Lithium Battery Runtime Explained How to Calculate How Long Your Battery Will Last

Watt hours measure the total energy stored in a battery. The relationship is Energy Wh equals Voltage V multiplied by Amp Hours Ah

For example a 48V lithium battery for solar storage rated at 100Ah stores 48 multiplied by 100 equals 4800Wh or 4.8kWh

A 12V 100Ah battery stores only 12 multiplied by 100 equals 1200Wh or 1.2kWh

Despite having the same 100Ah rating the 48V system stores four times more energy. This is why watt hours rather than amp hours should be used when comparing batteries of different voltages.

The basic runtime calculation is straightforward Runtime hours equals Battery Energy Wh divided by Load Power W

For example a 4800Wh battery powering a 400W load would theoretically last 4800 divided by 400 equals 12 hours

However this is the idealized theoretical runtime. Real world runtime is shorter because of depth of discharge limits and system efficiency losses. The practical formula is Runtime hours equals Battery Capacity Wh multiplied by DoD multiplied by System Efficiency divided by Load Power W

Where DoD is depth of discharge expressed as a decimal and System Efficiency accounts for battery inverter and wiring losses.

The first step is to determine how much power the load draws. List every device that will run from the battery and record its wattage. Common loads include：
LED lighting 10 to 50W
Laptop and phone chargers 50 to 100W
Refrigerator 100 to 200W average
Television 50 to 150W
Router and modem 10 to 30W
Small water pump 100 to 400W
Microwave 1000 to 1500W intermittent
Air conditioner 500 to 1500W

Add the wattages of all devices that will run simultaneously. This total is the continuous load power. If you need to estimate energy consumed over several hours rather than continuous load multiply each appliance wattage by its daily operating hours to get watt hours per day.

For mixed AC and DC loads remember that AC loads pass through an inverter which introduces efficiency losses.

Depth of discharge defines how much of the battery stored energy is safe to use. A LiFePO4 battery pack typically allows an 80 percent DoD meaning 80 percent of nameplate capacity is usable. The remaining 20 percent is reserved to protect cycle life.

System efficiency typically ranges from 85 percent to 95 percent depending on inverter quality battery age and wiring length. A reasonable planning assumption is 90 percent for a well designed system.

Using the practical formula a 4800Wh battery at 80 percent DoD and 90 percent efficiency powering a 400W load delivers 4800 multiplied by 0.80 multiplied by 0.90 divided by 400 equals 8.64 hours

This is a more realistic estimate than the theoretical 12 hour figure.

A 48V 100Ah battery stores 48 multiplied by 100 equals 4800Wh

Usable energy at 80 percent DoD and 90 percent efficiency 4800 multiplied by 0.80 multiplied by 0.90 equals 3456Wh

Powering a continuous 400W critical load 3456 divided by 400 equals 8.64 hours

Powering a 1000W load 3456 divided by 1000 equals 3.46 hours

For multi day autonomy multiply the daily energy requirement by the number of backup days. If critical loads consume 1500Wh per day and two days of backup are needed the required usable energy is 3000Wh. Dividing by the same DoD and efficiency factors gives a required nameplate capacity of approximately 3000 divided by 0.80 divided by 0.90 equals 4167Wh or about 4.2kWh

Runtime estimates vary between battery chemistries because usable capacity and efficiency differ significantly.

Lead acid batteries typically allow only a 50 percent DoD. Using a 12V 100Ah lead acid battery at 50 percent DoD and 85 percent efficiency yields 1200 multiplied by 0.50 multiplied by 0.85 equals 510Wh usable energy

Compared to the same size LiFePO4 battery at 864Wh usable the lead acid system delivers roughly 59 percent of the runtime. This is why comparing nameplate amp hours without considering DoD is misleading.

Lithium batteries also maintain a stable voltage throughout discharge while lead acid voltage drops steadily. A lead acid battery may trigger low voltage cutoff on sensitive electronics before its theoretical runtime expires.

Cold temperatures reduce lithium battery capacity. At 0C a LiFePO4 battery may deliver only 60 to 70 percent of its rated capacity. At high temperatures above 40C short term capacity may improve but long term degradation accelerates. Installing the battery within the manufacturer specified temperature range preserves rated runtime.

1. How long will a 100Ah lithium battery run a fridge
A typical 100Ah 12V LiFePO4 battery stores about 1200Wh. At 80 percent DoD and 90 percent efficiency a 150W refrigerator consuming roughly 50W on average will run approximately 17 hours. If the fridge draws 150W continuously during the compressor cycle runtime shortens to about 5.7 hours.

2. What is the difference between amp hours and watt hours
Amp hours measure current capacity at a given voltage while watt hours measure total energy. To convert amp hours to watt hours multiply by the battery voltage. A 12V 100Ah battery stores 1200Wh while a 48V 100Ah battery stores 4800Wh despite sharing the same amp hour rating.

3. How does depth of discharge affect battery runtime
Depth of discharge determines what percentage of the battery capacity is usable. A lead acid battery limited to 50 percent DoD delivers half its nameplate energy. A LiFePO4 battery at 80 percent DoD delivers 80 percent. Always use usable watt hours not nameplate capacity when comparing runtime.

4. Why is my battery runtime shorter than the calculator estimate
Common reasons include higher than expected load power cold temperatures which reduce capacity inverter efficiency losses undersized wiring voltage drops and battery aging. Recheck the actual wattage of connected loads and verify that the battery is operating within its recommended temperature range.

5. Can I connect multiple batteries to increase runtime
Yes. Connecting batteries in parallel increases amp hours and total watt hours while keeping voltage constant. For example two 12V 100Ah batteries in parallel create a 12V 200Ah system doubling runtime. Ensure all batteries are same voltage same chemistry and similar age before paralleling and follow the manufacturer maximum parallel string count.