Q: How long will a 100Ah battery run an appliance?

On a 12V system with LiFePO4 cells at 85% DoD and a 90% efficient inverter, a continuous 100W AC load will operate for roughly 9.1 hours.

Q: How do I calculate battery runtime from Ah and volts?

Multiply nominal voltage by Amp-hours to calculate total Watt-hours (Wh = V x Ah). Multiply by Depth of Discharge, then divide by load adjusted for inverter efficiency.

Q: How do I convert battery Wh to hours?

Divide usable Watt-hours by your active load in Watts. For example, 600 usable Wh divided by a 75W load yields 8 hours of runtime.

Q: Does an inverter reduce battery backup time?

Yes, inverters typically lose 8% to 15% of energy to heat during DC-to-AC conversion and draw quiescent idle power.

Q: What depth of discharge should I use for LiFePO4 batteries?

Recommended Depth of Discharge for Lithium Iron Phosphate (LiFePO4) is typically 80% to 90% to maintain high cycle life.

1. Battery Bank Configuration

Nominal Voltage (V)

Custom Voltage (V) Must be greater than 0

Rated Capacity (Ah) Enter a capacity > 0

Battery Chemistry (Preset)

Depth of Discharge - DoD (%) Usable limit specified by manufacturer. Must be between 1% and 100%

Condition / Temp Adjustment (%) e.g. 80% for winter cold or older cells. Must be 10% to 120%

Safety Reserve Margin (%) Emergency backup buffer kept untouched. Must be between 0% and 50%

2. Power Conversion & Inverter Efficiency

Inverter Efficiency for AC Loads (%) Applies exclusively to AC appliances (85-95%). Must be between 50% and 99%

DC-DC Converter Efficiency (%) Set to 100% if wired direct to battery terminals. Must be between 50% and 100%

Inverter Continuous Rating (W) (Optional)

Inverter Surge Rating (W) (Optional)

3. Connected Electrical Loads

Specify appliance wattages, AC or DC power supply, and duty cycle (percentage of time actively drawing power).

4. Optional Solar Recharging Input Click to expand

Solar Array Peak (Watts)

Peak Sun Hours (Hours/Day)

Charge Controller Eff. (%)

Computed Estimate

Estimated Battery Runtime

Enter system details to calculate runtime

Nominal Energy 0 Wh Rated capacity

Usable Battery Energy 0 Wh After DoD & Reserve

Appliance Load 0 W Active terminal draw

Battery-Side Demand 0 W Includes conversion loss

Load Breakdown AC: 0W | DC: 0W Appliance power split

Daily Energy Consumption 0 Wh/day Across 24-hour cycle

Battery Energy Allocation Usable vs Reserve

Usable

Res.

DoD Cap

How to Use This Solar Battery Runtime Calculator

Set Up Your Battery Bank: Choose your system voltage (12V, 24V, 48V, or custom) and enter capacity in Amp-hours (Ah). Select chemistry to set a realistic Depth of Discharge (DoD).
Enter Inverter Efficiency: If you are powering standard household AC electronics, set your inverter efficiency (typically 85% to 92%). Direct DC electronics bypass this loss entirely.
Add Electrical Appliances: Select preset devices (like laptops, fans, or refrigerators) or add custom items. Specify active running wattage and duty cycles.
Optional Solar Recharge: Input your solar panel array size and expected sunlight hours to see whether your solar panels can offset daily discharge.
Review Sizing Insights: View total runtime in hours/minutes, battery-side watt demand, and export or copy your final calculation.

How the Calculation Works (Mathematical Formulas)

This calculator avoids the common mistake of dividing nominal watt-hours by appliance wattage without accounting for separate conversion paths. It calculates usable energy and battery demand in distinct thermodynamic stages:

Stage 1: Usable Battery Energy

Usable Wh = [Voltage (V) × Capacity (Ah)] × (DoD % / 100) × (Health % / 100) × (1 - Reserve % / 100)

Stage 2: Power Demand at the Battery Terminals

AC Battery Demand (W) = ∑ [Appliance Watts × Qty × (Duty % / 100)] / Inverter Efficiency

DC Battery Demand (W) = ∑ [Appliance Watts × Qty × (Duty % / 100)] / DC-DC Efficiency

Total Battery Demand (W) = AC Battery Demand (W) + DC Battery Demand (W)

Stage 3: True Backup Runtime

Estimated Runtime (Hours) = Usable Energy (Wh) / Total Battery Demand (W)

Common Off-Grid & Backup Battery Use Cases

Camper Vans & RVs

Commonly powered by a 12V 100Ah LiFePO4 battery running a 45W 12V fridge (35% duty cycle), LED lighting, and phone charging for multiple days off-grid.

Work-From-Home Resiliency

Keeps a Wi-Fi router (15W–25W) and a laptop (45W–65W) operational for 12 to 24+ hours during prolonged municipal power outages.

Medical CPAP Emergency Backup

A standard CPAP running on 12V DC draws only 25W–35W without humidification, delivering over 25 hours of backup from a single 100Ah lithium battery.

Worked Calculation Example

Example System: 12V, 100Ah LiFePO4 battery at 90% DoD, 10% reserve, and 90% inverter efficiency powering a 60W laptop (AC) and a 20W LED strip (DC).

Nominal Energy = 12V × 100Ah = 1200 Wh
Usable Wh = 1200 Wh × 0.90 × (1 - 0.10) = 972 Wh
AC Battery Demand = 60W / 0.90 = 66.67 W
DC Battery Demand = 20W / 1.00 = 20.00 W
Total Battery Demand = 66.67 W + 20.00 W = 86.67 W
Estimated Runtime = 972 Wh / 86.67 W = 11.21 Hours (~11 hours, 13 minutes)

Frequently Asked Questions

1. How long will a 100Ah battery run an appliance?

Runtime depends on voltage and appliance draw. On a 12V system, a 100Ah battery holds 1,200 watt-hours nominal. With a LiFePO4 battery at 85% depth of discharge through a 90% efficient inverter, a continuous 100W load runs for approximately 9.1 hours.

2. How do I calculate battery runtime from Ah and volts?

Multiply volts by amp-hours (V × Ah = Wh) to get nominal Watt-hours. Multiply by usable Depth of Discharge (e.g. 0.85 for lithium), then divide by your device's running wattage adjusted for inverter loss.

3. How do I convert battery Wh to hours?

Divide the total usable Watt-hours by your load wattage. For example, 600 usable Wh divided by a 75W load gives 8 hours of runtime.

4. Does an inverter reduce battery backup time?

Yes. Power inverters lose roughly 8% to 15% of energy as heat during DC-to-AC conversion and also consume 5W to 25W of continuous idle standby power.

5. What depth of discharge should I use for LiFePO4 batteries?

Most manufacturers recommend 80% to 90% Depth of Discharge (DoD) to maximize cell lifespan across 3,000 to 5,000 cycles.

6. Can I run multiple appliances at the same time?

Yes. This calculator adds the average wattages of all running appliances, provided their combined peak draw does not exceed your inverter or battery BMS limits.

7. Can a solar panel extend battery runtime?

Yes. Daytime solar replenishment directly offsets ongoing power consumption, maintaining battery levels and significantly extending backup duration.

8. Why is real-world runtime different from the calculator estimate?

Real-world conditions vary due to temperature drops, cell aging, wire resistance, inverter standby consumption, and variable compressor cycling.

9. How do I estimate the runtime of a CPAP machine or mini-fridge?

Apply a duty cycle factor. A 100W mini-fridge running 35% of the time draws an average of 35W. CPAP machines draw much less power if heated humidification is switched off.

10. What happens if the appliance's starting surge is too high?

If an appliance motor's starting spike exceeds your inverter's surge threshold or trips your battery's BMS overcurrent protection, the system will instantly shut down.