Battery Sizing Calculator
Calculate how many batteries you need for solar. Size off-grid, grid-tied backup, and whole-home battery systems with chemistry comparison, temperature derating, and NEC 706 compliance checks — free.
Battery Sizing Calculator
Enter your daily load, autonomy requirement, and battery chemistry to get battery count, inverter size, and estimated system cost.
| LFP | Lead-Acid | NMC |
|---|
| Property | LFP (LiFePO4) | Lead-Acid (AGM/FLA) | NMC (Li-Ion) |
|---|---|---|---|
| Usable DoD | 80–90% | 50% | 80–85% |
| Round-Trip Efficiency | 95–98% | 75–85% | 90–95% |
| Cycle Life | 3,000–10,000 | 400–1,000 | 1,000–2,500 |
| Calendar Life | 10–15 years | 3–5 years | 7–10 years |
| Self-Discharge | 2%/month | 5%/month | 2%/month |
| Weight | ~7 kg/kWh | ~25 kg/kWh | ~6 kg/kWh |
| Installed Cost | $400–$800/kWh | $150–$400/kWh | $500–$900/kWh |
| $/kWh Delivered | $0.06–$0.15 | $0.20–$0.50 | $0.12–$0.25 |
| Charge Below 0°C | ✗ No | ✓ Yes (slow) | ✗ No |
| Maintenance | None | FLA: water topping | None |
| Thermal Safety | Excellent | Good | Moderate |
| Best For | All solar storage | Tight budget, short-term | Compact / lightweight |
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Three Battery System Modes
Size battery storage for any application — off-grid homesteads, backup power for critical loads, or whole-home coverage during extended outages.
Off-Grid
Standalone systems with no grid connection. Autonomy is set in days (1–10) to ensure the battery bank can sustain the full load through cloudy periods without solar charging.
- Multi-day autonomy sizing
- Full daily load coverage
- Conservative safety margins
Grid-Tied Backup
Grid-connected systems sized to power critical loads during outages. Autonomy is set in hours (2–72) — enough to ride through typical utility outages without oversizing the battery bank.
- Hours-based autonomy
- Critical load focus
- Cost-optimized bank size
Whole-Home Backup
Full household coverage including HVAC, appliances, and EV charging. Sized for extended outages where the entire home load must be sustained without grid support.
- Full home load coverage
- High peak/surge load handling
- Extended autonomy planning
Why Solar Professionals Use This Calculator
Go beyond a simple kWh estimate. Account for chemistry, temperature, and real-world losses before you spec a battery system.
Chemistry-Specific Sizing
LFP, Lead-Acid, and NMC each have different depth of discharge limits, round-trip efficiency, and temperature derating curves. The calculator applies the right factors for each chemistry automatically.
Temperature Derating
Battery capacity drops significantly in cold conditions. Enter your installation temperature (-20°C to +50°C) and the calculator adjusts usable capacity accordingly — critical for outdoor and garage installs.
Battery Bank Configuration
Outputs exact series and parallel string counts for your target system voltage, plus a wiring diagram and 15-year degradation projection — ready to hand off to the homeowner or AHJ.
How It Works
From load input to battery configuration in five steps.
Select System Mode
Choose Off-Grid, Grid-Tied Backup, or Whole-Home Backup. The mode sets the autonomy input type — days for off-grid, hours for backup systems.
Enter Load & Autonomy
Input daily energy consumption (kWh/day), your required autonomy period, and peak surge load in kW. These three numbers define the gross energy the battery bank must deliver.
Configure Battery Chemistry & Voltage
Select LFP, Lead-Acid, or NMC and your target system voltage (12V, 24V, 48V). Enter individual battery capacity in Ah. The tool auto-fills chemistry-specific DoD and efficiency defaults.
Set Environmental Conditions
Enter installation temperature and climate conditions. Cold environments reduce usable capacity — the calculator applies interpolated temperature derating factors per chemistry to give an accurate bank size.
Get Results
View required capacity (kWh installed and usable), battery count, series × parallel configuration, inverter size recommendation, estimated cost, and a 15-year degradation projection.
Built for Every Solar Professional
Solar Installers & System Designers
Spec battery banks with the right chemistry, voltage, and configuration for each project. Generate the series/parallel wiring layout and NEC 706 compliance callouts before ordering equipment.
Solar Sales Representatives
Show homeowners exactly how many batteries they need, what it will cost, and how long the system will last. The chemistry comparison table helps customers understand the trade-offs between LFP and lead-acid without a technical deep dive.
Off-Grid & Rural Electrification
Size battery banks for remote properties with no grid access. Use multi-day autonomy settings and conservative safety margins to ensure the system survives extended cloudy periods in any climate.
Calculation Methodology
Seven sequential adjustments from gross energy demand to final installed capacity.
Gross Energy Requirement
Daily Load (kWh) × Autonomy Period (days or hrs) Starting point — total energy the battery bank must deliver across the full autonomy window before any derating factors are applied.
Depth of Discharge Adjustment
Gross Energy ÷ DoD (%) Accounts for the portion of battery capacity that cannot be discharged without damaging cycle life. LFP default is 80%, lead-acid 50%, NMC 80%.
Round-Trip Efficiency Loss
DoD Result ÷ Round-Trip Efficiency (%) Energy lost in the charge/discharge cycle. LFP: ~95% RTE, NMC: ~92%, lead-acid: ~80%. Higher losses mean a larger bank is needed to deliver the same usable energy.
System Efficiency (Inverter + Wiring)
RTE Result ÷ (Inverter Eff. × Wiring Eff.) Inverter and wiring losses are applied together. A 95% inverter and 97% wiring efficiency compounds to ~92% — roughly 8% more capacity is needed to compensate.
Temperature Derating
System Loss Result ÷ Temperature Factor Capacity drops in cold conditions. At -10°C, LFP retains about 80% capacity; lead-acid drops to ~65%. The calculator uses interpolated lookup tables per chemistry from -20°C to +50°C.
Safety Margin & Bank Configuration
Temp Result × Safety Margin → Series × Parallel Strings A 1.1–1.3× safety margin is applied, then battery count is calculated: series strings to reach target voltage, parallel strings to reach required amp-hours. Inverter size = peak load × 1.25.
LFP vs Lead-Acid vs NMC
The right chemistry depends on your budget, climate, and cycle requirements. Here's how the three compare.
| Spec | LFP (LiFePO₄) | Lead-Acid (FLA/AGM) | NMC (Li-Ion) |
|---|---|---|---|
| Cycle Life | 3,000–6,000+ cycles | 500–1,200 cycles | 1,000–3,000 cycles |
| Usable DoD | 80–90% | 50% | 80% |
| Round-Trip Efficiency | ~95% | ~80% | ~92% |
| Cost per kWh | $300–$600 | $100–$250 | $400–$800 |
| Thermal Safety | Excellent — no thermal runaway risk | Good | Requires BMS protection |
| Maintenance | None | FLA: monthly water checks | None |
| Best For | Long-term solar storage | Low-budget off-grid | Space-constrained installs |
Pro Tips for Battery Sizing
Size for winter, not summer
Battery capacity drops in cold weather and solar production drops in winter. If you size for summer conditions, you'll have an undersized system when you need it most. Use the coldest expected installation temperature in the calculator.
Don't discharge lead-acid past 50%
Consistently discharging lead-acid past 50% DoD cuts cycle life in half. If the project needs deep daily cycling, LFP at 80–90% DoD is more cost-effective over the system's lifetime despite the higher upfront cost.
Check the 120% busbar rule before adding storage
Adding a battery inverter to a grid-tied system means another backfeed breaker on the panel. Run the busbar rule check before spec'ing the inverter size — a panel upgrade could add significant cost to the project.
FLA batteries need ventilation — per NEC 706.30
Flooded lead-acid batteries produce hydrogen gas during charging. NEC 706.30 requires ventilation in the battery enclosure. Factor in venting costs when comparing FLA to sealed AGM or LFP options for indoor installs.
Frequently Asked Questions
How many batteries do I need for solar?
It depends on your daily energy use, how many hours or days of backup you need, and your battery chemistry. A typical home using 30 kWh/day wanting 1 day of off-grid backup with LFP batteries (80% DoD, 95% RTE) needs roughly 40 kWh of installed capacity — about 8 × 5 kWh LFP batteries. Use this calculator with your actual numbers for an accurate bank size.
What is depth of discharge and why does it matter?
Depth of discharge (DoD) is the percentage of a battery's total capacity that can be used without damaging cycle life. LFP batteries can safely reach 80–90% DoD. Lead-acid batteries should not go below 50% DoD — doing so regularly can halve their cycle life. A 100 Ah lead-acid battery at 50% DoD delivers only 50 Ah of usable energy, so you need twice as many batteries to match an LFP bank at 80% DoD.
Which battery chemistry is best for solar storage?
LFP (LiFePO₄) is the best choice for most solar storage applications. It has the highest cycle life (3,000–6,000+ cycles), the best thermal safety, no maintenance, and the lowest lifetime cost per kWh cycled despite higher upfront cost. Lead-acid is cheaper upfront but has lower DoD, shorter cycle life, and requires maintenance (FLA). NMC has higher energy density but higher cost and thermal risk. Use the chemistry comparison table in the calculator to compare for your specific project.
How does temperature affect battery capacity?
Cold temperatures reduce usable battery capacity. At 0°C, LFP retains about 90% of rated capacity; lead-acid drops to around 80%. At -20°C, LFP is at roughly 65% and lead-acid at 40%. This means a battery bank sized for summer conditions can be significantly undersized in winter. Always enter your coldest expected installation temperature in the calculator to get an accurate bank size for your climate.
What system voltage should I use — 12V, 24V, or 48V?
48V is the standard for most residential and commercial solar storage systems. Higher voltage means lower current for the same power, which reduces wire sizing requirements and heat losses. 12V and 24V systems are typically limited to small off-grid cabins or RV/marine applications under 3 kW. For any system above 1–2 kW, 48V is almost always the right choice.
Does this calculator follow NEC Article 706?
The calculator includes NEC Article 706 compliance callouts — including the FLA ventilation requirement under NEC 706.30 and inverter sizing guidance. It is a sizing tool, not a permit package. Always verify your final design against the NEC edition adopted by your local AHJ, and consult a licensed electrician for permit submission.
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