Battery & Storage•UPS & Battery Storage•Updated September 2026

Battery Capacity & Sizing Calculator

Calculate battery storage capacity in Watt-hours (Wh), Kilowatt-hours (kWh), and Amp-hours (Ah). Evaluate usable battery bank capacity across LiFePO4 and Lead-Acid chemistries, or size battery capacity and unit count for your specific electrical load and runtime.

Input Parameters

Calculator Mode

Battery Chemistry & Type

Modern deep-cycle standard with flat discharge voltage. Safely delivers 80% to 90% usable capacity without significant cycle-life degradation.

Nominal Battery Voltage *Volts (V)

Single unit nominal voltage (typically 12V, 24V, or 48V).

Quick Select:

Rated Battery Capacity *Amp-hours (Ah)

Manufacturer nameplate capacity rating.

Unit:

Battery Count (Units in Bank) *units

Set to 1 for a standalone battery, or 2+ for a connected bank.

Single battery configuration (Standalone unit).

Estimated Usable Energy

1,020 Wh

1.02 kWh usable at 85% Depth of Discharge (12V Bank)

Total Nominal Energy

1,200 Wh

(1.20 kWh)

Bank Capacity (Ah)

100 Ah

@ 12V DC

Depth of Discharge

85%

LiFePO4

Typical Cycle Life

3,000–5,000+ cycles

Expected longevity

Active Formula Breakdown

Nominal Energy = 12V × 100Ah = 1,200 Wh (1.20 kWh) | Estimated Usable Energy (85% DoD) = 1,200 Wh × 0.85 = 1,020 Wh (1.02 kWh)

Need to calculate exact backup runtime for an existing battery?

If you already own a specific battery bank and want to determine runtime hours across varying household loads, use our dedicated UPS & Battery Backup Run-Time Hours Calculator.

Educational diagram comparing nominal battery capacity in Watt-hours versus usable energy across LiFePO4 lithium at 85% depth of discharge and lead-acid AGM at 50% depth of discharge.

Nominal vs. Usable Capacity: While both batteries may carry an identical 12V 100Ah (1,200 Wh) nameplate rating, chemistry depth-of-discharge (DoD) constraints dictate real usable energy. A LiFePO4 battery safely delivers ~85% (1,020 Wh), whereas lead-acid AGM should not exceed 50% discharge (600 Wh) to protect plate life.

Understanding Nominal vs. Usable Battery Capacity

When evaluating deep-cycle batteries for off-grid solar, RV boondocking, or home emergency backup, the single most common sizing mistake is assuming that 100% of a battery's nameplate capacity can be drawn into appliances. A battery labeled "100 Amp-hours at 12 Volts" stores 1,200 Watt-hours (Wh) of raw chemical energy, but the amount of electrical energy you can extract safely depends on its internal chemistry.

LiFePO4 Lithium

80% – 90%

Safely discharges 80% to 90% of rated capacity daily. Delivers 3,000 to 5,000+ full charge-discharge cycles without rapid degradation.

Lead-Acid (AGM / Gel)

50% Max

Industry standard guidelines recommend a maximum 50% depth of discharge. Draining lead-acid below 50% accelerates lead sulfate crystallization and destroys plate life.

Lithium-Ion (NMC)

80% Typical

Used in consumer portable power stations and electronics. High energy density with an optimal operating window of 10% to 90% state of charge.

Because of this chemistry differential, a homeowner replacing a 200Ah lead-acid bank (100Ah usable) only requires approximately 120Ah of LiFePO4 capacity to achieve identical usable energy, while saving roughly 70% in physical battery weight.

Mathematical Formulas & Calculation Methodology

Battery capacity calculations rely on fundamental electrical physics. Below are the deterministic equations used by our engine to calculate energy storage, conversion efficiency, and load sizing.

Formula

E_nom = V × Ah | E_usable = E_nom × DoD | Ah_req = (P × t) ÷ (V × η_inv × DoD)

Variables & Constants

Nominal Energy CapacityE_nom (Wh / kWh)

Total stored theoretical electrical energy in the battery or battery bank.

Bank VoltageV_bank (Volts (V))

Nominal DC operating voltage of the battery or series-connected bank.

Bank CapacityAh_bank (Amp-hours (Ah))

Nominal charge storage capacity at the rated discharge rate.

Depth of DischargeDoD (% (decimal))

Fraction of battery capacity safely discharged to maintain cycle life (e.g. 0.85 for LiFePO4, 0.50 for Lead-Acid).

Inverter Efficiencyη_inv (% (decimal))

DC-to-AC conversion efficiency of the inverter (typically 0.85 to 0.92 for quality pure sine wave inverters).

Load PowerP_load (Watts (W))

Continuous power consumed by connected electrical appliances.

Watt-hours (Wh) is calculated by multiplying circuit Voltage (V) by current capacity over time in Amp-hours (Ah).
To convert milliamp-hours (mAh) to Amp-hours (Ah), divide by 1,000 (e.g. 20,000 mAh = 20 Ah).
Inverter efficiency (η_inv) must be included when sizing DC battery storage for AC 120V household appliances; direct 12V DC loads bypass this conversion step.

Worked Example: Evaluating a 12V 100Ah Battery

Example Scenario: A homeowner is comparing a 12V 100Ah LiFePO4 battery against a 12V 100Ah AGM lead-acid battery, and plans to connect four units into a 48V bank.

Calculate Nominal Stored Energy

E_nom = 12V × 100Ah = 1,200 Wh (1.20 kWh)

Multiply the nominal 12V rating by the 100Ah capacity. This represents the total electrical energy stored inside the battery cells.

Apply Chemistry Depth of Discharge (LiFePO4 vs. Lead-Acid)

LiFePO4: 1,200 Wh × 0.85 = 1,020 Wh | Lead-Acid: 1,200 Wh × 0.50 = 600 Wh

Because deep-cycle lead-acid degrades rapidly when discharged beyond 50%, only 600 Wh is usable. A LiFePO4 battery safely delivers 1,020 Wh, yielding 70% more usable energy from identical 100Ah nominal ratings.

Evaluate Multi-Battery Bank Wiring (Example: 4 Batteries)

Series: 4 × 12V = 48V @ 100Ah (4,800 Wh) | Parallel: 12V @ 4 × 100Ah = 400Ah (4,800 Wh)

Series wiring multiplies voltage for higher-power inverters, while parallel wiring multiplies Amp-hours. Both configurations produce 4,800 Wh nominal and 4,080 Wh usable energy.

Result: From identical nominal 12V 100Ah ratings, LiFePO4 provides 1,020 Wh of usable energy versus 600 Wh for lead-acid. Wiring four batteries in series creates a 48V 100Ah bank delivering 4,080 Wh usable energy.

Battery Bank Wiring: Series vs. Parallel Architecture

Multiple batteries can be interconnected to create higher voltage or greater Amp-hour capacity. However, how you wire them radically changes electrical circuit parameters:

Wiring Type	How to Connect	Bank Voltage	Bank Capacity (Ah)	Total Stored Wh
Series	Positive (+) to Negative (-)	Adds Up (V × N)	Unchanged (1 × Ah)	Multiplies (Wh × N)
Parallel	Positive (+) to (+), Neg (-) to (-)	Unchanged (1 × V)	Adds Up (Ah × N)	Multiplies (Wh × N)
Series-Parallel	Pairs in series, then paralleled	Increases (e.g. 24V)	Increases (e.g. 200Ah)	Multiplies (Wh × N)

Critical Multi-Battery Safety Rule

Never connect batteries of differing chemistries, differing voltages, unequal capacities, or differing ages in series or parallel. A degraded or lower-capacity battery will cause premature cell reversal, overcharging, severe balance drift, and thermal runaway risks. Always use identical batteries from the same production batch and verify individual voltages match within 0.05V before connecting them into a bank.

Real-World Variables & Environmental Deratings

Theoretical nameplate capacity ratings are measured under laboratory test conditions (typically 77°F / 25°C at a slow C/20 discharge rate). Real-world installations must account for the following environmental and operational factors:

LiFePO4 Usable Depth of Discharge

Default: 85%

Typical Range: 80% to 90%

Practical Engineering Impact: Premium lithium cells can be discharged to 90% for occasional outages, but an 80% to 85% limit extends cycle life to 4,000+ full cycles.

Lead-Acid Usable Depth of Discharge

Default: 50%

Typical Range: 40% to 50%

Practical Engineering Impact: Discharging lead-acid batteries beyond 50% causes irreversible plate sulfation and reduces typical cycle life from 500 cycles down to under 200.

Inverter DC-to-AC Efficiency

Default: 85%

Typical Range: 80% to 93%

Practical Engineering Impact: Quality pure sine wave inverters achieve 88% to 92% at optimal load, but light loads and standby idle power lower real-world average efficiency to ~85%.

Operating Ambient Temperature

Default: 77°F (25°C)

Typical Range: 32°F to 104°F (0°C to 40°C)

Practical Engineering Impact: Cold ambient temperatures below 32°F temporarily reduce deliverable lead-acid capacity by 20% to 40% and can trigger BMS low-temp charging lockouts on lithium.

Discharge Rate (Peukert Effect)

Default: C/10 to C/20 Rate

Typical Range: C/2 to C/100

Practical Engineering Impact: Heavier discharge rates (> C/5) significantly diminish delivered lead-acid capacity. LiFePO4 cells maintain over 95% rated capacity even at 0.5C to 1C rates.

Parameter	Model Default	Typical Field Range	Practical Engineering Impact
LiFePO4 Usable Depth of Discharge	85%	80% to 90%	Premium lithium cells can be discharged to 90% for occasional outages, but an 80% to 85% limit extends cycle life to 4,000+ full cycles.
Lead-Acid Usable Depth of Discharge	50%	40% to 50%	Discharging lead-acid batteries beyond 50% causes irreversible plate sulfation and reduces typical cycle life from 500 cycles down to under 200.
Inverter DC-to-AC Efficiency	85%	80% to 93%	Quality pure sine wave inverters achieve 88% to 92% at optimal load, but light loads and standby idle power lower real-world average efficiency to ~85%.
Operating Ambient Temperature	77°F (25°C)	32°F to 104°F (0°C to 40°C)	Cold ambient temperatures below 32°F temporarily reduce deliverable lead-acid capacity by 20% to 40% and can trigger BMS low-temp charging lockouts on lithium.
Discharge Rate (Peukert Effect)	C/10 to C/20 Rate	C/2 to C/100	Heavier discharge rates (> C/5) significantly diminish delivered lead-acid capacity. LiFePO4 cells maintain over 95% rated capacity even at 0.5C to 1C rates.

National Electrical Code (NEC) & Storage Safety Disclaimers

National Electrical Code (NEC Article 480 / NFPA 70): Stationary battery storage installations must comply with local building and electrical codes, including proper conductor ampacity sizing, overcurrent protection devices (fuses/breakers), and physical disconnects.
DC Overcurrent Protection: High-capacity battery banks can deliver thousands of short-circuit Amperes. Every battery bank must include a properly rated Class T or ANL fuse mounted as close as practical to the positive terminal.
Flooded Lead-Acid Hydrogen Outgassing: Unsealed flooded lead-acid batteries emit flammable hydrogen gas during charging and require dedicated exterior mechanical or natural ventilation to prevent explosive atmospheres.
Lithium BMS Cutoff: Quality LiFePO4 batteries feature an internal Battery Management System (BMS) that disconnects the battery under low-voltage, over-current, or freezing charging conditions. System design must prevent sudden load dropouts on critical circuits.
Professional Engineering Advice: Calculations provided on CalcMyPower.com are for preliminary planning and educational purposes. Always consult a licensed master electrician or certified solar installer before purchasing or energizing stationary battery banks.

Frequently Asked Questions About Battery Capacity & Sizing

How do you calculate battery capacity in Watt-hours (Wh)?

To calculate battery energy capacity in Watt-hours (Wh), multiply the nominal battery voltage in Volts (V) by the rated capacity in Amp-hours (Ah): Energy (Wh) = Voltage (V) × Capacity (Ah). For example, a standard 12V 100Ah battery stores 1,200 Watt-hours of nominal energy (12V × 100Ah = 1,200Wh or 1.2 kWh).

What is the difference between Amp-hours (Ah) and Watt-hours (Wh)?

Amp-hours (Ah) measures electrical charge volume (current over time), whereas Watt-hours (Wh) measures total energy capacity (work performed). Ah is voltage-dependent; a 100Ah battery at 12V stores 1,200Wh, while a 100Ah battery at 48V stores 4,800Wh (four times as much energy). Comparing batteries by Watt-hours (Wh) provides an accurate comparison across different system voltages.

Why is usable battery capacity less than nominal capacity?

Every battery chemistry has a safe depth of discharge (DoD) limit to prevent permanent cell degradation. Traditional lead-acid batteries (AGM, Gel, Flooded) should not be discharged beyond 50% DoD to prevent rapid plate sulfation and cycle-life loss. Modern Lithium Iron Phosphate (LiFePO4) batteries safely deliver 80% to 90% of their nominal capacity. Therefore, a 100Ah lead-acid battery provides approximately 50Ah (600Wh usable), whereas a 100Ah LiFePO4 battery provides 85Ah (1,020Wh usable).

Does wiring batteries in series or parallel increase battery capacity?

Wiring batteries in parallel increases total Amp-hour capacity while keeping system voltage identical. For example, two 12V 100Ah batteries wired in parallel create a 12V 200Ah bank (2,400Wh). Wiring batteries in series increases system voltage while keeping Amp-hour capacity identical. Two 12V 100Ah batteries in series create a 24V 100Ah bank (2,400Wh). In both configurations, total stored energy in Watt-hours is identical.

How do you size a battery bank for a home power outage or off-grid solar?

To size a battery bank, first calculate daily or hourly energy consumption in Watt-hours: Load (Watts) × Hours of Runtime. If using an AC inverter, divide load energy by inverter efficiency (typically 85% or 0.85). Next, divide by your battery chemistry's recommended depth of discharge (0.85 for LiFePO4, 0.50 for Lead-Acid) to determine required nominal Watt-hours. Finally, divide nominal Watt-hours by your DC system voltage (12V, 24V, or 48V) to determine required battery Amp-hours.

What is Peukert's Law and how does it affect battery runtime?

Peukert's Law describes how the usable capacity of a lead-acid battery decreases as the rate of discharge increases. Lead-acid batteries are typically rated at a slow 20-hour discharge rate (C/20). If discharged rapidly in 1 to 2 hours (such as running a high-wattage microwave or space heater through an inverter), internal resistance causes heat and chemical bottlenecks, reducing actual delivered capacity by 20% to 40%. LiFePO4 lithium batteries experience negligible Peukert losses and maintain rated capacity under heavy loads.

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