General Generator FAQs

How does generator sizing actually work?

Generator sizing comes down to three numbers:

• Running load – the watts (or kW) your home uses while everything is running normally.
• Starting (surge) load – the temporary spike when motors start, like AC compressors and pumps.
• Headroom – extra capacity (typically 20–30%) so the generator isn’t running at the ragged edge.

A good rule of thumb: add up the running watts for everything you plan to use during an outage, then add the single highest motor starting load. Choose a generator so your expected peak load is around 70–80% of its rated output. This gives room for noise in the numbers, future loads, and hot-weather derating.

There’s no one-size-fits-all answer. Two homes with the same square footage can have very different electrical loads depending on fuel type (gas vs. electric appliances), number of AC units, well pumps, EV chargers, and so on.

What size generator do most homes need?

It depends on whether you want to power just essentials or nearly everything in the house:

• Essentials-only (via portable or smaller standby): Often 7–12 kW is enough to run a smaller central AC or several window units, refrigerator, some lighting, and key outlets — as long as you’re strategic about what runs at the same time.
• “Most of the house” with one central AC: Many typical 1,500–2,500 sq ft homes with gas heat and gas water heaters end up in the 16–24 kW standby range.
• Larger homes or two+ AC systems: Homes with 3,000+ sq ft, multiple large AC units, or significant electric heat often require 24 kW and up, and sometimes 30 kW+ depending on priorities.

These are typical ranges, not guarantees. The safest approach is to use a load-based calculator (like the one on SizeMyGenerator.com) and then have a licensed electrician confirm the final size and transfer switch setup for your specific home.

Do I need a “whole-house” generator, or can I just back up essentials?

Not everyone needs or wants a whole-house system. You have three main options:

• Portable generator + manual transfer switch – Lower cost. You select a limited set of circuits (fridge, a few rooms, possibly a smaller AC or window units, pumps). You manually manage loads.
• Standby generator with limited-circuit transfer switch – Automatic startup, but only certain circuits are backed up (often 8–16 circuits).
• Whole-house transfer switch with load management – The generator is connected to your entire panel, and smart modules shed lower-priority circuits automatically when needed.

Many homeowners are happy with a well-planned essentials-only setup, especially if outages are rare. In areas with frequent or extended outages, a properly sized whole-house system can be worth the extra cost and complexity.

Can I rely on square footage alone to pick a generator size?

No. Square footage is a rough proxy at best. Load is driven much more by:

• Number and size of AC units and heat pumps.
• Whether heat, water heating, and cooking are gas or electric.
• Presence of wells, sump pumps, EV chargers, shop equipment, or pools.
• How many rooms you truly need powered during an outage.

Rules like “X kW per Y square feet” can easily be off by 30–50%. They’re useful only as a starting point. A quick load-based calculation or using a generator sizing calculator will always be more accurate.

How do I calculate what size generator I need?

There are two basic approaches: a quick estimate and a more detailed load calculation.

Quick estimate:

1. List the rooms and systems you care about during an outage (HVAC, fridge, well or sump pump, medical equipment, office, etc.).
2. Note the amps or wattage from labels or manuals for each item.
3. Convert amps to watts using watts = volts × amps (for most household loads, 120 V or 240 V).
4. Add up all the running watts for the items you plan to run at the same time.
5. Identify the appliance with the highest starting surge (often a central AC, well pump, or large compressor).
6. Add that starting surge to your running total, then add 20–30% headroom.

More precise method: electricians follow NEC-style load calculations that account for continuous vs. non-continuous loads, diversity factors, and your actual panel configuration. That’s more work by hand, which is why a calculator plus a quick professional review is usually the most efficient path.

What’s the difference between running watts and starting watts?

Running watts (sometimes labeled “rated watts” or “continuous kW”) are what an appliance needs to keep running. Starting watts (or “surge watts”) are the brief spike when the motor first kicks on.

Many motor loads draw 2–3× their running current for a fraction of a second at startup. For example, a 1,000 W well pump might momentarily need 2,000–3,000 W to start. Modern soft-start kits and variable-speed equipment can reduce this spike, but it still needs to be considered in sizing.

Generators are usually rated for a continuous output (for example, 20 kW) plus a slightly higher short-term surge rating. Your goal is to keep combined loads — including the biggest startup event — within what the generator can comfortably deliver.

Is it worse to undersize or oversize a generator?

Undersizing creates immediate problems: nuisance tripping, brown-outs when large loads start, and in the worst case, damage to sensitive electronics if voltage sags. If your generator is obviously too small for your chosen loads, you’ll be forced into inconvenient manual load shedding (flipping breakers on and off).

Oversizing is safer electrically but more expensive. Very light loading can also lead to poor fuel economy and, on some engines, sub-optimal operating conditions over long periods. Most homeowners aim for a balanced size with a modest buffer rather than the extremes.

Should I still talk to an electrician if I use an online calculator?

Yes. A calculator is an excellent way to narrow the range quickly and understand trade-offs, but it doesn’t replace on-site evaluation. A licensed electrician or generator installer will:

• Confirm your loads and priorities.
• Check panel capacity, grounding, and bonding.
• Verify gas service capacity for natural gas or tank sizing for propane.
• Handle permits, inspections, and transfer switch selection.

Think of the calculator as the “research and planning” step and the electrician as the final authority who makes sure everything is safe and code-compliant.

Ready to see what this looks like with your actual loads? Open the generator sizing calculator and plug in your home’s details for a tailored recommendation.

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