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When the power goes out at dinner time and the fridge light stays dark, the question is practical: what size inverter will run the things that matter? An inverter size chart turns the watts printed on appliance labels into the inverter size you need, by matching running watts and surge watts to the continuous output and surge output of the inverter. This guide walks through the chart step by step, using only the figures printed on the labels of the appliances you own.
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The Quick Answer
Read the running watts and the surge watts from the label of each appliance you want to run, add the running watts together, and pick an inverter whose continuous output covers that total and whose surge output covers the largest startup draw. Match the inverter voltage to the battery voltage, and choose pure sine wave output for appliances with motors or electronic controls. The steps below show how to work through the chart for your own loads.
Find the Running Watts on Each Label
Every appliance that plugs into an inverter carries a label, usually on the back or the base, that lists volts, amps and watts. Running watts are the watts the appliance draws while it runs at a steady pace, and that figure is the starting point for the chart. If the label lists volts and amps but no watts, multiply them together for the running watts. A plug-in watt meter between the appliance and the outlet measures real use over time, which is the best figure if the label is worn or hard to read.
Write the running watts of every appliance you plan to run into a list before you shop, and keep the list with the labels it came from. A fridge, a freezer, a microwave, a phone charger and a set of lights all count, even the small loads, because the chart adds them together and small draws add up across a whole evening of use.
Allow for Surge at Startup
Running watts are only part of the chart. Motors pull a short spike of extra power when they start, called surge, and the inverter has to cover that spike or it shuts off. Fridges, freezers and power tools all surge at startup, while lights, phone chargers and televisions draw a steady load with little or no surge. Read the surge watts from the appliance label or the manual, and if the label lists no surge figure, look for the startup draw in the manual before you size the inverter.
The surge output of the inverter has to cover the largest startup draw among the loads you run together. If several motors start at the same moment, their surges add together, so plan for the worst moment, not the average moment. A fridge compressor that kicks in while a freezer compressor is already running is the classic case: the chart has to cover the surge of each motor that starts while the rest keep running.
Add the Loads You Run Together
An inverter size chart is a list, not a guess. Add the running watts of every appliance you plan to run at the same time, and keep the surge watts of the motors in a separate column so the startup spike stays visible. The total running watts set the continuous output you need, and the largest surge sets the surge output you need. Loads you run at different times do not add together, so a microwave you run alone needs less inverter than the same microwave run with the fridge, the freezer and the lights on at the same time.
Be honest about what runs together. An outage evening usually means the fridge, the freezer, the lights and a phone charger all at once, with the microwave or a power tool added for short spells. A camping setup is lighter: lights, a phone charger and a small fan, with the fridge cycling on and off through the night. Write the list for the heaviest moment you expect, then size to that moment.
Match the Inverter to the Battery
The inverter also has to match the battery that feeds it. Read the battery voltage from the battery label and pick an inverter built for that voltage, because an inverter built for a higher voltage will not work on a lower-voltage bank, and one built for a lower voltage can be damaged by a higher-voltage bank. The battery also has to feed the inverter enough current for the loads, so check the battery manual for the current it can supply and the cable sizes it calls for, and follow the manual for fuses and connections.
Choose Pure Sine Wave or Modified Sine Wave
Inverters make AC output in different forms: pure sine wave and modified sine wave. Pure sine wave output matches the smooth power from a household outlet, and it suits appliances with motors, compressors and electronic controls. Modified sine wave output is a rougher stepped wave that suits simple loads such as lights and basic chargers, but some motors run hot or noisy on it and some electronic controls refuse to run on it at all. When the appliance manual calls for pure sine wave output, take that as the rule and choose pure sine wave.
For a fridge, a freezer or a CPAP, pure sine wave output is the safer choice, because the compressor or the control board is sensitive to the shape of the power. For lights and a phone charger alone, modified sine wave output will do, but many owners pick pure sine wave output for the whole setup and stop worrying about which load needs it.
Inverter Size Chart
Use the chart below as a worksheet. Read the running watts and the surge watts for each load from its own label, write them in, and add the columns. The surge column matters most for loads with a motor, so mark those loads before you add anything.
| Load | Running watts | What to check |
|---|---|---|
| Fridge | Read the label | Compressor start-up draws a short spike |
| Chest freezer | Read the label | Stays cold longer when the power is off |
| Microwave | Read the label | Runs at a steady draw while cooking |
| Phone charger | Read the label | Small draw, easy to add to any setup |
| Lights | Read the label | Add the watts of every bulb you run together |
| Power tools | Read the label | Motors pull a spike when they start |
| CPAP | Read the label | Check the label for the draw with the humidifier on |
| Television | Read the label | Electronic controls prefer pure sine wave output |
The chart gives a shape, not a size: the labels on your own appliances fill in the figures. The labels on the fridge and the freezer matter most, because their compressors set the running total and the surge peak. When a label is missing, a plug-in watt meter measures the real draw, and the manual gives the surge figure for motors.
Ways to Keep the Size Down
- Run the microwave alone, with the fridge and freezer already cold, so the heavy load never shares the inverter with a startup surge.
- Stagger motor starts: let the fridge compressor settle before starting the freezer, so the surges never stack.
- Move small loads off the inverter where you can: a fan or a USB charger that runs from the battery skips the inverter entirely.
- Pre-chill the fridge and freezer before an expected outage, and keep the doors shut, so the compressors cycle less often and the inverter carries a lighter load.
- For camping and vans, run the fridge on DC and keep the inverter for the loads that truly need AC.
FAQ: power away from the grid
Can an inverter be too big for the load?
Yes, in the sense that a larger inverter draws more power from the battery even with a small load, so size to the loads you actually run. A bigger inverter also needs a battery bank that can feed it, so match the inverter to the battery as well as to the loads.
What happens if the inverter is too small?
It shuts off when the load passes its continuous output, or when a motor starts and the surge passes its surge output. The appliance stops, and the inverter may need a reset before it runs again. Size for the largest startup draw, not just the running watts.
Do I need a pure sine wave inverter?
Appliances with motors and electronic controls run best on pure sine wave output, and some controls refuse to run on modified sine wave output at all. Simple heating loads and basic chargers are less fussy, but pure sine wave output is the safer choice when the manual calls for it.
Can I run a fridge and a freezer on the same inverter?
Add the running watts of each appliance and plan for the compressors starting at the same moment, which adds the surges together. A larger inverter, or staggering the starts, keeps the load inside what the inverter can supply. See the best inverters for a refrigerator for options.
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