How to use this inverter size calculator
- Total running watts, add up the wattage of every appliance you want to power at the same time and enter the total.
- Surge / headroom margin, leave it at 20% for a typical mix, or raise it if you run motors, pumps or air conditioners.
- Power factor, keep the default 0.8 for a normal home load, or use the figure on your equipment if you have it.
- Read the required power, required VA rating and recommended size, they update instantly as you type.
Nothing is submitted or stored: the numbers never leave your device, so you can plan a solar, battery or backup setup privately.
How to size an inverter
Sizing an inverter comes down to two numbers: running watts and surge watts. Running watts are the steady power your appliances use while they operate. Surge watts are the much larger, short burst some appliances pull at the instant they switch on. Motors in fridges, freezers, pumps, washing machines and air conditioners can spike to several times their running watts for a fraction of a second, and an inverter that only matches the running figure may stall or trip.
Inverters are rated in VA (volt-amps) and kVA, not just watts, because they have to supply apparent power, not only the real power that does work. The two are linked by the power factor. For a typical mixed household load the power factor is about 0.8, so the conversion is VA = W ÷ power factor. That means 800 watts of load needs roughly 1000 VA before you add any margin.
On top of that you should add about 20% headroom so the inverter is not running flat out, which keeps it cooler, more efficient and ready for the next motor start. This calculator does both steps for you: it applies your margin to the running watts, divides by the power factor to get VA, then rounds up to the next common inverter size. If your loads are mostly motors, bump the margin higher and check the inverter's published surge rating as well as its continuous rating.
Common appliance wattages
- LED bulb: 5-12 W
- Laptop: 50-90 W
- Ceiling fan: 60-80 W
- TV (LED): 80-150 W
- Refrigerator (running): 100-250 W (surge 600-1200 W)
- Microwave: 800-1200 W
- Water pump (0.5 HP): 370 W (surge 1100-2000 W)
- Window air conditioner: 900-1500 W (high surge)
- Electric kettle: 1500-2000 W
Add together only the items you would realistically run at the same time. The running watts in this list are for guidance, your appliance labels are the most accurate source, and remember that anything with a motor draws a much higher surge than its running figure suggests.
Note: This calculator is a planning guide for general use. It does not replace a qualified electrician or the manufacturer's specifications. For permanent installations, solar systems and anything safety critical, have your design checked by a licensed professional.
Frequently asked questions
What size inverter do I need?
Add up the running watts of everything you want to power at once, add about 20% headroom, then divide by the power factor (usually 0.8) to get VA. For example 800 W with 20% headroom is 960 W, which is about 1200 VA, so a 1200 VA inverter is the smallest sensible choice.
What is the difference between watts and VA?
Watts measure real power that does work, while VA (volt-amps) measure apparent power, the total the inverter must supply. They are linked by the power factor: VA = watts ÷ power factor. Inverters are rated in VA or kVA, so you size in VA, not just watts.
What is power factor and why does it matter?
Power factor is the ratio of real power to apparent power, typically around 0.8 for a mixed home load. A lower power factor means the inverter has to supply more VA for the same watts, so it pushes your required rating up.
Why do I need extra headroom for motors and fridges?
Motor loads like fridges, pumps and air conditioners draw a surge of two to six times their running watts for a moment when they start. Sizing only to running watts can stall or trip the inverter, so you add headroom and check the inverter's surge rating.
Is bigger always better for an inverter?
Not always. An oversized inverter costs more and can run less efficiently at light loads, while an undersized one overheats and shuts down. Aim for a rating comfortably above your peak need, usually the next standard size up from your calculated VA.