending on what the system is for. A backup generator, a battery backup, a whole-home heat pump, a single room’s air conditioner, and the running cost of a system you already have each answer a genuinely different real-world question. Picking the wrong calculator for the question you actually have wastes time at best and leads to a wrong-sized purchase at worst — this guide walks through which one to reach for first.

Backup Power: Generator or Battery, and Why That’s Not the Same Question

If the goal is keeping the lights (and the fridge, and maybe the well pump) running during an outage, the first decision is fuel-powered versus battery-stored, since they’re sized completely differently:

• A generator needs to cover every appliance’s continuous running watts, plus the single largest startup surge on top — a motor (a well pump, a refrigerator compressor) draws significantly more power for a moment when it starts than it does while running. The Generator Sizing Calculator adds up running watts across everything you’d want powered simultaneously, then adds the largest single starting-watts spike, not every appliance’s spike added together.
• A battery backup is sized differently because it answers a duration question, not a peak-power question: given a load, how long will stored energy actually last, or given a target runtime, how big a battery do you need? The Battery Backup Calculator accounts for the fact that not every kWh of rated battery capacity is usable — depth-of-discharge limits and inverter conversion losses both take a real share off the top of the rated number.

Practical takeaway: these aren’t competing tools for the same question — a generator answers “can I run everything at once,” while a battery answers “how long will this specific load last.” A real backup-power plan often uses both: a generator sized for peak simultaneous load, and a battery sized for how long you need to bridge a gap before the generator (or utility power) is available again.

Everyday Heating and Cooling: Whole-Home vs. Single-Room

Once backup power is a separate question, everyday climate control splits by scope — a whole home versus a single room needs a different calculator entirely:

• A whole-home heat pump is sized from square footage, using a square-feet-per-ton rule of thumb that varies by climate zone and insulation quality — a home in a mild, well-insulated climate needs meaningfully less capacity per square foot than one in an extreme climate with poor insulation. The Heat Pump Sizing Calculator takes total conditioned area, climate zone, and insulation quality together, since none of the three alone gives an honest capacity estimate.
• A single room’s air conditioner is sized differently — not from a per-square-foot ratio, but from a published room-size-to-BTU reference table, adjusted for ceiling height, occupancy, sun exposure, and whether the room includes a kitchen (appliance heat load adds real cooling demand). The BTU / Air Conditioner Sizing Calculator handles this room-specific sizing, distinct from a whole-home capacity estimate.

Practical takeaway: don’t try to extrapolate a single room’s BTU need into a whole-home estimate by simple multiplication, or vice versa — the two calculators use genuinely different underlying methods (a reference table with adjustment factors versus a square-footage ratio), not the same formula at different scales.

Once a System Is Sized: What It Actually Costs to Run

Sizing answers “how big,” not “how much will this cost me monthly” — that’s a separate calculation built on a system’s actual efficiency rating, not its capacity:

The HVAC Operating Cost Calculator converts a system’s BTU output and SEER efficiency rating into an actual electricity cost, given how many hours a day and days it runs and your electricity rate. A higher SEER rating means the same cooling output for less electricity — which is exactly why a more efficient, often more expensive unit can pay for itself in lower running costs over its lifetime.

Practical takeaway: run this calculator with a candidate system’s real SEER rating (not just its BTU capacity) before comparing purchase prices across options — a cheaper unit with a lower SEER rating can cost more over a few years of operation than a pricier, more efficient one.

Putting It Together

Start by identifying which real question you’re actually asking: backup power during an outage (generator for peak simultaneous load, battery for how long a given load lasts), everyday climate control (heat pump for a whole home by square footage and climate, BTU sizing for a single room), or the ongoing cost of running a system you’re sizing or already own. Each of these five calculators answers one of those questions specifically — running the wrong one for your actual situation produces a technically correct number that answers a question you weren’t asking.