Use this Off-Grid Battery Calculator to estimate the nominal battery bank energy and amp-hour capacity needed for a cabin, home, workshop, farm, or remote solar system. Enter daily energy use, desired autonomy, battery voltage, usable depth of discharge, system efficiency, reserve margin, and peak load.
The result separates the energy you actually need from the larger rated battery bank required to cover losses, discharge limits, and a planning reserve. It is a sizing starting point, not an installation approval.
Calculate a practical storage target from daily load, no-charge days, discharge allowance, system losses, reserve margin, and the nominal voltage of your planned battery bank.
Off-grid battery sizing starts with energy, not battery count. First estimate how many watt-hours your loads use in a normal day. Then multiply by the number of days you want the battery bank to carry the system without meaningful charging. The calculator increases that usable-energy requirement to account for your chosen depth of discharge and overall delivery efficiency, then adds an optional reserve margin.
If you are still building a daily load estimate, compare individual appliance runtime with the Battery Runtime Calculator and convert battery ratings with the Battery Capacity Calculator. Better load data produces a more useful off-grid result.
Defaults show a 51.2V off-grid example. Replace them with your own measured or documented values.
For 5,000 Wh/day and 2 autonomy days, the loads need 10,000 Wh of delivered energy. At 80% planned DoD, 90% delivery efficiency, and 15% reserve, the estimated nominal bank is about 15.97 kWh, or 312.0 Ah at 51.2 V.
Important: This Off-Grid Battery Calculator estimates energy capacity and a simplified peak battery current. It does not size an inverter, BMS, busbar, fuse, breaker, cable, solar array, charge controller, or generator. Check the Wire Gauge Calculator only after you know the design current and cable run, then verify the result against applicable codes and equipment documentation.
The useful number is not simply “how many batteries.” A reliable estimate connects your real energy use, desired autonomy, usable battery window, losses, bank voltage, and peak-power demands.
Measure or estimate each load in watt-hours per day. A 100 W device used for 5 hours contributes about 500 Wh/day. Refrigerators, pumps, and other cycling loads are better estimated from measured daily energy than from nameplate watts alone.
Check appliance runtimeAutonomy is the period the battery bank should support the planned load without enough new energy from solar, wind, generator, or another charger. Longer autonomy quickly increases storage size, cost, weight, and charging requirements.
Compare solar bank sizingRated battery energy is not the same as planned delivered energy. Your chosen discharge window keeps a reserve inside the battery, while inverter and electrical losses reduce how much stored energy reaches the load.
Review LiFePO4 sizingWatt-hours express energy across any system voltage. Amp-hours depend on voltage, so the same kWh bank requires fewer Ah at a higher nominal voltage. Bank architecture still has to match the inverter, batteries, BMS, charger, and manufacturer rules.
Plan series and parallel banksThe calculator uses transparent energy equations so you can verify the result by hand and see exactly which assumption changes the battery-bank target.
The first step is the energy that must actually reach the loads during the selected autonomy period. If daily energy use is 5,000 Wh and the target is two no-charge days, the loads require 10,000 Wh. That is delivered energy, not yet the rated battery size.
Next, divide by delivery efficiency and planned depth of discharge. Dividing by 90% efficiency increases the stored-energy requirement because some energy is lost before reaching the load. Dividing by 80% DoD increases the rated bank again because only 80% of the nominal energy is planned for use.
The reserve margin is applied after those adjustments. A 15% reserve multiplies the base nominal storage by 1.15. Reserve is intentionally separate from DoD: DoD describes the planned usable share of the battery, while reserve is extra design headroom for uncertainty or changing loads.
Finally, amp-hours are calculated from the resulting nominal watt-hours and the entered nominal bank voltage. This is why a bank's Ah number cannot be compared across voltages without also considering volts.
The peak-current estimate uses your peak simultaneous load and the same delivery-efficiency assumption. It is only a scale check. Motor startup, inverter surge behavior, DC loads, BMS limits, conductor temperature, voltage drop, and protective-device requirements can change the actual design current.
A load audit is usually more valuable than guessing battery size. Smart plugs, inverter logs, energy meters, or appliance energy labels can help build a daily Wh estimate. Include inverter standby power, pumps, communications equipment, refrigeration, lighting, electronics, water systems, and seasonal loads that matter during the period you are designing for.
The U.S. Department of Energy explains that energy storage is not 100% efficient and distinguishes energy capacity from power capacity. That is why this calculator treats stored kWh and peak watts as different planning questions. See the DOE overview of solar energy and storage.
More autonomy can improve resilience, but it also makes the battery bank substantially larger. In a solar system, compare storage autonomy with local weather, seasonal production, generator availability, and how quickly the array can recharge the bank after a low-sun period. Oversizing storage without enough charging power can leave a large bank chronically undercharged.
For a solar-specific cross-check, use the solar storage sizing tool after this calculation and compare the assumptions rather than expecting identical results from different models.
A correct kWh result does not prove that a particular battery can supply the required current. Check continuous discharge current, short-duration surge, charging current, low- and high-temperature protection, supported series/parallel configurations, and communication requirements for the exact battery model.
Victron's current off-grid home guidance illustrates that real system sizing considers losses and an allowed depth of discharge. Treat manufacturer examples as examples, not universal settings for unrelated batteries.
Storage and charging should be designed together. A larger bank can support longer autonomy, but it also needs enough solar, generator, alternator, wind, hydro, or grid charging capacity to recover in the time available. Charging limits from the battery and BMS can also cap how quickly the bank can accept energy.
After selecting a tentative bank, use the charge-time calculator to compare recovery times, then verify the charge profile and limits against the battery and charger manuals.
| Planning change | What it changes | Typical effect on calculated bank | What to verify next |
|---|---|---|---|
| Higher daily Wh | More energy consumed every day | Bank Wh and Ah rise proportionally | Load audit and efficiency opportunities |
| More autonomy days | Longer operation without recharge | Bank size rises proportionally | Solar recovery time and backup charging |
| Lower planned DoD | More capacity kept in reserve | Larger rated bank required | Battery documentation and operating goals |
| Lower efficiency | More energy lost before the load | Larger rated bank required | Inverter efficiency and standby consumption |
| Higher bank voltage | Same energy represented at fewer Ah | kWh unchanged; required Ah decreases | Inverter, BMS, charger, and wiring compatibility |
| Higher peak watts | More instantaneous power demand | Energy sizing may stay similar; current requirement rises | Inverter surge, BMS, cables, fusing, voltage drop |
Accuracy and safety: Real capacity changes with temperature, age, state of charge, cell balance, discharge rate, BMS behavior, wiring losses, and equipment cutoffs. For critical medical, communications, water, navigation, fire-safety, or emergency loads, use conservative reserves and qualified system design. This page does not replace electrical codes, product manuals, or professional engineering.
These examples show how the same method scales from a small cabin to a larger remote property. They are arithmetic demonstrations, not recommended configurations.
A small cabin uses 1,500 Wh/day and needs 1.5 days of autonomy. With 80% DoD, 90% efficiency, and 15% reserve, the required nominal storage is about 3.59 kWh. At 25.6 V, that is roughly 140 Ah.
The battery bank is only part of the design. A cabin with a pump or compressor may need much higher short-term power than its modest daily energy suggests.
Assume 8,000 Wh/day, two autonomy days, 80% DoD, 90% efficiency, and 20% reserve. The calculator returns about 26.67 kWh of nominal storage. At 51.2 V, that is about 521 Ah.
Before buying batteries, confirm whether the solar array and other charging sources can realistically recover that energy after the design low-sun period.
A remote site using 12,000 Wh/day with three autonomy days, 80% DoD, 88% efficiency, and 20% reserve needs about 61.36 kWh nominal in this model. At 51.2 V, that is about 1,198 Ah.
At this scale, load reduction, generator strategy, seasonal solar production, inverter architecture, battery-module limits, and professional design can materially change the best system solution.
If you want to compare the same storage requirement at different voltages, convert between Wh and Ah with the Battery Capacity Calculator. For a 48V-class lithium installation, the 48V Lithium Battery Calculator provides a voltage-specific cross-check. Those tools complement the Off-Grid Battery Calculator rather than replacing a complete system design.
Common questions about battery-bank energy, autonomy, depth of discharge, voltage, losses, and how to interpret the calculator without overestimating its precision.
Estimate daily energy in Wh, multiply by the desired autonomy days, divide by delivery efficiency, divide by the planned usable depth of discharge, then add any reserve margin. Divide the final nominal Wh by bank voltage to convert the result to Ah. The Off-Grid Battery Calculator performs these steps automatically.
There is no universal number. It depends on weather, seasonal generation, generator availability, acceptable load shedding, the reliability you need, and how quickly your charging system can recover the bank. Compare the cost of extra storage with the cost and practicality of additional generation or backup charging.
Use the nominal voltage specified for the complete battery bank. Many LiFePO4 systems commonly use nominal values such as 12.8V, 25.6V, or 51.2V, but not every product follows the same convention. Use your actual battery or bank documentation rather than assuming a label is exact.
Use a planned usable percentage that matches the battery documentation, warranty conditions, BMS behavior, temperature conditions, and your reserve strategy. The calculator accepts 1% to 100% because it is a math tool, not because every battery should be operated across that full range.
The delivery-efficiency field is intended to represent a simplified overall loss allowance, which can include inverter conversion. It does not separately calculate changing inverter efficiency, idle consumption, cable losses, transformer losses, or DC conversion losses. If those are significant, incorporate measured data into your load estimate or efficiency assumption.
Because watt-hours equal volts multiplied by amp-hours. For the same energy requirement, increasing nominal voltage reduces the Ah number. That does not mean the energy requirement disappears; the kWh stays the same. Higher voltage also affects current, wiring, equipment compatibility, and safety requirements.
Not safely by itself. Individual battery count depends on each battery's voltage, Ah, supported series/parallel limits, BMS communication, maximum string count, balancing requirements, current limits, and inverter compatibility. Use the required bank Wh and Ah as the target, then map that target to products permitted by their manufacturer documentation.
No. Battery storage and solar generation are connected but different sizing problems. Solar array sizing depends on daily energy, location, seasonal solar resource, panel orientation, system losses, controller limits, and the required recharge window. Use the Solar Battery Bank Calculator and supporting solar guidance for the next stage.
Run the calculator with measured daily energy, a deliberate autonomy target, the correct nominal bank voltage, and realistic loss and reserve assumptions. Then verify the result against charging capacity, peak-power requirements, battery documentation, wiring, protection, and local electrical requirements.
Recalculate Your Battery Bank