Battery Bank Sizing for Off-Grid
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Calculate
Enter the total continuous load power in watts
Enter the desired off-grid autonomy period in hours (e.g. 24 h = 1 day, 72 h = 3 days)
Typical DoD: 50% for AGM/lead-acid, 80% for LiFePO4
Overall system efficiency including inverter losses — typically 85–95%
Ah rating of one battery at the system voltage
Overview
A Battery Bank Sizing for Off-Grid calculator estimates how much battery storage is required to run an off-grid electrical system for a target autonomy period. This page uses a fixed off-grid sizing model: it starts with the required load energy, applies the selected autonomy period, adjusts for system efficiency losses, adjusts again for usable depth of discharge, and then converts the result into required Wh, kWh, Ah, and optional battery count.
This makes the calculator useful for off-grid cabins, remote systems, backup-independent solar systems, telecom shelters, RV and van systems, and other battery-powered setups where real storage margin matters. Usable battery capacity depends strongly on chemistry and DoD assumptions, and Victron's published guidance highlights the practical difference between AGM and LiFePO4 in exactly this way.
Accurate off-grid battery bank sizing ensures reliable autonomy, avoids undersized or oversized installations, and helps balance cost, weight, and performance for systems that must survive without grid support.
How to Use This Calculator
Enter load power — in W.
Enter autonomy time — in h.
Select system voltage — choose from 12 V, 24 V, 48 V.
Enter depth of discharge (dod) — in %.
Enter system efficiency — in %.
Enter battery unit capacity — in Ah.
Click "Calculate" — get required battery capacity, required energy (wh), required energy (kwh).
Use the result to support your engineering design and analysis decisions.
Inputs & Outputs
Inputs
- •Load Power (W)
- •Autonomy Time (h)
- •System Voltage (V) — Options: 12 V, 24 V, 48 V
- •Depth of Discharge (DoD) (%)
- •System Efficiency (%)
- •Battery Unit Capacity (Ah)
Outputs
- •Required Battery Capacity (Ah)
- •Required Energy (Wh) (Wh)
- •Required Energy (kWh) (kWh)
- •Usable Battery Capacity (Ah)
- •Recommended Battery Count (batteries)
Formula
Calculator Formula
This calculator uses a fixed off-grid battery-bank sizing model.
Step 1: Required load energy
E_load = P × t
Where:
- E_load = required load energy in watt-hours (Wh)
- P = load power in watts (W)
- t = autonomy time in hours (h)
If the user enters daily energy directly, that value becomes E_load.
Step 2: Apply autonomy requirement
If autonomy is entered in days:
E_autonomy = E_load_daily × D
Where:
- E_autonomy = total required energy over the off-grid autonomy period
- E_load_daily = daily energy demand in Wh/day
- D = autonomy days
If autonomy is entered directly in hours, this step is already reflected in Step 1.
Step 3: Adjust for system efficiency
E_adjusted = E_load / η
Where:
- E_adjusted = adjusted required energy in Wh
- η = overall system efficiency as a decimal
Step 4: Adjust for depth of discharge
E_bank = E_adjusted / DoD
Where:
- E_bank = required nominal battery-bank energy in Wh
- DoD = usable depth of discharge as a decimal
Step 5: Convert to required amp-hours
Ah_required = E_bank / V_system
Where:
- Ah_required = required battery-bank capacity in amp-hours
- V_system = system voltage
Step 6: Convert to battery count
Battery Count = ⌈Ah_required / Ah_battery⌉
Rounded up to the next whole battery.
Step 7: Energy conversions
kWh_required = E_bank / 1000
This means the page follows one exact path:
Load → Autonomy → Efficiency Adjustment → DoD Adjustment → Required Wh/kWh → Required Ah → Battery Count
This is the fixed decision model used on the page.
Variable Reference
| Variable | Meaning | Units |
|---|---|---|
| P / loadPower | Load power | W |
| t / autonomyHours | Autonomy time | h |
| V_system / systemVoltage | System voltage | V |
| DoD / depthOfDischarge | Depth of discharge | % |
| η / efficiency | System efficiency | % |
| Ah_battery / batteryAh | Single battery capacity | Ah |
| E_load | Required load energy | Wh |
| E_adjusted | Efficiency-adjusted energy | Wh |
| E_bank | Required nominal bank energy | Wh |
| Ah_required | Required battery capacity | Ah |
| kWh_required | Required energy in kWh | kWh |
| Battery Count | Number of batteries needed | — |
What is Battery Bank Sizing for Off-Grid
Battery bank sizing for off-grid is the process of determining how much stored electrical energy is required to keep a system running without grid support for a target period. In off-grid design, the question is not just how many batteries exist on paper, but how much usable energy the system can realistically deliver after accounting for discharge limits, efficiency losses, and actual runtime expectations. That is why off-grid battery sizing should be done from energy demand and autonomy, not from battery count alone. Practical battery behavior also changes with chemistry, age, discharge rate, and temperature, which is why real-world design requires margin. Victron's published guidance and manuals repeatedly emphasize these practical usable-capacity considerations.
Sizing Model
This calculator follows one exact path:
Load → Autonomy → Efficiency Adjustment → DoD Adjustment → Required Wh/kWh → Required Ah → Battery Count
This is the fixed model used on the page. It starts with the required load energy, applies the requested autonomy period, adjusts for system efficiency losses, adjusts again for usable depth of discharge, and then converts the result into required Wh, kWh, Ah, and battery count.
Why Usable Capacity Matters
Nameplate capacity is not the same as usable capacity. A 200 Ah lead-acid battery at 50% DoD delivers only 100 Ah of usable energy. A 200 Ah LiFePO4 battery at 80% DoD delivers 160 Ah. This difference is significant when sizing an off-grid battery bank for a specific autonomy target.
Victron Energy notes a practical reference of about 50% DoD for AGM and about 80% DoD for LiFePO4, while also emphasizing that system design must consider real operating conditions including temperature, discharge rate, and battery age.
Engineering Applications
Off-grid battery bank calculations are widely used across many applications:
- Off-grid solar systems — sizing battery storage to cover nighttime and cloudy-day loads
- Cabin and remote-site power — ensuring reliable power without grid connection
- RV and van systems — balancing weight, space, and runtime for mobile off-grid installations
- Telecom sites — maintaining DC power for communication equipment during extended outages
- Generator-supported systems — sizing battery banks to bridge gaps between generator run cycles
In all cases, accurate off-grid battery bank estimation directly impacts system reliability, cost efficiency, and safety.
Units
This calculator uses:
| Unit | Purpose |
|---|---|
| W (watts) | Load power |
| h (hours) | Autonomy time |
| Wh (watt-hours) | Energy storage |
| kWh (kilowatt-hours) | Large energy totals |
| V (volts) | System voltage |
| Ah (amp-hours) | Battery capacity |
| count | Number of batteries |
The core battery equations are unit-neutral: Ah stays Ah, Wh stays Wh, kWh stays kWh, V stays V. Imperial vs metric does not materially change the core off-grid sizing formula.
Practical Tips
When sizing an off-grid battery bank, always start with the actual energy requirement (Wh), not just Ah. This ensures the system voltage is properly accounted for.
For lead-acid batteries, use a conservative 50% DoD to maximize cycle life. Deep discharges significantly reduce the number of charge cycles a lead-acid battery can deliver.
For lithium (LiFePO4) batteries, 80% DoD is a common practical assumption. Some manufacturers allow up to 90–100% DoD, but this may reduce long-term cycle life.
For efficiency, account for all losses in the system: inverter efficiency (typically 90–95%), wiring losses (1–3%), and charge controller losses (2–5%). A combined efficiency of 85–90% is realistic for most off-grid systems.
For off-grid autonomy, conservative assumptions are usually more useful than optimistic ones. Real-world conditions — cloudy days, higher-than-expected loads, aging batteries — all reduce effective capacity.
Important: This calculator provides a strong first-pass off-grid sizing estimate. Final battery bank design should always consider manufacturer specifications, temperature derating, battery aging, surge requirements, charging source capacity, and solar array or generator recharge capability.
Key Facts
- This calculator uses one fixed off-grid sizing model — it sizes from energy demand, applies autonomy, then corrects for efficiency and depth of discharge.
- Usable capacity is not the same as nominal capacity — Victron explicitly describes about 50% practical DoD for AGM and about 80% for LiFePO4.
- System efficiency losses from inverters, wiring, and charge controllers typically range from 5% to 15%.
- Battery performance changes with temperature, discharge rate, age, and charge state — conservative assumptions are usually more useful than optimistic ones.
- A 48 V system requires half the current of a 24 V system for the same power, reducing cable losses.
- For stationary off-grid and energy storage installations, NFPA 855 provides installation-safety context.
Applications
- Off-grid solar battery sizing
- Cabin and remote-site power design
- Backup-independent battery systems
- Telecom and remote monitoring systems
- RV / van / mobile off-grid systems
- Generator-supported battery storage planning
- Comparing 12 V, 24 V, and 48 V systems
- Estimating physical battery count for autonomy targets
Example Calculation
Example Calculation
Given:
- Load power = 600 W
- Autonomy time = 10 h
- System voltage = 24 V
- Depth of discharge = 80%
- Efficiency = 90%
- Battery size = 200 Ah
Step 1: Required load energy
E_load = 600 × 10 = 6,000 Wh
Step 2: Adjust for efficiency
E_adjusted = 6,000 / 0.90 = 6,666.67 Wh
Step 3: Adjust for DoD
E_bank = 6,666.67 / 0.80 = 8,333.33 Wh
Step 4: Convert to Ah
Ah_required = 8,333.33 / 24 = 347.22 Ah
Step 5: Convert to battery count
Battery Count = ⌈347.22 / 200⌉ = 2 batteries
Result:
- Required Battery Capacity: 347.22 Ah
- Required Energy Storage: 8,333.33 Wh (8.33 kWh)
- Usable Battery Capacity: 277.78 Ah
- Recommended Battery Count: 2 batteries
Interpretation: In this example, a bank below about 347 Ah at 24 V would be undersized for the stated assumptions. A two-battery solution at 200 Ah each provides 400 Ah total nominal capacity, which gives practical headroom above the 347 Ah minimum requirement. The final adequacy still depends on real discharge behavior, battery chemistry, and recharge capability.
Standards & References
- Victron Energy Battery Sizing Guide — practical depth-of-discharge and usable capacity references for AGM and LiFePO4 chemistries
- NFPA 855 — Standard for the Installation of Stationary Energy Storage Systems
- NFPA 70E Article 320 — battery and battery-room-related safety considerations
- IEEE 1188 — Recommended Practice for Maintenance, Testing, and Replacement of VRLA Batteries for Stationary Applications
- IEEE 485 — Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications
Limitations
- This calculator is a first-pass off-grid sizing tool, not a full system-engineering package.
- It does not replace manufacturer discharge curves, temperature derating, battery aging analysis, inverter surge review, charger sizing, solar-array sizing, generator recharge planning, or cable-loss review.
- Real off-grid performance depends on temperature, battery age, discharge rate, load variability, charging conditions, and chemistry.
- Victron's technical materials emphasize that battery behavior changes significantly under real conditions, and charging time also depends on depth of discharge and charge current.
- Final performance depends on actual operating conditions — treat this as a strong first-pass off-grid sizing tool.
Common Mistakes to Avoid
- Sizing from raw Ah alone without first calculating required Wh over the actual autonomy period.
- Ignoring depth of discharge, which can make a bank appear far more capable than it is in usable operation.
- Forgetting efficiency losses, especially in inverter-backed off-grid systems.
- Applying the same assumptions to AGM and LiFePO4 even though practical usable DoD often differs — Victron's published material is especially helpful on this point.
- Failing to check whether the recharge source (solar array, generator) can actually refill the bank in time.
- Sizing only for average load and ignoring real autonomy expectations or surge behavior.
Frequently Asked Questions
What does this Battery Bank Sizing for Off-Grid calculator do?
What formula does this calculator use?
Why does depth of discharge matter so much?
Why does efficiency matter in off-grid sizing?
Is Ah enough by itself to size an off-grid battery bank?
Is a larger battery bank always better?
Does imperial or metric mode change the result?
Can this calculator replace installation standards or manufacturer guidance?
Frequently Used Together
Engineers often use these calculators in combination for complete project workflows:
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Calculate
Enter the total continuous load power in watts
Enter the desired off-grid autonomy period in hours (e.g. 24 h = 1 day, 72 h = 3 days)
Typical DoD: 50% for AGM/lead-acid, 80% for LiFePO4
Overall system efficiency including inverter losses — typically 85–95%
Ah rating of one battery at the system voltage