What Size Generator Do I Need for My House? Sizing Guide and Load Worksheet

Quick answer: Most homes need a 5,000-8,000 watt portable generator to run essentials, or a 14-24 kW standby generator to power the whole house. The right number depends on what you run at the same time, and especially on your central air conditioner or heat pump, which has the biggest starting surge.

Asking “what size generator do I need for my house” is the right first question, because buying too small means tripped breakers in the middle of an outage, and buying too large means paying thousands for capacity you will never use. This guide walks through the math, shows typical sizes by home size, and includes a simple worksheet you can fill in at your kitchen table.

Want a second set of eyes on your numbers? You can request a free generator load assessment and quote using the form on this page.

Starting Watts vs Running Watts: The Core Concept

Every appliance has two numbers that matter. Getting them straight is the key to sizing correctly.

• Running (rated) watts: the steady power an appliance uses once it is operating. Lights, TVs and routers draw only their running watts.
• Starting (surge) watts: the extra burst a motor-driven appliance needs for a second or two to get going. Compressors, well pumps and sump pumps are the usual culprits.

A central air conditioner might run at 3,500 watts but need 7,000 or more for a moment at startup. Your generator must be able to handle that peak without stalling or tripping.

The practical rule: add up the running watts of everything you want on at once, then add the single largest starting surge on top. Do not add every surge together, since motors rarely start at the same instant.

Essentials vs Whole-Home: Decide What You Want to Power

Before any math, decide on your goal. The two common approaches have very different size and cost profiles.

Essentials-only backup

This covers the loads you need to stay safe and comfortable for a few days. It usually pairs a portable generator with a manual inlet and interlock, or a standby unit feeding a small critical-loads panel.

• Refrigerator and freezer
• Lights in main living areas
• Furnace blower or boiler pump
• Sump pump or well pump
• Phone chargers, router, a TV or laptop
• A window AC unit or a few space-heater-free comforts

Whole-home backup

This keeps nearly everything running, including central air, an electric range, the water heater and the dryer. It almost always means a permanently installed standby generator with an automatic transfer switch. For costs and what to expect during the project, see our guides on whole house generator cost and standby generator installation cost.

Many homeowners land in the middle: whole-home convenience, but with a load management device that temporarily shuts off a big appliance (like the dryer) when the AC kicks on, which lets you choose a smaller unit.

Typical Generator Size by Home Square Footage

These ranges are general estimates for planning. Your actual need depends on climate, fuel type, and whether you have electric heat, a heat pump, a pool pump or an EV charger.

• 1,500 sq ft: about 9-14 kW for most whole-home setups (one 2-3 ton AC).
• 2,000-2,500 sq ft: about 14-18 kW, sometimes 20 kW with a larger AC.
• 3,000 sq ft: about 18-22 kW, often with two AC zones.
• 3,500-4,000 sq ft: about 22-26 kW or more, particularly with multiple HVAC systems or electric appliances.

For essentials only, a 5,000-8,000 watt portable or a smaller 7-10 kW standby unit is often enough regardless of home size.

Square footage is only a starting point. A 2,000 sq ft home in Phoenix with a 5-ton AC may need more generator than a 3,000 sq ft home in a mild climate with gas heat.

Your central air conditioner, not your square footage, is usually what decides generator size.

Generator Load Worksheet: Calculate Your Number

Use the approximate figures below if you cannot find an exact nameplate rating. These are typical estimates and vary by model and age.

Step 1: List your loads

• Refrigerator: about 700 running / 2,200 starting watts
• Chest freezer: about 500 running / 1,500 starting
• Sump pump (1/3 hp): about 800 running / 1,300-2,000 starting
• Well pump (1/2 hp): about 1,000 running / 2,100-3,000 starting
• Furnace blower: about 600-1,000 running / 1,500-2,000 starting
• Central AC (3 ton): about 3,500 running / 6,000-9,000 starting
• Electric water heater: about 4,000-4,500 running (no surge)
• Electric range (large burner/oven): about 2,000-5,000 running
• Electric dryer: about 5,000 running
• Microwave: about 1,000-1,500 running
• LED lighting (whole house): about 200-500 running
• TV, router, chargers: about 300-500 running

Step 2: Follow the math

1. Write down every appliance you want running at the same time.
2. Add the running watts of all of them for a subtotal.
3. Find the largest starting surge among them (usually the AC or well pump) and add only the extra above its running watts.
4. Add 20-25% headroom so the generator is not running at its limit.
5. Convert to kW by dividing watts by 1,000 and compare to available generator sizes.

Example: essentials plan

Refrigerator (700) + sump pump (800) + furnace blower (800) + lights and electronics (700) = 3,000 running watts. The biggest surge is the fridge or pump at about 2,000 starting, so add roughly 1,300 extra for a 4,300 watt peak. With 25% headroom, that is about 5,400 watts, so a 6,500-7,500 watt portable fits comfortably.

Example: whole-home plan

Add a 3-ton AC (3,500 running, roughly 6,000 extra surge capacity needed), electric water heater (4,000) and the essentials above (3,000) and you are near 10,500 running watts before the surge and headroom. That points toward an 18-22 kW standby unit, or a smaller one if load management is installed.

Common Sizing Mistakes to Avoid

• Ignoring the surge: a generator rated for your running load can still fail to start the AC.
• Oversizing “just in case”: a very large unit costs more upfront and can run inefficiently at light loads.
• Forgetting future loads: an EV charger, heat pump or hot tub added later can push you past capacity.
• Skipping fuel and altitude factors: propane typically delivers less output than natural gas, and high elevation or heat can derate a generator.
• Guessing instead of measuring: your electrician can read your panel and service size to confirm real demand.

Fuel choice and installation details also affect which model makes sense, and a planned guide on natural gas vs propane generators will cover that tradeoff. If you are comparing a permanent unit to a battery system, our breakdown of home battery backup system cost shows how those options stack up.

A generator that is too small fails when you need it most; one that is too big wastes thousands.

Get a Professional Load Assessment

A worksheet gets you close, but a licensed pro can measure actual demand, check your electrical panel and service size, and account for local codes and fuel availability. That confirms the right kW before you spend real money.

A good assessment typically covers:

• Your panel capacity and which circuits matter most
• The exact nameplate data on your HVAC, pumps and appliances
• Whether load management could let you choose a smaller unit
• Placement, fuel supply and transfer switch requirements

Once you have a size, the next step is understanding the install itself. Our guide to whole house generator installation explains the process and timeline, and you can browse more on our home backup power hub. Rules on permits and transfer switches vary by city and utility, so local requirements will apply.

Ready to confirm your numbers? Request a free, no-obligation generator quote through the form on this page and a vetted provider can assess your load.

Frequently Asked Questions

What size generator do I need to run a whole house?

Most whole-home setups fall between about 14 kW and 24 kW, depending on square footage, central AC size and whether you have electric appliances. A load calculation or professional assessment gives the most accurate answer for your home.

Can a 5,000-watt generator run a house?

A 5,000-watt generator can usually run essentials like a refrigerator, lights, a furnace blower and electronics. It generally cannot start a central air conditioner or power an electric range or dryer at the same time.

How many kW do I need to run central air?

A typical 3-ton central AC runs at roughly 3.5 kW but may need 6-9 kW to start. Larger systems need more, so the AC is usually the biggest factor when sizing a generator for summer outages.

Is it better to oversize a generator?

A modest cushion of 20-25% is smart. Going far beyond that adds cost and can reduce efficiency at light loads, so right-sizing based on your actual loads is the better approach.