Quick Answer
To calculate battery runtime, multiply your battery's total watt-hours (Wh) by its safe depth of discharge (DoD) and your inverter's efficiency, then divide by the appliance's running watts. The formula is: Runtime (hours) = (Nominal Battery Wh × DoD × Inverter Efficiency) ÷ Appliance Watts. For example, a 1,000 Wh lithium battery (100% DoD) running through an 85% efficient inverter will power a 100W laptop for about 8.5 hours.
Defining the Variables
Before doing the math, you need to understand the four numbers that dictate how long your battery will actually last:
- Nominal Battery Capacity (Wh): This is the total energy the battery holds. If your battery is rated in Amp-hours (Ah), multiply Ah by the battery voltage (V) to get Wh. (e.g., 12V × 100Ah = 1,200 Wh).
- Depth of Discharge (DoD): You cannot drain most batteries to zero without damaging them. Lead-acid batteries typically have a 50% usable DoD, while Lithium Iron Phosphate (LiFePO4) batteries safely allow 80% to 100% DoD.
- Inverter Efficiency: Batteries store DC power, but most household appliances need AC power. The inverter that converts DC to AC consumes energy in the process, typically losing 10% to 15% (meaning 85% to 90% efficiency).
- Appliance Running Watts: This is the continuous power your device draws. Be careful not to use the "starting watts" (surge) for runtime calculations, as surges only last a few seconds.
What Most Runtime Estimates Miss
Many basic calculators simply divide total battery capacity by appliance wattage. That approach will leave you in the dark because it ignores real-world physics:
- Inverter Idle Draw: Even when your appliances are turned off, an inverter left powered on will consume a small amount of power (often 10W to 50W) just to stay active. Over a 24-hour period, this "phantom draw" can drain a small battery completely.
- Peukert's Law (for Lead-Acid): If you draw power very quickly from a lead-acid battery, its effective capacity shrinks. A 100Ah lead-acid battery drained over 20 hours will provide much more total energy than the same battery drained in 2 hours.
- Temperature Degradation: Batteries kept in freezing conditions (below 32°F / 0°C) can lose 20% to 50% of their usable capacity temporarily.
- Cycling vs. Continuous Loads: A 1,000W refrigerator does not run continuously. It cycles on and off, meaning its average hourly draw might only be 150W to 200W.
Battery Runtime Reference Table
The table below shows estimated runtimes for common appliances based on a standard 12V 200Ah LiFePO4 battery (2,400 Wh nominal).
Assumptions: 90% usable DoD (2,160 Wh), 85% inverter efficiency. Total usable energy = 1,836 Wh.
| Appliance Load | Running Watts | Estimated Runtime |
|---|---|---|
| Wi-Fi Router + LED Lamp | 30 W | 61.2 hours |
| CPAP Machine (Heater off) | 40 W | 45.9 hours |
| Laptop Charging | 65 W | 28.2 hours |
| LCD Television (55-inch) | 120 W | 15.3 hours |
| Full-Size Refrigerator (Average) | 150 W | 12.2 hours |
| Portable Space Heater (Low) | 750 W | 2.4 hours |
| Microwave Oven | 1,200 W | 1.5 hours |
Illustrative Worked Example
Let's walk through a realistic, step-by-step calculation.
The Scenario: You have a 24V 100Ah AGM Lead-Acid battery and you want to run a 200W desktop computer setup during a power outage. Your inverter is rated at 88% efficiency.
Step 1: Calculate Nominal Watt-Hours
24 Volts × 100 Amp-hours = 2,400 Wh
Step 2: Apply Safe Depth of Discharge (DoD)
Because this is an AGM lead-acid battery, draining it below 50% regularly will ruin its lifespan.
2,400 Wh × 0.50 (DoD) = 1,200 Usable Wh
Step 3: Apply Inverter Efficiency
The inverter loses 12% of the power during the DC-to-AC conversion.
1,200 Usable Wh × 0.88 (Efficiency) = 1,056 Effective Wh
Step 4: Calculate Final Runtime
Divide the effective watt-hours by the appliance's running watts.
1,056 Effective Wh ÷ 200 Watts = 5.28 hours
Result: Your computer will run for roughly 5 hours and 15 minutes before the battery reaches its safe 50% discharge limit.
Practical Checklist for Sizing Your Battery
Before buying a battery or relying on one for an emergency, follow these steps:
- Read the nameplate: Check the exact wattage on the back of your appliance. If it only lists Amps and Volts, multiply them (Amps × Volts = Watts).
- Check your battery chemistry: Confirm if you have Lead-Acid (50% DoD) or Lithium (80-100% DoD).
- Account for surge: Ensure your inverter's peak rating can handle the startup surge of appliances with motors (like fridges or pumps), even if the running watts are low.
- Use a calculator: For multi-appliance scenarios, use the WattSizing Calculator to model your daily load profile.
FAQs
Why is my battery dying faster than the math suggests?
The most common culprits are inverter idle draw (which consumes power 24/7 if left on), cold temperatures reducing battery capacity, or appliances drawing more power than their label suggests as they age.
Can I run a 1,500W heater on a 1,000Wh battery?
Technically yes, but only for about 30 to 40 minutes. High-draw appliances like space heaters deplete small batteries incredibly fast and can trigger the battery's internal safety shutoff if the discharge rate is too high.
Should I use starting watts or running watts for this calculation?
Always use running watts to calculate runtime. Starting watts (surge) only last for 1 to 3 seconds when a motor turns on. However, your inverter must be sized large enough to handle that brief starting surge.
How do I calculate runtime for a 12V DC appliance?
If you are plugging a 12V device directly into a 12V battery (like a 12V car fridge), you can skip the inverter efficiency step. The formula becomes simply: (Nominal Wh × DoD) ÷ Appliance Watts.
Sources
- U.S. Department of Energy - Battery Storage for Homes
- National Renewable Energy Laboratory (NREL) - Battery Lifespan and Degradation
CTA
Want runtime estimates that match your real appliances without doing the math by hand? Use the WattSizing Calculator to model battery capacity, inverter losses, and your specific daily load profile in one easy workflow.
