Battery Storage Calculator

Battery capacity works like a water tank — the bigger the tank (amp-hours) and higher the pressure (voltage), the more energy you can store. But unlike water flowing at a steady rate, electrical devices draw power in bursts. Your laptop might spike to 90W when processing, then drop to 30W when idle, making real-world runtime different from calculated averages.

The calculator multiplies amp-hours by voltage to get watt-hours of storage, then divides by your device power draw to estimate runtime. A 100Ah 12V battery stores 1,200 watt-hours — enough to run a 150W load for 8 hours at perfect efficiency. But real systems lose 10-20% to inverter conversion, wire resistance, and battery chemistry limits.

Battery storage assumes you can use 100% of stored energy, but that damages most battery types. Lead-acid batteries should only discharge to 50% capacity, lithium batteries to 20%. This 'usable capacity' cuts your runtime in half compared to the nameplate rating, which is why experienced installers size battery banks at twice their calculated needs.

Use this calculator when sizing battery banks for off-grid solar systems, RV electrical setups, or home backup power systems. It helps determine whether your existing batteries can handle a camping trip, or how much storage you need for emergency power during outages.

The calculator works best for steady loads like LED lighting, refrigerators, or electronics. It's less accurate for variable loads like power tools, microwaves, or air conditioners that cycle on and off. For these intermittent loads, estimate average power consumption over time rather than peak wattage.

Avoid using this calculator for automotive starting batteries, which are designed for high-current bursts, not sustained discharge. Deep-cycle batteries used in solar, marine, and RV applications store more energy and handle repeated discharge cycles. The runtime formulas apply only to deep-cycle battery chemistries like AGM, gel, or lithium.

The biggest mistake is ignoring discharge limits — using 100% of battery capacity ruins lead-acid batteries and voids lithium warranties. Lead-acid batteries lose years of lifespan if discharged below 50%, while lithium batteries should stay above 20%. Size your battery bank for twice your calculated runtime to stay in the safe zone.

Another common error is adding device nameplate ratings instead of actual consumption. A 1000W microwave doesn't run continuously at full power — it cycles on and off. Measure actual power draw with a kill-a-watt meter rather than trusting device labels, which often show maximum rather than typical consumption.

Temperature kills battery performance, but most people ignore it. Lead-acid batteries lose 50% capacity at 0°F, while lithium batteries can lose 20% capacity at freezing temperatures. If your battery bank lives in an unheated garage or RV, multiply your calculated capacity by 0.8 to account for cold weather performance degradation.

Battery runtime follows the simple formula: Runtime = (Battery Capacity × Voltage × Efficiency) ÷ Device Power. A 100Ah 12V battery with 85% efficiency powering a 150W load gives: (100 × 12 × 0.85) ÷ 150 = 6.8 hours of runtime.

The key conversion is amp-hours to watt-hours. Amp-hours measure charge storage, but devices consume power in watts. Multiply amp-hours by system voltage to get watt-hours: 100Ah × 12V = 1,200Wh. Then divide by device wattage: 1,200Wh ÷ 150W = 8 hours at perfect efficiency.

Efficiency losses stack multiplicatively, not additively. A 90% efficient inverter feeding a 95% efficient device charger gives 85.5% total efficiency (0.90 × 0.95), not 185%. Wire losses add another 2-5% loss, and cold temperatures can reduce lithium battery capacity by 20% or more. Account for these real-world factors by using conservative efficiency estimates.

Peukert's effect makes high-current draws reduce usable battery capacity below the nameplate rating. A 100Ah lead-acid battery delivers 100Ah at the 20-hour rate (5A draw) but only 80Ah at the 5-hour rate (20A draw). Lithium batteries have minimal Peukert losses, maintaining nearly full capacity even at high discharge rates.