Smart-Storage Sizing Engine: Battery Runtime & Capacity Calculator
Battery Storage & Sizing Calculations
Sizing energy storage banks for off-grid solar systems and calculating standby capacity for backup arrays requires balancing chemistry specifications, voltage options, and inverter overhead. Whether you are **calculating battery backup times**, selecting battery layouts, or analyzing **voltage drop metrics** across **off-grid DC circuits**, establishing exact load characteristics is key.
Computes run duration based on capacity, system voltage, depth of discharge, and AC/DC loads.
Computes required Amp-hours (Ah) to sustain load demands for a targeted duration.
When sizing AC loads running off battery banks, inverter conversion efficiency must be accounted for. Standard off-grid inverters typically operate at a conversion efficiency of \(\eta \approx 85\%\) (a coefficient of 0.85). This conversion overhead increases the power load on the battery pack: running an AC load of 85W actually draws exactly: \(\frac{85\text{W}}{0.85} = 100\text{W}\) of continuous DC power from the storage array.
Battery Sizing Variables Glossary
V) (e.g. 12V, 24V, 48V).Ah).W) and adjusted for inverter loss.W).NEC & Battery Chemistry Sizing Rules
Selecting battery chemistries involves evaluating safety factors, thermal constraints, and cycle life limits. Operating storage blocks safely requires obeying regulatory continuous guidelines.
Lithium Iron Phosphate (LiFePO4) Sizing Advantages
Modern LFP chemistry offers significant installation advantages under NEC Article 706. Unlike lead-acid batteries, LiFePO4 chemistry maintains a flat voltage curve and can be safely discharged to 100% depth of discharge without compromising cell integrity. A standard 12V 100Ah LFP battery yields a full 1.2 kWh of usable energy capacity, with standard lifetimes ranging from 3,000 to 5,000 cycles.
Lead-Acid Chemistry 50% Limit Rule
Sealed Lead-Acid (SLA) or flooded lead-acid batteries degrade rapidly if deeply discharged. Discharging past 50% Depth of Discharge causes irreversible plate sulfation and cell capacity loss, reducing cycles to fewer than 300. Sizing a lead-acid bank to handle a 1.2 kWh load requires using a battery rated for at least 2.4 kWh (200Ah at 12V) to keep the depth of discharge at 50% or less.
Frequently Asked Questions
How long will a 100Ah battery last?
For a 12V 100Ah battery (providing 1200Wh of nominal energy) running a continuous 100W load, the runtime depends on chemistry. A LiFePO4 battery supports 100% depth of discharge (DoD) and will last exactly 12 hours (\(\frac{12\text{V} \times 100\text{Ah} \times 1.0}{100\text{W}} = 12\text{h}\)). A Lead-Acid battery is limited to 50% DoD to prevent degradation, yielding exactly 6 hours of runtime (\(\frac{12\text{V} \times 100\text{Ah} \times 0.5}{100\text{W}} = 6\text{h}\)).
Why does battery type change the runtime calculation?
Lead-acid battery chemistry degrades quickly if discharged below 50% depth of discharge (DoD), meaning only half of the rated capacity is usable. LiFePO4 chemistry has a stable voltage curve and supports 100% DoD for 3,000+ cycles without cell damage, doubling the available runtime for the same Ah capacity.
What is the conversion efficiency of a standard off-grid inverter?
Standard off-grid solar inverters run at an efficiency rating of approximately 85% (\(\eta = 0.85\)). Because of this overhead conversion loss, you must divide your AC load by 0.85 to find the actual current draw from the battery. For instance, an AC load of 85W pulls a constant 100W of DC power from your batteries.