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To size backup power, add up the running watts of everything you want to power at the same time, then add the single highest starting-watt jump from whichever appliance has a compressor or motor (like a fridge or well pump). That combined number is the minimum continuous output your generator or power station needs. The right total varies a lot by household, a small apartment covering just a fridge, lights, and chargers needs far less than a house running a well pump, sump pump, or furnace blower, so there’s no single number that fits every home. The process below walks you through calculating your own.
Why Load Calculation Matters Before You Buy Anything
Backup power gear is sold by watts, but most first-time buyers pick a generator or power station based on price or brand instead of doing the math first. That’s backwards. Undersize your system and it trips, stalls, or damages equipment when a compressor kicks on. Oversize it and you’ve spent money on capacity you’ll never use. A load calculation, done on paper before you shop, tells you exactly what wattage range to look for.
This guide walks through the process step by step. It’s an educational overview, not a substitute for a licensed electrician’s assessment, especially if you’re planning any permanent connection to your home’s wiring.
Running Watts vs. Starting Watts: The Two Numbers You Need
Every electrical device has two wattage figures that matter for this calculation.
- Running watts (rated watts): The steady amount of power a device draws once it’s already on and operating normally.
- Starting watts (surge watts): A short, higher burst of power some devices need for a second or two when they first switch on. This applies mainly to anything with a motor or compressor, such as refrigerators, freezers, well pumps, sump pumps, and air conditioners.
Devices without motors, lights, phone chargers, routers, laptops, and most electronics, generally have a starting draw close to their running draw, so you can treat those as a single number.
Step 1: List Every Device You Want to Power
Walk through your home and write down every item you’d realistically want running during an outage. Be honest about what you actually need versus what would be nice to have; a longer list means a bigger, more expensive system.
- Refrigerator and/or freezer
- Well pump or sump pump, if applicable
- Furnace blower or space heater
- A few lights (LED bulbs draw far less than incandescent)
- Phone and device chargers
- Wi-Fi router and modem
- Medical equipment, such as a CPAP machine or oxygen concentrator
- Window air conditioner or box fan
Step 2: Find the Wattage of Each Device
The most reliable source is the nameplate or spec label on the device itself, usually on the back or bottom, or the manufacturer’s product manual. Look for a wattage figure directly, or calculate it from amps and volts using this formula:
Watts = Amps × Volts
Most U.S. household outlets run at 120 volts. So a device pulling 5 amps draws roughly 600 watts (5 × 120). If the label only lists amps, this formula gets you an estimate; the nameplate wattage, when listed, is more accurate. Wattage varies by manufacturer, model, and age, so always use the figure printed on your own device rather than a generic number from another source.
Step 3: Add Up Running Watts
Add the running-watt figures for every device on your list that you’d run simultaneously. This total is your baseline continuous load.
Step 4: Add the Largest Single Starting-Watt Surge
Look at just the motor- or compressor-driven devices on your list, refrigerator, freezer, well pump, sump pump, air conditioner, and find the one with the highest starting-watt figure. Add only that single highest surge number on top of your running-watt total from Step 3. You don’t need to add every device’s surge separately, because in practice these motors rarely all start at the exact same instant, but you do need to cover the worst-case single jump.
Illustrative example only, not a real appliance combination: Say your running-watt total comes to 1,500 watts, and the largest starting-watt surge among your motor-driven devices adds another 800 watts. Your target minimum continuous output would be roughly 2,300 watts. Your own numbers will differ based on your specific appliances.
Step 5: Add a Safety Margin
Once you have your combined figure, it’s common practice to shop for equipment rated somewhat above that number rather than exactly at it, since running a generator or power station at its absolute ceiling for hours at a time adds wear and reduces the margin for error if you add a device later. How much margin to build in is a personal and budget decision; there’s no single official percentage that applies to every household.
Practical Checklist
- List every device you’d want to run during an outage.
- Find each device’s running-watt figure from its nameplate or manual.
- Identify which devices have motors or compressors and check their starting-watt figures.
- Add all running watts together.
- Add only the single highest starting-watt surge on top.
- Build in a margin above that total before shopping for equipment.
- Confirm any permanent connection to household wiring with a licensed electrician.
- Plan generator placement at least 20 feet from your home, per CPSC and CDC guidance, before you ever need to use it in an actual outage.
Generator Safety Is Part of the Load Conversation
Sizing a generator correctly doesn’t help if it’s used unsafely. The Consumer Product Safety Commission and the CDC both advise keeping portable generators outside and at least 20 feet away from windows, doors, and vents, with the exhaust directed away from the home, since carbon monoxide from a generator can build to lethal levels indoors even with doors or windows open. Both agencies also recommend battery-powered or battery-backup carbon monoxide alarms on every level of the home and outside sleeping areas. Battery-based power stations don’t produce exhaust and don’t carry this specific risk, which is one reason some households choose them for indoor-safe backup power, though they still have their own charging and battery-safety considerations.
Limitations of This Method
- This calculation gives you a planning target, not a guarantee of performance under every real-world condition, such as extreme heat, extreme cold, or an aging appliance drawing more than its rated watts.
- Wattage figures on older appliances may be less accurate than current nameplates; when in doubt, use a plug-in watt meter to measure real draw if you have access to one.
- This guide does not cover sizing for whole-home standby generators tied into a transfer switch, which typically requires a licensed electrician’s load calculation specific to your home’s electrical panel.
- Battery-based power stations have a limited total energy capacity (measured in watt-hours) in addition to a wattage limit; a device may fit within the wattage ceiling but still drain the battery faster than expected. Runtime depends on the specific battery capacity of the unit you choose.
Who Should Be Extra Careful
Households relying on powered medical equipment, such as a CPAP machine, oxygen concentrator, or powered wheelchair, should build in extra margin and consider a backup plan that doesn’t depend on a single device, since an equipment failure during an outage carries higher stakes. Households with well water should account for the well pump’s starting surge carefully, since pumps are often the single largest jump in a home’s load calculation. Anyone considering a permanent electrical connection between a generator and household wiring should use a licensed electrician and a proper transfer switch rather than improvising a connection, which can create a shock hazard for utility workers and violate electrical code.
Frequently Asked Questions
How many watts do I need to run a refrigerator during an outage?
It depends on the specific refrigerator, both size and age affect running watts and starting-watt surge. Check the nameplate on your own unit for its rated running watts, and consult the manufacturer’s documentation for its starting-watt surge, since these figures vary too much by model to state as a single number.
Can I just add up all the starting watts for every device instead of just the highest one?
You could, and it would give you an extra-conservative, higher target. Many households instead add only the single largest starting-watt surge on top of total running watts, on the reasoning that multiple compressor-driven appliances rarely start at the exact same instant. Which approach you choose depends on your own risk tolerance and budget.
Is a battery-based power station safer than a fuel generator for this kind of planning?
Battery-based power stations don’t produce exhaust, so they avoid the carbon monoxide risk that the CDC and CPSC warn about with fuel-powered generators, which is a meaningful safety advantage, particularly for indoor or close-proximity use. They come with their own tradeoffs, including total energy capacity and recharge time, so the right choice depends on your specific needs and how long you expect outages to last.
Do I need an electrician to use a portable generator or power station?
Running individual devices directly off a portable generator or power station with its own outlets generally does not require an electrician. However, any setup that connects a generator to your home’s electrical panel or wiring, such as through a transfer switch, should be installed or inspected by a licensed electrician to meet code and avoid safety hazards.
Related Reading
- Off-Grid Living: A Beginner’s Guide to Power, Water, and Self-Sufficiency covers solar, battery banks, and generator basics in more depth for households planning a longer-term power setup.
- Prepping for Beginners: How to Get Started in 7 Practical Steps walks through the broader readiness picture that backup power fits into.
- Food and Water Storage Guide is worth pairing with backup power planning, since a working refrigerator only protects your food supply as long as your power plan holds up.
- Emergency Preparedness Family Plan covers how to fold backup power decisions into a broader household plan.
Disclaimer
This article is for general educational purposes only and does not constitute electrical, engineering, or safety certification advice. Wattage figures vary by manufacturer, model, and age, always confirm exact figures using your own device’s nameplate or manufacturer documentation before making a purchase. Any permanent connection between a generator and household electrical wiring should be designed and installed by a licensed electrician in accordance with local code. Blueprint Prepared is an independent publication and is not affiliated with any generator or power station manufacturer named or implied in this article.