Inverter Sizing Calculator
Add your own appliances with their exact wattage, or use the presets, to find the correct inverter size including surge wattage for motors and compressors.
Running Watts Aren't the Whole Story
Every inverter spec sheet lists a continuous power rating, and most people size their inverter against that number alone by adding up appliance wattages. That works fine for resistive loads like lights and heaters, but it breaks down the moment a motor is involved. Fridges, water pumps, and air conditioners all draw a short burst of power — sometimes 2-3 times their running wattage — just to get the motor spinning. Undersizing for that surge is the most common reason an inverter trips or shuts down the instant a compressor kicks on, even though it handled the same appliance fine once it was running.
This calculator estimates surge load by taking your single largest motor-driven appliance and applying a 2.5x multiplier to it, then adding that to the running total of everything else. It's a simplification — exact surge ratings vary by specific motor — but it's a far more realistic starting point than ignoring surge entirely, which is what most basic inverter calculators do.
Why the Safety Margin Matters
A 20-25% safety margin on top of your calculated load isn't padding for its own sake. Grid or generator voltage isn't always perfectly stable, inverters lose a little efficiency as they age, and most households add appliances over time rather than removing them. Sizing exactly to today's load means re-doing this calculation the next time you buy a window AC unit or a second fridge.
Once you have your target inverter size, it needs to be paired with a battery that can actually deliver that output — check your storage capacity against the Battery Backup Sizing Calculator to make sure both numbers line up before buying either component.
Hybrid vs. Standalone Inverters
The sizing math on this page applies to either type, but which one you actually need depends on your setup. A standalone inverter just converts battery DC power to AC for your appliances — simple, and usually cheaper. A hybrid inverter does that plus manages solar charging and grid interaction in one unit, which is what you want if you're building an integrated solar-plus-battery system rather than a battery-only backup. Buying a hybrid inverter for a battery-only backup setup means paying for solar-management circuitry you'll never use; buying a standalone inverter when you're planning to add solar later usually means replacing it. Decide which setup you're actually building before shopping by size alone.
Frequently Asked Questions
What size inverter do I need for my home?
Add up the running watts of everything you'll power at once, then add a safety margin of about 25%. If you're running any motors or compressors, check surge wattage too, since starting loads can be 2-3 times higher than running loads.
What is surge wattage and why does it matter?
Surge wattage is the brief spike in power draw when a motor-driven appliance first starts, such as a fridge compressor, water pump, or air conditioner. An inverter sized only for running watts can shut down or fail when a large motor starts, even if it handles the running load fine.
Why does the calculator round up to standard inverter sizes?
Inverters are manufactured in standard capacities like 1kW, 2kW, 3kW, 5kW, 8kW, and 10kW. Rounding your requirement up to the nearest standard size reflects what you'd actually be able to buy.
Is a bigger inverter always better?
Not necessarily. Oversizing wastes money upfront and can slightly reduce efficiency at low loads, since inverters run most efficiently near their rated capacity. The goal is matching size to your actual load plus a reasonable safety margin, not maximizing capacity.
What if my appliance isn't on the preset list?
Use the "Add Your Own Appliance" field above the checklist to enter its exact name and wattage from the nameplate or user manual, and mark whether it has a motor or compressor. It's included in the calculation exactly like a preset item.