How to Size a Power Inverter for Off-Grid, RV & Backup Systems
Learn how to calculate continuous wattage, surge startup wattage, waveform selection (Pure Sine Wave vs. Modified), and DC cable sizing.
Step-by-Step Procedure
Calculate Total Simultaneous Continuous Load
Add up the wattage of every device that might run at the exact same time.
Determine the Highest Motor Surge Load
Check for appliances with AC motors or compressors (refrigerators, pumps, power tools). Multiply running watts by 2× to 4× to find peak starting surge watts.
Apply a 20% to 25% Safety Headroom
Multiply continuous load by 1.20 to prevent the inverter from running at 100% capacity in hot ambient conditions.
Choose Pure Sine Wave
Select a Pure Sine Wave inverter for clean grid-quality power compatible with sensitive electronics and induction motors.
Worked Practical Example
Home Emergency Backup Inverter
Running a refrigerator (180W run, 1000W surge), internet router (20W), 3 LED lights (30W), and a TV (80W).
- •Continuous load: 180W + 20W + 30W + 80W = 310 W
- •Add 20% headroom: 310 W × 1.20 = 372 W continuous requirement
- •Peak surge: 1000W (refrigerator startup) + 130W (other loads) = 1,130 W surge
- •Inverter selection: A 1,000W continuous / 2,000W surge Pure Sine Wave inverter is the ideal commercial size.
Common Mistakes to Avoid
- ✕Buying a modified sine wave inverter for refrigerator compressors (causes buzzing, overheating, and motor failure).
- ✕Using undersized DC battery cables for high inverter currents.
Frequently Asked Questions
How many DC Amps does a 2000W inverter draw at full load?
On a 12V battery system: 2,000 W / (12 V × 0.88 efficiency) ≈ 189 Amperes! This requires heavy 2/0 or 4/0 AWG copper cable and a 250A fuse.