| Appliance | Running Watts | DC Draw |
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Sizing an Inverter
An inverter must handle both the steady running watts of your appliances and the brief surge when motors and compressors start. Add up the running watts, then apply a surge factor for the toughest load. Pick an inverter rated above that figure so it never clips or shuts down under a startup spike.
DC Draw Is Higher Than AC Watts
Because the inverter is not perfectly efficient and runs at low voltage, the current pulled from a 12V battery is large. A 600 watt load can draw nearly 60 amps on the DC side, which is why inverter cables are thick and need a fuse close to the battery.
Runtime and Battery Size
Runtime is usable battery energy divided by the inverter input power. Lead-acid banks give up only half their rating, so real runtime is shorter than the raw amp-hours suggest. The helper sizes the battery for a target number of hours so you can plan a bank rather than guess.
Frequently Asked Questions
What surge factor should I use?
Resistive loads like heaters need little headroom; mixed loads about 1.5 times; motors, pumps, and compressors can spike to 2 to 3 times running watts.
Pure sine or modified sine?
Pure sine is safer for electronics, motors, and medical devices. Modified sine is cheaper but can buzz or damage sensitive gear.
Why does my battery drain so fast?
High-wattage AC appliances pull heavy DC current. Large loads like microwaves and heaters need a sizeable battery bank to run for any length of time.