How Many Watts to Power a Home: A Sizing Guide for Installers & Homeowners

Sizing a power system for a home boils down to two critical numbers: running watts and surge watts. Running watts are the continuous power needed to keep lights and appliances humming. Surge watts are the momentary jolt required to start motor-driven equipment like an air conditioner or well pump. This guide provides the decision-making framework for installers, developers, and homeowners to accurately size a backup generator or solar and battery system, ensuring it meets real-world project demands without costly errors.

A typical American home might need 5,000–7,500 running watts, but could demand up to 12,000 surge watts the moment a central AC unit kicks on. Getting this calculation right is non-negotiable for system reliability and NEC compliance.

Your Quick Guide to Home Wattage Needs

Before diving into a detailed load calculation, a solid baseline estimate is essential for framing the conversation, whether you're sizing a Cummins backup generator or designing a complete solar and battery system with components from FranklinWH or Sungrow. This table provides a practical starting point for initial project planning.

Typical Home Wattage Requirements at a Glance

Home Size	Typical Continuous Watts	Estimated Surge Watts (Peak)	Primary Use Case & Target Audience
Small Home / Cabin	2,000–4,000 W	4,000–6,000 W	For Homeowners: Powering essentials like a refrigerator, lights, and a few outlets via a critical loads sub-panel.
Medium Home (1,500-2,500 sq ft)	5,000–7,500 W	8,000–12,000 W	For Installers: Sizing for whole-home backup, including HVAC, kitchen appliances, and well pump.
Large Home (2,500+ sq ft)	7,500–12,000+ W	12,000–20,000+ W	For Developers & EPCs: Powering large homes with multiple AC units, an EV charger, and other high-demand appliances.

Critical Mistake To Avoid: These figures are estimates. The actual watts needed depend on specific appliances, lifestyle, and climate. A home in Phoenix running central air will have a drastically different power profile than a same-sized home in Seattle. A formal load calculation is a mandatory step for any accurate, NEC-compliant system design.

With this overview, let’s tackle the fundamental difference between watts and watt-hours. Understanding this is critical to avoiding the most common and costly sizing mistakes in both generator and battery projects.

Watts vs. Watt-Hours: Getting This Right is Non-Negotiable

Before sizing any home power system, you must nail the difference between Watts (W) and Watt-hours (Wh). Confusing these two is the single most common and costly mistake in project planning, leading to undersized systems that fail when needed most.

Let's break it down:

• Watts (W) = Power: The rate of electricity use at any given second. An air conditioner kicking on is a high-wattage event.
• Watt-hours (Wh) = Energy: The total amount of electricity used over time. It measures total consumption over hours or days.

Why This Spells Success or Failure for Your Project

Sizing a system based on one metric while ignoring the other is a recipe for disaster.

• Watts (kW) determine your inverter or generator size. You must add the running watts of all simultaneous loads, plus the single largest startup surge. This peak demand dictates the raw power your equipment must deliver instantly.
• Watt-hours (kWh) determine your battery bank size. This is about endurance. Your total daily energy consumption dictates the storage capacity needed to power you through the night or a multi-day outage.

Expert Tip for System Sizing: Always think: Watts (kW) are for your power source (inverter/generator), and watt-hours (kWh) are for your energy storage (batteries). A powerful inverter is useless if your battery dies in two hours, and a massive battery bank from a brand like the FranklinWH system can't help if your inverter chokes on the startup surge from your well pump.

A Quick Look at Average Home Energy Use

For real-world context, the average U.S. household consumes around 10,332 kilowatt-hours (kWh) per year, averaging 861 kWh per month.

This breaks down to a daily average of 26 to 33 kWh—a critical number for sizing battery capacity for multi-day backup. You can explore more data on average household energy usage for a deeper dive. This average shows why measuring energy (kWh) over time is vital. While your home might only pull 5-7 kW of power at any moment, that adds up to a huge energy figure your battery system must handle.

How To Calculate Your Home's Exact Power Needs

Moving past estimates, a precise load calculation is the most important step in designing any power system. This data-backed power budget prevents the two most common and expensive mistakes: overspending on an oversized system or installing an undersized system that fails under load.

Step 1: Inventory Your Appliances (The Energy Audit)

Grab a notepad or spreadsheet and walk through the home, listing every appliance and device to be powered during an outage.

• Kitchen: Refrigerator, freezer, microwave, coffee maker, dishwasher, electric oven/range.
• Living Areas: TVs, sound systems, computers, routers, lighting.
• Utility/Basement: Washing machine, electric dryer, sump pump, well pump.
• HVAC: Central air conditioner, furnace blower motor, window AC units.

Step 2: Find Running And Starting Watts

For each item, find two key numbers: running watts (steady power use) and starting/surge watts (momentary power burst for motors). This surge can be 2-3x the running wattage and is the top reason undersized systems fail. Find these numbers on the appliance's nameplate, in the owner’s manual, or on the manufacturer's website.

For a deeper dive, check out our complete guide on how to calculate power consumption.

Common Appliance Wattage and Surge Demand

Appliance	Typical Running Watts	Typical Starting/Surge Watts
Refrigerator/Freezer	700 W	2,200 W
Central Air Conditioner	3,500 W	5,000 W
Window AC Unit (10k BTU)	1,200 W	1,800 W
Sump Pump (1/2 HP)	1,050 W	2,150 W
Well Pump (1/3 HP)	750 W	1,500 W
Microwave (1000W)	1,000 W	N/A
Electric Clothes Dryer	5,400 W	6,750 W
Washing Machine	500 W	1,150 W
Coffee Maker	1,000 W	N/A
Television (LED)	100 W	N/A

Step 3: Calculate Your Total Load

1. Calculate Total Running Watts: Add the running watts for everything you plan to use at the same time.
2. Determine Peak Surge Watts: Identify the single appliance with the highest starting wattage. Add that number to your total running watts. Do not add all starting watts together. This final number is your peak demand.

For Installers and EPCs: Size your generator or inverter against the calculated peak demand. Per NEC guidelines, always add a safety buffer of at least 20-25% to this total. It ensures system longevity, prevents nuisance tripping, and protects client investments.

Hidden Cost/Compliance Watchout: The Hidden Loads

It’s easy to miss these critical, hardwired appliances, but they can be a huge drain.

• Well Pumps: A non-negotiable load for rural properties with a significant surge demand.
• Sump Pumps: Absolutely critical for preventing basement flooding during storm-related outages.
• Medical Equipment: Life-sustaining devices like CPAP machines must be prioritized.
• EV Chargers: A Level 2 EV charger is a massive continuous load, often pulling 7,200 watts or more. Integrating balanced power regulation systems is essential for managing this load without overloading the system.

Sizing Your Generator and Solar System Correctly

With your load calculation complete, it's time to select the hardware. This is where data-driven decisions separate a reliable, high-performing system from one that fails. Let the math guide the machine, not the other way around.

Translating Watts to Generators

Sizing a backup generator is straightforward. You need two figures: total running watts and peak surge watts.

Critical Sizing Rule: Always build in a 20-25% buffer on top of your calculated peak demand. If your peak surge load is 8,000 watts, select a generator rated for at least 10,000 surge watts. This safety margin prevents the generator from redlining, extending its life and protecting sensitive electronics.

Every generator has two ratings:

• Running (Rated) Watts: The continuous power it can produce. This must exceed your simultaneous running load.
• Starting (Surge) Watts: The maximum power burst it can deliver for a few seconds. This must exceed your peak surge demand.

For a detailed walkthrough, see our guide on how to size a whole-house standby generator.

Sizing for Solar and Battery Storage

For a solar and battery system, the logic is split between power and energy.

1. Sizing Your Solar Inverter (Power - kW)
The inverter is the heart of the system, converting DC power from panels to AC power for the home. Its size is determined by your peak power demand (in watts). It must handle your simultaneous running load plus the largest surge. If your running load is 6,000 watts and peak surge is 10,000 watts, you need an inverter from a brand like Sungrow rated for at least 6kW continuous output and 10kW surge capacity.

2. Sizing Your Battery Bank (Energy - kWh)
Battery capacity is about stamina, sized based on your total daily energy consumption (in kWh). It answers, "How long do I need this to last?" A home consuming 30 kWh per day would need a battery system from a manufacturer like FranklinWH with at least that much usable capacity for 24-hour backup. Even guides for optimizing off-grid solar power setups can provide insights into managing energy storage.

The Importance of Expert Verification

While DIY calculations are a great start, always get a second opinion from a professional. A NABCEP-certified designer at Portlandia Electric Supply can verify your numbers against NEC standards, account for real-world factors like voltage drop, and ensure your final design is powerful, safe, and code-compliant. This review is cheap insurance against expensive mistakes.

Real-World Sizing Scenarios for Different Homes

Let's apply these concepts to three common scenarios we help contractors and homeowners plan for daily.

Scenario 1: The Small Home with an Essentials-Only Backup Plan

Goal: A cost-effective way to keep critical loads running during an outage.
Appliance Load List:

• Refrigerator/Freezer: 700 W running, 2,200 W surge
• Sump Pump: 1,050 W running, 2,150 W surge
• Lights & Outlets: 500 W running
• Internet Router & Modem: 20 W running
• Furnace Fan/Blower: 750 W running, 1,500 W surge

Calculation:

1. Total Running Watts: 700 + 1,050 + 500 + 20 + 750 = 3,020 W
2. Highest Surge: Refrigerator at 2,200 W.
3. Peak Demand: 3,020 W (running) + 2,200 W (surge) = 5,220 W

Equipment Recommendation: A generator rated for at least 4,000 running watts and 6,000 surge watts is a perfect match, providing a safe 20-25% buffer.

Scenario 2: The Medium Family Home Aiming for Whole-Home Backup

Goal: Maintain normal life during an outage, including central A/C.
Appliance Load List (in addition to essentials):

• Central Air Conditioner (3-ton): 3,500 W running, 5,000 W surge
• Electric Water Heater: 4,500 W running (non-simultaneous use)
• Microwave: 1,000 W running
• Television & Electronics: 400 W running
• Washing Machine: 500 W running, 1,150 W surge

Calculation:

1. Total Running Watts (without water heater): 3,020 W (essentials) + 3,500 W (AC) + 1,000 W + 400 W + 500 W = 8,420 W
2. Highest Surge: A/C unit at 5,000 W.
3. Peak Demand: 8,420 W (running) + 5,000 W (surge) = 13,420 W

Pro Tip for Installers: Exclude the electric water heater from the simultaneous load. Smart energy management systems or an interlock kit can prevent it from running with the A/C, dramatically reducing the required generator/inverter size and saving the client thousands.

Equipment Recommendation: A standby generator rated for at least 15,000 surge watts (15 kW). For solar, a hybrid inverter with 12 kW continuous output and a battery bank of 30 kWh or more is required. Learn more about whether solar panels can power a whole house.

Scenario 3: The Large Modern Home with High-Draw Appliances

Goal: Total energy independence, including EV charging.
Appliance Load List (in addition to medium home loads):

• Level 2 EV Charger: 7,200 W running
• Electric Oven: 2,400 W running
• Second HVAC zone: 2,000 W running

Calculation:

1. Total Running Watts: 8,420 W (medium home) + 7,200 W (EV) + 2,400 W (oven) + 2,000 W = 20,020 W
2. Highest Surge: Central A/C at 5,000 W.
3. Peak Demand: 20,020 W (running) + 5,000 W (surge) = 25,020 W

Equipment Recommendation: A liquid-cooled standby generator of 25 kW or more. A solar solution requires stacking multiple inverters and a massive battery array (40-60 kWh+ using multiple BYD or FranklinWH units). Advanced load management is non-negotiable.

Your Top Questions About Home Power Sizing, Answered

Can I Just Use My Utility Bill to Figure Out My Wattage Needs?

Partially. Your utility bill provides total energy consumption in kilowatt-hours (kWh), which is perfect for sizing a battery bank. However, it does not show your peak power demand in watts (kW). That peak moment when multiple large appliances run simultaneously can only be determined by a proper load calculation, which is essential for sizing a generator or solar inverter.

What’s the Big Deal If I Undersize My Generator or Inverter?

Undersizing is a critical failure. An undersized system is designed to fail when you need it most.

• Generators: An undersized unit will struggle, causing voltage drops that can damage sensitive electronics and will constantly trip its own breaker.
• Solar Inverters: An inverter will shut down ("clip") if the home tries to pull more power than its rating, leaving you without power even with a full battery.
  Always size for your peak load plus a 20-25% buffer.

Do I Really Need to Power My Entire House During an Outage?

No, and for most projects, a critical load sub-panel is a smarter, more cost-effective strategy. This separate breaker box powers only the essentials: fridge, freezer, furnace fan, well pump, and key outlets. This dramatically reduces power demand, allowing for a smaller, more affordable generator or solar and battery system, saving thousands. The average person in the U.S. and Canada uses a massive 15.9 megawatt-hours a year as of 2023. You can see more about these global electricity trends on ember-energy.org. A targeted critical load strategy is a practical response to this high usage.

How Does Adding an EV Charger Change My Calculations?

An EV charger is a game-changer. A Level 2 charger is a massive, sustained load of 7,000 to 12,000 watts.

Pro Tip for Installers: If a client wants to charge an EV during an outage, you must add that load to your peak demand calculation. Modern smart panels and energy storage systems can automatically manage this with "load shedding," pausing the charger when a larger load like an AC unit starts. This feature is a key differentiator between a system that works flawlessly and one that constantly trips.

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From initial design to final commissioning, having the right equipment and expert support ensures project success. At Portlandia Electric Supply, we provide in-stock inventory of the generators, inverters, and batteries you need, backed by our NABCEP-certified team to guarantee your system is designed for rock-solid, code-compliant performance. Request a Quote from our team today to get your next project started.