How Long Will a 12V Battery Last Using a Power Inverter?

The 3 Critical Factors Determining Runtime

Calculating power inverter runtime isn't just about battery capacity - it's a dance between three key players:

• Battery capacity: Measured in amp-hours (Ah), but we'll need to convert this to watt-hours for accurate calculations
• Load wattage: The actual power draw of your connected devices
• System efficiency: Typically 80-90% for modern inverters, plus battery discharge limitations

The Power Math Simplified

Let's break down the calculation process:

1. Convert battery capacity to watt-hours: 12V × Ah rating
2. Apply battery discharge limit (70% for lead-acid): Multiply by 0.7
3. Account for inverter efficiency (use 85% as default): Multiply by 0.85
4. Divide by device wattage to get hours

Why Your Actual Results Might Differ

Ever wonder why field tests rarely match theoretical calculations? These hidden factors play bigger roles than most users realize:

• Temperature swings: Battery capacity drops 1-2% per °F below 70°F
• Peak loads: Motor-starting surges can temporarily double power draw
• Cable resistance: Poor wiring can waste up to 15% of your power

Pro Tip: The 80/20 Rule for Battery Safety

Never drain batteries completely. For lead-acid models:

• Stop at 50% discharge for occasional use
• Limit to 30% discharge for daily cycling

Real-World Application: Emergency Power Setup

Imagine needing to power a 150W CPAP machine through an 8-hour night:

1. Total watt-hours needed: 150W × 8h = 1,200Wh
2. Account for 85% inverter efficiency: 1,200 ÷ 0.85 = 1,412Wh
3. Add 30% safety margin: 1,412 × 1.3 = 1,836Wh
4. Convert to battery capacity: 1,836Wh ÷ 12V = 153Ah

You'd need two 100Ah deep-cycle batteries connected in parallel to safely meet this requirement.

Optimizing Your Power System

• Match inverter size to your typical load - oversized units waste power
• Use pure sine wave inverters for sensitive electronics
• Consider lithium batteries if weight and cycle life are concerns