Battery Capacity Calculator
Work out stored energy, required battery size, or runtime — enter voltage, Ah, load and efficiency, and read the result straight off the panel below.
Battery Capacity Formula
Every mode above comes from the same small set of relationships between voltage, current, power, and energy.
kWh = Wh ÷ 1000
Ah = Wh ÷ V
Runtime (h) = Usable Battery Wh ÷ Load (W)
A 12V 100Ah battery stores approximately 12 × 100 = 1,200Wh (1.2kWh) nominal — the label figure. Actual usable energy is lower once depth of discharge and system losses are factored in, which the calculator above does automatically.
How to Calculate Battery Capacity
To size a battery for a load, start with the energy the load needs, then work backward to amp-hours at your chosen system voltage.
- Multiply load power (W) by runtime (h) to get raw energy needed (Wh).
- Divide by inverter efficiency and battery efficiency to account for conversion losses.
- Divide by your permitted depth of discharge, to size the nominal battery rather than just the usable portion.
- Divide by battery voltage to get required Ah.
This separates three numbers that get conflated: theoretical capacity (the full Ah rating), usable capacity (what you can safely draw within your DoD), and recommended nominal capacity (the size to actually buy).
Battery Capacity Examples
Defaults below: 90% inverter efficiency, 95% battery efficiency, 50% depth of discharge — matching the calculator.
How to Calculate Battery Runtime
A 12V 100Ah battery (1,200Wh nominal) running a 200W load, at 90% inverter efficiency, 95% battery efficiency, and 50% DoD: usable energy ≈ 513Wh, runtime ≈ 2.6 hours.
Real-world runtime rarely matches this exactly. Battery age, ambient temperature, discharge rate, and how spiky the load is (motors and compressors draw more than their steady rating on startup) all shift the result — usually downward.
How to Size a Battery for an Inverter
- Add up the wattage of everything you'll run simultaneously.
- Decide how long that load needs to run.
- Apply inverter efficiency, battery efficiency, and your DoD.
- Divide by system voltage for required Ah, then round up to an available battery or bank size.
Also confirm the battery — and its BMS, if lithium — can deliver the peak current the inverter draws, especially during surge. Capacity and deliverable current are separate specifications.
How to Size a Solar Battery
Adjust for DoD and system losses as above, then account for seasonal drops in solar production and runs of consecutive cloudy days. Solar battery sizing should never be based on inverter power alone — a large inverter can sit next to a modest battery bank, and vice versa, depending entirely on the energy that needs storing.
Ah vs Wh vs kWh
Ah measures electrical charge capacity alone — no voltage involved. Wh measures stored energy directly (Wh = V × Ah), making it the more useful figure across different-voltage batteries. kWh is simply 1,000 Wh.
A 12V 100Ah battery stores 1,200Wh; a 24V 100Ah battery stores 2,400Wh — double the energy on an identical Ah rating. Comparing batteries by Ah alone, without checking voltage, gives a misleading picture.
Understanding Battery Voltage
System voltage sets the current a load draws: I = P ÷ V. A 1,000W load on 12V draws roughly 83A before losses; the same load on 48V draws about 21A. Higher voltages mean thinner cabling and less resistive loss — one reason larger systems move to 24V or 48V.
Nominal voltage also isn't fixed in practice — a "12V" lead-acid battery may sit anywhere from ~12.6–12.8V full to ~11.8–12V near empty. The calculator's voltage input is a planning figure, not a guarantee under load.
Understanding Depth of Discharge
DoD is the percentage of nominal capacity that's been used. A 100Ah battery at 50% DoD has had roughly 50Ah drawn from it.
- Nominal capacity — the full rated Ah on the label.
- State of charge — how full the battery currently is.
- Depth of discharge — the inverse of SoC.
- Usable capacity — the portion designed to be drawn before recharge, per the chemistry's recommended DoD.
There's no single DoD figure for every battery — it varies by chemistry, brand, and cycling pattern, so it's a user-adjustable input rather than a constant.
Battery Efficiency
No battery returns 100% of the energy put into it — charging and discharging both lose energy as heat, varying with age, chemistry, and discharge rate.
- Battery efficiency — round-trip losses inside the battery.
- Inverter efficiency — DC/AC conversion losses.
- Overall system efficiency — both combined, plus wiring losses.
Treat calculator output as a planning estimate, not a lab-measured guarantee for your specific hardware.
Battery Chemistry and Capacity
Lead-Acid
Flooded, AGM, and gel variants. Flooded needs periodic water top-ups and ventilation; AGM and gel are sealed and maintenance-free. Heavier per Wh than lithium, lower upfront cost, and typically discharged more conservatively — the exact recommended DoD depends on the specific battery, not a universal number.
Lithium-ion / LiFePO4
Higher usable capacity relative to nominal rating, longer cycle life in many deep-cycle uses, and much lower weight. Requires a BMS for over-charge/discharge and cell-balance protection, different charge profiles, and a higher cost per Ah. Check the datasheet for exact figures.
Battery Bank: Series and Parallel
Series
Voltage adds, Ah stays the same. Two 12V 100Ah batteries in series ≈ 24V 100Ah, ≈2,400Wh total.
Parallel
Ah adds, voltage stays the same. Two 12V 100Ah batteries in parallel ≈ 12V 200Ah, ≈2,400Wh total.
Series-parallel combines both — four 12V 100Ah batteries can form two series pairs (24V 100Ah each) wired in parallel for ≈24V 200Ah (≈4,800Wh). Only combine batteries matched in type, age, and capacity — mixed batteries unbalance charging and shorten the life of the whole bank.
| Voltage | Capacity | Nominal Energy |
|---|---|---|
| 12V | 50Ah | 600Wh |
| 12V | 100Ah | 1,200Wh |
| 12V | 200Ah | 2,400Wh |
| 24V | 100Ah | 2,400Wh |
| 24V | 200Ah | 4,800Wh |
| 48V | 100Ah | 4,800Wh |
| 48V | 200Ah | 9,600Wh |
Battery Capacity vs Battery Power
Capacity (Ah/Wh) is total stored energy. Power (W) is how fast that energy can be delivered right now. A battery can hold plenty of energy and still be unsuitable if it can't safely deliver the required current — check its continuous/peak discharge rating and BMS limit, not just its Ah figure. A "5,000W inverter needs a 5,000Wh battery" rule of thumb skips the variable that actually sizes a battery: runtime.
How to Use the Calculator
- Choose the mode that matches your question — Energy, Required Ah, Runtime, or a direct Wh/Ah conversion.
- Enter battery voltage, and load power or Ah depending on the mode selected.
- Enter desired backup time, if sizing a battery.
- Enter inverter efficiency.
- Enter battery efficiency and permitted depth of discharge, if applicable to the mode.
- Select Calculate.
- Read the result off the panel, along with the assumptions used to reach it.
Real-World Battery Sizing
- Peukert effect (mainly lead-acid) — fast discharge delivers noticeably less total capacity than the slow-discharge rating.
- Temperature — cold reduces usable capacity and available current.
- Battery age — capacity fades over service life.
- Cable losses — undersized or long runs waste energy as heat.
- Surge loads — motors and compressors draw far more than steady wattage at startup.
For final engineering decisions, use the manufacturer's discharge curves — this calculator is built for planning-stage sizing.
Common Battery Sizing Mistakes
- Ignoring inverter efficiency when sizing a battery.
- Assuming the full nameplate Ah is usable, ignoring DoD.
- Confusing Ah with Wh across different voltages.
- Shopping on Ah alone, ignoring voltage.
- Ignoring Peukert effects at fast discharge rates.
- Ignoring battery age and capacity fade.
- Ignoring temperature effects on usable capacity.
- Ignoring surge current versus continuous rating.
- Mixing incompatible chemistries or ages in one bank.
- Ignoring the battery's charging requirements.
Battery Safety Tips
- Always connect with correct polarity.
- Use appropriately rated fuses or breakers.
- Size cables for the expected current.
- Ventilate battery types that can vent gas, particularly flooded lead-acid.
- Treat exposed terminals as live at all times.
- Use charging equipment matched to the battery chemistry.
- Respect BMS limits on lithium batteries.
- Never mix incompatible battery types, ages, or capacities.
Always follow the battery and inverter manufacturer's specifications and applicable electrical codes. Results from this calculator are estimates for planning purposes — actual performance depends on chemistry, temperature, discharge rate, battery age, and system losses.
Frequently Asked Questions
How many Wh is a 12V 100Ah battery?
Approximately 1,200Wh (1.2kWh) nominal.
How do I calculate battery backup time?
Divide usable battery energy (Wh, after efficiency and DoD adjustments) by the load in watts.
How many batteries do I need for a 5kW inverter?
That depends on required runtime, not inverter wattage alone — use the Required Ah mode with your actual load and backup time.
What is the difference between Ah and Wh?
Ah measures charge capacity alone; Wh measures stored energy and includes voltage, making it the more directly comparable figure.
Does higher Ah mean a better battery?
Not necessarily — a higher-Ah battery at a lower voltage can store less total energy than a lower-Ah battery at a higher voltage.
What does battery DoD mean?
Depth of discharge — the percentage of nominal capacity that has been drawn from the battery.
Can I use this calculator for lithium batteries?
Yes — adjust the DoD input to reflect the higher usable capacity typical of lithium chemistries per the manufacturer's datasheet.
Why is actual runtime lower than the calculated figure?
Temperature, battery age, discharge-rate effects (Peukert), wiring losses, and surge loads all reduce real-world runtime below the simplified formula's result.
How many batteries do I need for a 48V bank?
Typically four 12V batteries in series per string, then additional strings in parallel to reach the required Ah.
Is battery capacity the same as battery power?
No — capacity describes total stored energy, while power describes the rate energy can be delivered at any moment.