Free Tool
Inverter Size Calculator
Add the devices you want to run at the same time and get a recommended inverter size that covers running load, motor start-up surges and sensible headroom.
Size an inverter to the combined running watts of everything switched on at once, then make sure its surge rating covers your highest start-up spike. Motors in fridges, pumps and power tools can briefly draw three to five times their running watts, so the surge figure often decides the inverter, not the continuous rating.
Interactive Tool
Inverter Size Calculator
As a rule of thumb, we recommend 24V systems above 1,500W and 48V systems above 3,000W.
Recommended inverter size
500W
215W running total, 665W surge peak
Estimated max DC input at 12V: 49A
Shop inverters and chargersEstimates include 20% headroom and allow for motor start-up surges. You can size the DC cable for this current with our cable size calculator.
How this is calculated
We add up the running watts of everything you plan to power at once, then add 20% headroom so the inverter is never working flat out. Running an inverter at its limit shortens its life and triggers overload cut-outs, so a little breathing room is always worth it.
Devices with motors or compressors, such as fridges, pumps and power tools, briefly draw several times their running wattage when they start. We assume only the largest of these starts at any one moment and add that surge on top of your running total. Most quality inverters can surge to roughly twice their continuous rating for a second or two, so we check the surge peak against double the inverter rating.
The recommended size is the smallest standard inverter that satisfies both checks. The DC input current estimate assumes 85% inverter efficiency, which is why higher system voltages (24V or 48V) make sense for bigger inverters. Lower current means thinner cables, smaller fuses and less heat.
Frequently asked questions
What size inverter do I need?
Add up the running watts of all appliances you will use simultaneously, then add around 20 percent headroom. Separately, check the peak surge of your highest-draw motor appliance stays within the inverter’s surge rating.
What is surge or peak power?
The brief spike of power an appliance draws at start-up, especially anything with a motor or compressor. A fridge rated 150W running can surge to 600W or more for a second. Inverters quote a continuous rating and a higher short-term surge rating.
Do I need a pure sine wave inverter?
For most modern electronics, yes. Pure sine wave matches mains power and runs everything safely. Modified sine wave is cheaper but can hum, run motors hot, and upset sensitive electronics, medical devices and some chargers.
Can an inverter be too big?
A much larger inverter than you need wastes energy through higher idle draw and costs more up front. Size it to your real load plus sensible headroom rather than buying the biggest available.
