4kWh), a 2000W inverter is ideal. Formula: Inverter Wattage ≤ (Battery Voltage × Ah Rating × 0. Factor in surge power needs but prioritize sustained loads. This free power inverter calculator helps you determine the correct inverter size and battery bank capacity for your specific needs. Describe what you want changed, added, or compared. Do not include. After hands-on testing and side-by-side comparison, I confidently recommend the BELTTT 2000W Pure Sine Wave Inverter as your best-sized inverter for a 12-volt battery—perfect when power quality and capacity really matter.