Is a hurricane or blizzard threatening your local power grid? Use our free generator size calculator to determine exactly what size engine you need to survive a blackout. If you buy a generator that is too small, your refrigerator compressor will immediately stall the engine. If you buy one that is too big, you are wasting thousands of dollars on gasoline. This tool adds up running Watts and startup surges to find your perfect mechanical match.
Select Your Essential Appliances
Check the boxes for the items you absolutely MUST run simultaneously during a power outage.
Recommended Generator Size
0 Watts
(Look for a generator with at least this 'Starting / Surge' rating)
Total Running Watts
0 W
Highest Motor Surge
+0 W
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How to Use This Backup Power Estimator
Buying the right generator requires balancing mathematics and emergency planning. First, go through the checklist and select the appliances you absolutely cannot live without during a blackout. If you live in a flood zone, the sump pump is mandatory. If you have medical insulin, the refrigerator is mandatory.
The calculator instantly adds up the continuous running Watts of all selected items. More importantly, it scans your list to find the single largest electric motor (usually the fridge or AC). It calculates the massive "Surge" required to kickstart that motor, and outputs the final mathematical capacity you need to look for when shopping at the hardware store.
The Surge Wattage Trap
The biggest mistake homeowners make during hurricane season is ignoring "Surge Wattage" (also called Starting Watts). When an electric motor first turns on, it requires a massive, 2-second blast of electricity to overcome the physical inertia of standing still. Once it starts spinning, it settles down into a much lower "Running Wattage."
For example, a refrigerator only requires 700 Watts to stay running. However, the exact millisecond the compressor turns on, it demands nearly 2,200 Watts. If you buy a cheap 1,000 Watt generator, it will instantly stall and shut off the moment the fridge tries to start. You must always buy a generator based on the peak surge calculation.
Generator Size Examples
Let's look at a family trying to survive a 3-day winter blackout. They only want to run the essentials: the refrigerator (700W), a space heater for the bedroom (1500W), a few LED lights (100W), and the internet router to check the news (50W). Their total continuous load is 2,350 Watts.
However, using our calculator, the algorithm detects the refrigerator compressor. It adds the 1,500 Watt starting surge of the fridge to the total. The final result recommends a generator size of at least 3,850 Watts. The family can safely buy a standard 4,000-Watt portable unit and survive the storm comfortably.
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Emergency Power Tips for 2026
Never Use Suicide Cords: Do not build a double-sided male plug to force generator power into a wall outlet. This illegal practice bypasses your circuit breakers, can burn your house down, and can physically electrocute linemen working on the power poles outside. Hire an electrician to install a proper transfer switch.
Beware of Central Air: Trying to run a Central Air Conditioner on a portable generator is nearly impossible. A central HVAC unit requires a massive 7,000+ Watt surge just to start the outside fan. If you want full AC during a blackout, you must spend $10,000+ on a massive Standby Generac system.
Inverter Generators for Electronics: If you are plugging expensive laptops and OLED televisions into a generator, you must buy an "Inverter" generator (like the Honda EU series). Cheap, open-frame construction generators produce "dirty" electricity that will physically destroy sensitive microchips.
Frequently Asked Questions
What size generator do I need to run a house?
To run essential appliances (fridge, lights, microwave) during an outage, a 5,000 to 7,500 Watt portable generator is sufficient. To power an entire house including Central AC, you will need a massive 20,000+ Watt standby generator.
What is 'starting wattage' vs 'running wattage'?
Running wattage is what an appliance needs to stay on. Starting wattage (surge) is the massive, 2-second burst of extra electricity required to kickstart large electric motors, like a refrigerator compressor.
Can I run a generator indoors?
Absolutely NEVER. Gas and propane generators produce massive amounts of lethal carbon monoxide. They must be operated outdoors, at least 20 feet away from any windows or doors.
What is an inverter generator?
An inverter generator produces 'clean' sine-wave electricity that is safe for sensitive microchips. Standard open-frame generators have electrical spikes that can instantly destroy expensive televisions and laptops.
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How to Maximize the Value of This Tool
To get the absolute best results from this calculator, accuracy in your inputs is paramount. We suggest gathering all relevant documentation before you begin to ensure the numbers you enter are precise. If you are calculating a loan or mortgage, have your official interest rate quote, your exact down payment amount, and your loan term handy. If you are using a tax or paycheck tool, pull up your most recent pay stub and last year's tax return. Small discrepancies in inputs—like being a single percentage point off on an interest rate, or estimating a slightly inaccurate local tax rate—can compound into massive mathematical errors over a long period. Additionally, we highly encourage you to run multiple scenarios. Don't just calculate your expected outcome; change the variables slightly to see how a higher interest rate, a longer payoff period, or a different salary bracket affects your final bottom line. This "stress testing" approach is exactly how professional financial analysts use data to make robust, bulletproof plans. By exploring both the best-case and worst-case scenarios, you will be much better prepared for whatever the future holds, allowing you to make confident, data-driven decisions.
Stop guessing with emergency power. Scroll back up and calculate the exact engine wattage you need to survive the storm.