Battery Runtime Calculator

Knowing how long your battery will last is essential for off-grid power planning. Whether you're setting up a van build, preparing for power outages, or sizing a solar system, our battery runtime calculator helps you understand exactly how much power you have available.

Battery runtime depends on several interconnected factors: capacity (Ah), voltage, depth of discharge (DoD), and inverter efficiency. Understanding each of these helps you maximize your battery's potential and avoid unexpected power loss.

LiFePO4 batteries can safely discharge to 80-90% of their capacity, making them ideal for off-grid use. They also have a flat discharge curve, maintaining consistent voltage until nearly depleted, which means your devices run at full power until the battery is nearly empty.

Lead acid and AGM batteries should only be discharged to 50% to maximize lifespan. Regularly deep-discharging lead acid batteries will significantly reduce their cycle life from 500+ cycles to potentially under 200 cycles.

To calculate how long your battery will last, you need to understand two key conversions. First, convert your battery's amp-hour rating to watt-hours:

Watt-hours (Wh) = Amp-hours (Ah) × Voltage (V)

For example, a 100Ah battery at 12.8V (LiFePO4 nominal voltage) has 1,280Wh of total energy storage. However, you can't use all of this energy—you need to account for depth of discharge.

Usable Wh = Total Wh × Depth of Discharge

With 80% DoD for LiFePO4: 1,280Wh × 0.8 = 1,024Wh usable. Now you can calculate runtime:

Runtime (hours) = Usable Wh × Inverter Efficiency ÷ Load (W)

With a 100W load and 90% inverter efficiency: (1,024 × 0.9) ÷ 100 = 9.2 hours. This battery runtime calculator handles all these calculations automatically, giving you accurate estimates for your specific setup.

Choosing the right battery chemistry significantly impacts your available runtime. Here's a detailed comparison to help you understand which battery type best fits your needs:

LiFePO4 (Lithium Iron Phosphate) offers the best value for off-grid applications despite higher upfront costs. With 80-90% usable capacity, 2,000-5,000 cycle life, and virtually no maintenance, a 100Ah LiFePO4 battery effectively provides the same usable energy as a 200Ah lead acid battery. The flat discharge curve means consistent voltage and device performance throughout the discharge cycle. LiFePO4 batteries also handle partial charging well, with no memory effect.

AGM (Absorbed Glass Mat) batteries are a good middle ground between lead acid and lithium. They're sealed, maintenance-free, and more resistant to vibration—making them popular for RVs and marine applications. However, they share lead acid's 50% depth of discharge limitation and typically last 300-500 cycles when properly maintained.

Flooded Lead Acid batteries are the most affordable option but require regular maintenance (checking water levels, cleaning terminals) and proper ventilation due to hydrogen gas release during charging. They're best suited for stationary applications where weight isn't a concern and maintenance is accessible.

When calculating battery runtime, remember that a 200Ah LiFePO4 battery provides roughly 2,048Wh of usable energy, while a 200Ah lead acid battery only provides about 1,200Wh—despite having the same amp-hour rating.

Let's walk through some practical scenarios to help you understand how long your battery will last in real-world situations:

Weekend camping with 100Ah LiFePO4: Running LED lights (20W) for 5 hours, charging phones and tablets (20W average for 3 hours), and powering a 12V cooler (40W average running 50% of the time = 20W continuous) gives you about 180Wh daily usage. Your 100Ah battery (1,024Wh usable) would last over 5 days without recharging.

Working from a van with 200Ah LiFePO4: A laptop (50W for 8 hours = 400Wh), phone charger (15W for 2 hours = 30Wh), LED lights (20W for 4 hours = 80Wh), and a small 12V fridge (averaging 30W continuous = 720Wh/day) totals about 1,230Wh daily. A 200Ah battery (2,048Wh) would last about 1.5 days, meaning you'd need solar charging or shore power daily.

Emergency backup for essential loads: During a power outage, you might run a small chest freezer (50W average), LED lights (30W for 6 hours), and charge devices (20W for 2 hours). Daily usage would be about 700-800Wh. A 200Ah LiFePO4 battery bank could keep essentials running for 2-3 days.

Getting the most from your battery system requires attention to efficiency at every stage. Here are proven strategies to extend how long your battery will last:

Use 12V DC appliances when possible. Every time power passes through an inverter, you lose 10-15% to inefficiency. A 12V DC refrigerator connected directly to your battery bank is significantly more efficient than an AC fridge through an inverter. The same applies to lights, fans, and device chargers.

Right-size your inverter. Inverters are most efficient at 50-75% of their rated capacity. Running a 3000W inverter to power a 100W load means the inverter operates at only 3% capacity—where efficiency might drop to 75-80%. For typical loads under 500W, a 1000W inverter is more efficient.

Consider duty cycles. Many appliances don't run continuously. A refrigerator might cycle on for 15 minutes every hour (25% duty cycle), so a 100W fridge actually averages only 25W. Understanding this helps you more accurately estimate runtime.

Monitor battery voltage. A battery monitor or smart shunt helps you track actual energy usage and remaining capacity. Knowing your state of charge prevents over-discharge and helps you plan recharging needs.

Maintain optimal temperatures. Batteries perform best between 50-80°F (10-27°C). In cold weather, insulate your battery compartment. In hot weather, ensure adequate ventilation to prevent overheating, which degrades battery life.

Many people end up with inadequate battery capacity or unexpected power issues due to common calculation errors. Here's what to watch for when using a battery runtime calculator:

Forgetting inverter losses. If you calculate runtime based on raw battery capacity without accounting for inverter efficiency, you'll overestimate by 10-15%. Always factor in 85-95% efficiency for AC loads.

Ignoring depth of discharge. A 100Ah lead acid battery doesn't give you 100Ah of usable power. Discharging below 50% dramatically shortens battery life. Always calculate based on usable capacity, not total capacity.

Using peak wattage instead of average. A microwave rated at 1200W might only draw that during cooking. For runtime calculations, use the actual average draw over time. A refrigerator might be rated at 150W but only average 40W due to cycling.

Not accounting for parasitic loads. Inverters draw power even when nothing is connected (idle draw). A typical inverter might consume 10-20W just staying on. Charge controllers, battery monitors, and alarm systems also draw small amounts continuously.

Underestimating future needs. Battery systems are often expanded over time. Size your initial system with room to grow, or at least ensure your charging capacity can support additional batteries later.

Ignoring Peukert's effect on lead acid. Lead acid batteries deliver less capacity at higher discharge rates. A battery rated at 100Ah at the 20-hour rate might only deliver 80Ah at the 5-hour rate. LiFePO4 batteries are largely immune to this effect, which is another reason they're preferred for high-draw applications.