Daily Energy Demand
Enter the watt-hours your loads consume in a representative day. Measured energy is ideal, but a careful appliance-by-appliance estimate is a useful planning start.
Battery Sizing Calculator
Use this Battery Sizing Calculator to estimate the nominal amp-hour and kilowatt-hour capacity your battery bank needs from daily energy use, system voltage, days of autonomy, planned depth of discharge, conversion efficiency, and a reserve margin. It is designed for LiFePO4 planning in solar, RV, marine, home backup, portable, and off-grid systems.
The result is a transparent engineering estimate rather than a product recommendation. You can change one assumption at a time, compare 12.8V, 25.6V, and 51.2V examples, and then verify current limits, charging, cable size, and BMS compatibility before buying hardware.
What the Battery Sizing Calculator Helps You Plan
A useful battery estimate starts with energy demand, then accounts for how much of the battery you plan to use, conversion losses, backup time, and a practical buffer for real-world variation.
Battery capacity is often advertised in amp-hours, while appliances are usually described in watts or watt-hours. The core conversion is simple: watt-hours equal amp-hours multiplied by nominal voltage. Expressing the load in watt-hours lets you compare systems at different voltages without assuming that the same Ah rating always stores the same energy.
This page separates the decisions that are often mixed together. Daily energy describes consumption, autonomy sets backup duration, depth of discharge sets the planned usable share, efficiency accounts for losses, reserve adds breathing room, and nominal voltage converts the final energy target back into amp-hours.
If you are still building your load list, start with the Watt Hour Calculator to convert appliance watts and operating time into energy. If you already own a battery and want to estimate how long it will run a steady load, the Battery Runtime Calculator is the more direct tool.
Autonomy Days
Choose how many days the battery should cover before significant recharging. One day suits many daily-cycle systems; off-grid backup may need more depending on the charging source.
Usable Discharge Window
Depth of discharge is treated as a planning input, not a universal rule. Use the limits and operating guidance for the exact battery model you intend to install.
System Efficiency
Conversion losses mean the battery normally supplies more energy than the appliance receives. Enter a realistic efficiency for the inverter or delivery path you expect to use.
Reserve Margin
A reserve can absorb load growth, measurement error, aging, colder conditions, or days that are heavier than average. It is added after the base battery requirement is calculated.
Ah and kWh Outputs
The calculator shows both energy and amp-hours so you can compare battery banks sensibly across voltages and then check a suitable module configuration.
How to Size a Battery Bank Step by Step
The Battery Sizing Calculator is only as good as its inputs. Use the process below to turn real load information into a capacity target you can verify against battery and inverter specifications.
First, estimate daily energy rather than relying only on peak wattage. A 1,500W kettle may operate for minutes, while a 70W refrigerator can cycle throughout the day. Multiply each load's average running watts by its daily operating hours and add the watt-hours together. For cycling equipment, measured energy from a monitor is often more reliable than multiplying nameplate watts by 24 hours.
Second, decide how long the battery needs to carry those loads without meaningful charging. This is the autonomy setting. A solar system that normally recharges every day may use one day for basic sizing, while a remote cabin can require a larger buffer for poor weather. Autonomy is not the same thing as reserve margin: autonomy intentionally covers extra time; reserve margin is an additional percentage added to the calculated capacity.
Third, enter nominal bank voltage. The common LiFePO4 examples on this page use 12.8V, 25.6V, and 51.2V. Those values are common nominal pack voltages, but always use the rating on your battery documentation. If you are deciding between bank arrangements, the Series / Parallel Battery Calculator can help you understand how voltage and amp-hour capacity change as batteries are combined.
Solar and Off-Grid
Size storage around measured daily load, planned autonomy, seasonal charging conditions, and a realistic reserve. Then use the Solar Battery Bank Calculator for a more solar-focused cross-check.
Explore off-grid sizingRV and Camper
RV loads include refrigerators, fans, lights, water pumps, electronics, inverter standby draw, and occasional high-power appliances. Build the load budget from your travel routine rather than a generic battery size.
Use the RV calculatorMarine Systems
Navigation, communications, refrigeration, pumps, lighting, and house loads can create very different duty cycles. Energy sizing must be followed by marine-appropriate protection, wiring, mounting, and charging checks.
Open marine sizingHome Backup
Separate essential circuits from discretionary loads, estimate realistic outage duration, and decide whether charging from solar or the grid is expected during the backup period.
Plan home storageBattery Sizing Calculator for LiFePO4 Systems
Enter your expected load and design assumptions. The calculator returns the base energy requirement, adjusted nominal storage, recommended Ah, and an approximate module count at the selected bank voltage.
Calculate Required Battery Capacity
Formula: Required nominal Wh = Daily Wh × Autonomy ÷ Efficiency ÷ DoD × (1 + Reserve). Required Ah = Required nominal Wh ÷ Nominal Voltage.
Calculation Results
Load energy for autonomy
1.50 kWh
Daily Wh multiplied by autonomy days.
Base nominal storage
1.85 kWh
Storage required after efficiency and DoD adjustments, before reserve.
Recommended nominal storage
2.22 kWh
Base storage plus your selected reserve margin.
Required bank capacity
173.6 Ah
Recommended nominal storage divided by bank voltage.
Reference module count
2 modules
Rounded up from required Ah ÷ module Ah at the same bank voltage.
Planned usable fraction
81.0%
DoD × efficiency, before the reserve margin is added.
1,500 Wh × 1 day ÷ 90% efficiency ÷ 90% DoD × 1.20 reserve = 2,222 Wh ≈ 173.6 Ah at 12.8V.
Educational estimate only: The Battery Sizing Calculator models energy capacity, not equipment approval. Verify battery/BMS continuous and surge current, charger settings, cable size, fuse rating, temperature limits, mounting, and permitted series/parallel configurations before installation. The site's Wire Gauge Calculator can help with an initial conductor-size check, but manufacturer and applicable electrical requirements take priority.
Related Battery Tools
After sizing storage, check runtime, charging time, and conductor planning with these published tools.
Battery Runtime Calculator
Estimate how long a known battery can support a steady load.
Use CalculatorBattery Charge Time Calculator
Estimate recovery time from battery capacity and available charging power.
Use CalculatorWire Gauge Calculator
Run an initial cable-size and voltage-drop check after current is known.
Use CalculatorBattery Sizing Formula, Examples, and Practical Checks
The Battery Sizing Calculator uses transparent energy arithmetic so you can reproduce the result by hand and understand exactly which assumption changes the required bank size.
The Core Battery Sizing Formula
Required Wh = Daily Wh × Days ÷ Efficiency ÷ DoD × (1 + Reserve)
Efficiency, depth of discharge, and reserve are entered as percentages but used as decimal factors. With 90% efficiency and 90% planned DoD, the load can receive 0.90 × 0.90 = 81% of the nominal battery energy in this simplified model before reserve is added.
Required Ah = Required Wh ÷ Nominal Voltage
This second step converts energy back into the amp-hour rating commonly shown on batteries. It is why the same energy requirement produces fewer amp-hours at a higher bank voltage. If you only need to convert between Ah and Wh, the Battery Capacity Calculator provides a focused conversion.
Checks the Formula Does Not Replace
- Peak and surge current: a bank can have enough kWh yet still be unable to supply an inverter or motor surge.
- BMS limits: continuous charge/discharge current and low/high-temperature protection can limit usable system power.
- Charging capability: a larger battery bank also takes longer or requires more charging power to recover.
- Cables and fuses: size them from maximum expected current, run length, voltage drop, environment, and equipment limits.
- Battery combination rules: not every product permits arbitrary series or parallel configurations.
Victron's installation guidance, for example, instructs users to size battery cables for expected current and to fuse battery circuits appropriately. That is a manufacturer-specific example, not a universal substitute for the manual that came with your equipment. See the Victron Lithium Battery Smart installation guidance.
Depth of Discharge and Efficiency
DoD is a design choice linked to the exact chemistry, model, BMS behavior, warranty, and operating strategy. The calculator allows 1% to 100% because users may want to compare scenarios, but a mathematically valid input is not automatically a recommended operating point. Use your battery documentation for real limits.
Efficiency is also variable. Inverter efficiency can change with load, and a system may have standby consumption, wiring loss, DC-DC conversion, or other loads that a single percentage does not fully capture. If you have measured battery-side energy for a representative day, it can be better to size from that measured value rather than trying to model every individual loss.
Reserve Margin and Future Growth
A reserve is not the same as usable DoD. DoD reduces the fraction of nominal energy you plan to withdraw; reserve increases the nominal capacity you buy above the calculated minimum. A 20% reserve is simply a planning buffer chosen by the user, not a required universal standard.
Reserve can be helpful when your load estimate is uncertain, when you expect future appliances, or when you want more tolerance for aging and difficult conditions. If cost or weight is critical, compare several reserve values instead of accepting one default. For price planning after sizing, the Lithium Battery Cost Calculator can help convert capacity targets into a budget estimate.
| Scenario | Daily Load | Autonomy | Voltage | DoD / Efficiency | Reserve | Estimated Capacity |
|---|---|---|---|---|---|---|
| Small RV house bank | 1,200 Wh | 1 day | 12.8V | 90% / 90% | 20% | ≈ 1.78 kWh / 138.9 Ah |
| Cabin daily-cycle bank | 3,000 Wh | 1.5 days | 25.6V | 90% / 92% | 20% | ≈ 6.52 kWh / 254.7 Ah |
| Home essential-load backup | 6,000 Wh | 1 day | 51.2V | 90% / 92% | 25% | ≈ 9.06 kWh / 177.0 Ah |
Worked example: 1,500Wh/day at 12.8V
For 1,500Wh/day and one day of autonomy, 90% efficiency raises battery-side energy to 1,666.7Wh. Dividing by 90% planned DoD gives 1,851.9Wh nominal, and a 20% reserve raises the target to 2,222.2Wh. At 12.8V that is 173.6Ah. A 100Ah reference module would round to two modules only if the manufacturer permits the required configuration.
For charging planning, battery capacity is only half of the problem. A larger bank must be restored by solar, alternator, shore power, generator, or another charging source. Use the Battery Charge Time Calculator to estimate recovery time after you know the bank capacity and available charging power.
The U.S. Department of Energy notes that energy storage involves conversion losses and distinguishes energy capacity from power capacity. That distinction matters here: this calculator estimates how much energy to store, while the inverter, BMS, conductors, and protection devices determine how much power can be delivered safely at a given moment. See DOE's solar energy and storage basics for background on storage energy and power concepts.
Three Practical Battery Sizing Scenarios
These Battery Sizing Calculator examples are transparent calculations rather than testimonials. They show how the same sizing method changes with load, voltage, autonomy, and reserve.
RV example
Moderate 12V Travel Day
A 1,200Wh daily budget with one day of autonomy, 90% DoD, 90% efficiency, and 20% reserve needs about 1.78kWh nominal. At 12.8V that is roughly 139Ah. The next step is checking whether refrigerator cycling, inverter standby, and any high-power cooking loads are represented accurately.
Off-grid example
24V Cabin With Extra Autonomy
A 3,000Wh/day cabin sized for 1.5 days, 90% DoD, 92% efficiency, and 20% reserve needs about 6.52kWh nominal. At 25.6V that is about 255Ah. A larger energy target should be compared with the solar array and charging window so the bank can recover after poor-weather use.
Backup example
48V Essential-Load Backup
A 6,000Wh daily essential-load budget, one day of autonomy, 90% DoD, 92% efficiency, and 25% reserve produces roughly 9.06kWh nominal, or about 177Ah at 51.2V. This energy figure still does not confirm that the bank can support a large inverter surge or simultaneous appliance startup.
Battery Sizing Calculator Frequently Asked Questions
These answers cover the most common interpretation issues: Ah versus Wh, reserve margin, autonomy, voltage, real-world losses, and how to move from a calculator result to a safe system design.
How do I calculate the battery size I need?
Start with daily load energy in Wh. Multiply by the number of autonomy days, divide by system efficiency, divide by the planned depth-of-discharge fraction, then add your reserve margin. That gives required nominal Wh. Divide nominal Wh by nominal battery voltage to convert the result to Ah. The Battery Sizing Calculator performs those steps and keeps the assumptions visible.
Why should I size batteries in watt-hours before amp-hours?
Watt-hours measure energy and allow direct comparison across voltages. A 100Ah battery at 12.8V contains about 1,280Wh nominal, while 100Ah at 25.6V contains about 2,560Wh. Ah alone therefore does not describe total stored energy unless voltage is also known.
What depth of discharge should I enter for LiFePO4?
Use the operating limit or planning target appropriate to the exact battery, BMS, warranty, temperature range, and reserve strategy. The calculator's default is an example, not a manufacturer recommendation for every LiFePO4 battery. Check the datasheet and manual for the product you intend to use.
What is a good reserve margin for battery sizing?
There is no universal reserve percentage. A margin can cover uncertain loads, future additions, aging, or operating conditions, but the right value depends on your design priorities. Compare several scenarios—such as 10%, 20%, and 30%—and decide how much additional capacity, cost, and weight are justified.
Does a 48V battery bank need fewer amp-hours than a 12V bank?
For the same nominal energy, yes. Required Ah equals required Wh divided by voltage, so a higher-voltage bank needs fewer Ah to store the same Wh. That does not mean it needs less energy. It also does not automatically make a 48V architecture better for every application; equipment compatibility and system design still matter.
Can I use the result to choose the number of batteries?
You can use the result as a capacity target, but battery count depends on each module's voltage, Ah rating, allowed series/parallel configuration, BMS requirements, and whether the final bank voltage matches the system. Use the Battery Series Parallel Calculator to explore configuration arithmetic, then confirm the permitted arrangement in the manufacturer documentation.
Why can real runtime be shorter than the sizing estimate?
Real loads vary, inverter efficiency changes, standby draw consumes energy, battery temperature and aging affect available capacity, wiring has losses, and a BMS can disconnect before the theoretical energy target is reached. Better load measurements and conservative assumptions usually improve the estimate.
Does this calculator size my inverter, fuse, or cable?
No. The Battery Sizing Calculator estimates energy storage. Inverter, BMS, fuse, and cable sizing depend on power and current, including surge behavior, cable length, allowable voltage drop, temperature, insulation, and equipment ratings. Treat those as separate engineering checks.
How do I improve the accuracy of my battery size estimate?
Measure real daily Wh over several representative days, include inverter idle consumption, use the nominal bank voltage from the datasheet, use realistic efficiency, and model both typical and heavy-use scenarios. For seasonal solar systems, test winter and summer assumptions separately rather than relying on one annual average.
Turn the Capacity Estimate Into a Complete Battery Plan
Run the Battery Sizing Calculator with a typical day and a heavy-use day, then verify the result against your battery datasheet, BMS current limits, charging sources, inverter requirements, cable sizing, and protection. A good design is not just large enough—it is compatible from storage to load.
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