How Long Will a 100Ah Battery Last? LiFePO4 vs. Lead-Acid Sizing Manual

CALIBRATED: 2026.06.08 EST. 5 MIN READ
SYSTEM SNAPSHOT // CORE ANALYSIS
A 100Ah battery's true runtime is determined strictly by its chemistry and conversion efficiency. A 12V 100Ah LiFePO4 battery delivers a full 1200Wh of usable energy (100% DoD), whereas a Lead-Acid equivalent provides only 600Wh (50% DoD) to prevent severe chemical cell degradation.
NOMINAL VOLTAGE 12V DC (Nominal Specification)
BUDGET ESTIMATE $250 - $500 (Base Cell Blocks)

Designing an off-grid solar setup, preparing a home backup array, or upgrading an RV power system eventually leads to one critical question: how long will a 100ah battery last? A 100Ah rating is the industry standard for standalone storage blocks, but calculating its true field performance is rarely a matter of simple division. To build a fire-safe and reliable system, you must look past nominal marketing specs and account for battery chemistry limits, inverter overhead, and continuous load characteristics.

>_ TERMINAL PROMPT: Trying to calculate runtime for your specific appliances? Stop running manual linear physics. Input your real-world load profile directly into our official Smart-Storage Sizing Engine to project continuous runtime hours instantly.

The 100Ah Myth: Nominal vs. Usable Energy Storage

On paper, calculating the total energy capacity of a standard 12V 100Ah battery seems straightforward. By multiplying nominal voltage by charge capacity, you derive the total watt-hours of energy contained within the chemical cells:

\[ \text{Nominal Energy (Wh)} = 12\text{V} \times 100\text{Ah} = 1200\text{Wh} \]

This \(1200\text{Wh}\) (or \(1.2\text{kWh}\)) represents the theoretical absolute capacity. In a perfect, lossless environment, a continuous \(100\text{W}\) appliance would draw power for exactly 12 hours. However, in real-world DIY builds, extracting the full nominal capacity is governed strictly by the battery’s internal chemistry. Discharging a battery completely can destroy its molecular architecture, meaning your actual usable energy depends directly on the chemistry’s maximum safe Depth of Discharge (DoD).

LiFePO4 vs. Lead-Acid: Depth of Discharge (DoD) Physics

Battery chemistry choice alters your battery backup time calculator equations. The two dominant technologies in the off-grid sector exhibit fundamentally opposing structural tolerances:

Lithium Iron Phosphate (LiFePO4): Modern LFP cells feature robust crystalline structures capable of sustaining deep cyclical depletion. Under NEC Article 706 safety guidelines, premium LFP packs can be safely dispatched to a 100% Depth of Discharge (\(DoD = 1.0\)) without accelerating capacity fade. This means a 100Ah lithium battery delivers the full usable \(1200\text{Wh}\) of energy for thousands of duty cycles.

Lead-Acid (AGM, Gel, Flooded): Traditional lead-acid batteries suffer from severe chemical degradation if drained too deeply. Discharging past a 50% Depth of Discharge (\(DoD = 0.5\)) triggers rapid plate sulfation and active material shedding, shortening the cell’s lifespan from several years to just a few months. Therefore, a 100Ah lead-acid battery provides only half its rated capacity—exactly \(600\text{Wh}\) of usable energy.

Continuous Load (Watts) LiFePO4 Runtime (100% DoD) Lead-Acid Runtime (50% DoD) System Delivery Status
50 W 24.00 Hours 12.00 Hours Stable Flow
100 W 12.00 Hours 6.00 Hours Optimal Range
200 W 6.00 Hours 3.00 Hours Nominal Load
300 W 4.00 Hours 2.00 Hours Elevated Draw
500 W 2.40 Hours 1.20 Hours Heavy Demand
1000 W 1.20 Hours 0.60 Hours Peak Stress
>_ TERMINAL PROMPT: Trying to calculate runtime for your specific appliances? Stop running manual linear physics. Input your real-world load profile directly into our official Smart-Storage Sizing Engine to project continuous runtime hours instantly.

Accounting for Inverter Overhead & Conversion Losses

Another major factor that skew basic linear depletion models is inverter efficiency. When running standard household AC appliances from a DC battery bank, the current must pass through an off-grid inverter. This conversion process is never perfectly efficient.

Industrial off-grid inverters typically operate with a conversion efficiency coefficient of \(\eta = 0.85\) (85% efficiency). This internal conversion loss introduces an additional power penalty on the storage array. The physical formula to calculate the actual power drawn from the battery bank is:

\[ \text{Actual DC Power Draw (W)} = \frac{\text{AC Appliance Load (W)}}{\eta} \]

For example, running a continuous AC appliance labeled at 85W does not pull 85W from your cells. Accounting for the 15% thermal overhead loss (\(\frac{85\text{W}}{0.85}\)), it draws exactly 100W of continuous DC power from the array. This conversion loss must be factored into every 200ah battery runtime and 100Ah sizing model to avoid unexpected low-voltage cutouts.

Peukert’s Law: The Lead-Acid High-Current Penalty

When analyzing how long a battery bank will hold up, lead-acid systems face an added mathematical disadvantage known as Peukert’s Law. This chemical law proves that the faster a lead-acid battery is discharged, its available capacity drops exponentially.

While a 100Ah lithium battery delivers 100Ah of charge whether you draw 5 Amps or 50 Amps, a lead-acid battery rated for 100Ah is measured at a slow 20-hour discharge rate (5 Amps). If you connect a heavy load like a microwave pulling 50 Amps, internal resistance causes rapid heat buildup, and the effective capacity of the lead-acid battery instantly shrinks to around 60Ah, severely clipping your estimated backup margins.

Step-by-Step Battery Sizing Checklist

  • Sum your total continuous DC loads and separate them from your AC appliance profiles.
  • Divide all AC wattage requirements by 0.85 to account for standard inverter conversion loss.
  • Apply the appropriate LiFePO4 depth of discharge factor (1.0) or Lead-Acid limit (0.5) to determine true usable watt-hours.
  • Cross-verify cable runs with a voltage drop calculation to prevent energy loss through thin wiring.