Battery Bank Sizing Guide 2025
LiFePO4 has replaced lead-acid as the standard for DIY solar. This guide gives you the real math, honest brand comparisons, and real-world examples to size your battery bank correctly the first time—avoiding the two most common mistakes: 12V systems for large loads and underestimating phantom loads.
Battery Chemistry Comparison (2025)
LiFePO4: The New Standard
LiFePO4 (Lithium Iron Phosphate) has effectively replaced lead-acid for 95% of DIY solar, RV, and home backup projects. The combination of safety, longevity, and plummeting prices makes it the default choice.
Current Pricing (2025)
Honest Brand Comparison
EG4 / SOK
Home Backup / Cabins
LiTime / Redodo
Budget DIY / RV
Battle Born
Drop-in Retrofits
Epoch
Marine / Harsh Conditions
Lead Acid: Still Viable?
Generally No
A "cheap" $100 AGM offers only ~50% usable capacity and lasts ~500 cycles. Over 10 years, you'll buy 3-4 sets, costing significantly more than one LiFePO4 bank.
Exceptions
- • Extreme cold: Charging below -20°C without heating
- • Engine starting: High CCA still lead-acid's domain
DIY Cell Builds: No longer recommended. Pre-built server rack batteries with warranties are now nearly as cheap as raw parts.
Sizing Methodology
The "back of napkin" math often fails because it ignores efficiency losses. Use this formula:
Battery Size = (Daily Wh ÷ Inverter Efficiency) × Days of Autonomy × Temperature Factor
Key Variables
Days of Autonomy
Depth of Discharge (DoD)
| Feature | LiFePO4 | Lead Acid |
|---|---|---|
| Max DoD | 100% | 50% |
| Recommended DoD | 80% | 50% |
| Usable capacity | 80-100% | ~50% |
Temperature Derating
LiFePO4 capacity drops to ~80-90%
Capacity drops to ~50-60%
Series vs Parallel & Voltage Selection
Voltage Selection Rule
Configuration Limits
Series (12V → 48V)
Many budget BMS units limit series to 4 units. Warning: Series-connected 12V batteries can drift out of balance. A native 48V battery is always superior to four 12V in series.
Parallel
Preferred for capacity expansion. Keeps voltage safe and constant. Brands like SOK allow up to 10 units in parallel.
"Smart" Battery Issue
Some Bluetooth-enabled batteries struggle in parallel, entering protection mode with slight voltage differences. Check manuals for "Max Parallel" limits.
Load Analysis: The Phantom Menace
Most people calculate the fridge and TV but miss the phantom loads—the power your system uses just to stay "on."
Real-World Consumption
| Appliance | Active Draw | Daily Est. |
|---|---|---|
| Starlink (Standard) | 50-75W | 1,200-1,800 Wh |
| Starlink (High Perf) | 110-150W | 2,600-3,600 Wh |
| 12V RV Fridge | 40-60W (30% cycle) | 300-600 Wh |
| Residential Fridge | 100-200W (30% cycle) | 1,000-1,500 Wh |
| Gas Furnace Fan | 60-100W | 500-1,000 Wh |
| Induction Cooktop | 1,500W | 250-500 Wh |
| Laptop Charging | 60W | 150 Wh/charge |
High-Frequency Inverters (EG4 6000XP, cheap 3kW): Consume ~40-60W just sitting idle. That's 1.2 kWh per day—half a standard 200Ah battery!
Solution: Size your bank to cover this "idle tax" or use "Search Mode" (though this can disrupt clocks/WiFi).
Real-World Sizing Examples
Scenario A: Digital Nomad Van (12V)
Loads: Starlink (1,200Wh), Laptop (300Wh), 12V Fridge (500Wh), Lights/Fan (100Wh)
2,820 Wh ÷ 12.8V = 220 Ah used daily\n220 Ah × 1.5 days autonomy = 330 Ah needed
Common mistake: Buying just 200Ah. You'll drain it to 0% every night with Starlink running.
Scenario B: Off-Grid Cabin (48V)
Loads: Fridge, LED lights, well pump, TV, Starlink, coffee maker
5.7 kWh × 3 days autonomy = 17.1 kWh needed
Note: 15kWh is slightly under ideal 17.1kWh, but acceptable with backup generator for day 3.
Charging Requirements
Charge Rate Rules
e.g., 50A for 100Ah battery
Heat builds up. Stick to 0.5C for longevity.
Cost Analysis (2025)
| System Size | Configuration | Est. Cost | $/kWh |
|---|---|---|---|
| 2.5 kWh | 2× 100Ah 12V (Budget) | $350-400 | ~$150 |
| 5.1 kWh | 1× 48V Server RackBest Value | $1,100-1,300 | ~$235 |
| 14.3 kWh | 1× EG4 WallMount | ~$3,500 | ~$245 |
| 30 kWh | 6× Server Rack | ~$7,000 | ~$230 |
Safety & Installation
Fusing: The Class T Requirement
LiFePO4 batteries have incredibly low internal resistance, meaning they can dump thousands of amps in a short circuit.
ANL Fuses
OK: Small 12V banks
Dangerous: Large 48V banks (arc interruption capacity too low—can cause fire)
Class T Fuses
Mandatory for large banks. Can safely interrupt 20,000+ amps. Use on your main battery terminal.
Wire Sizing by Voltage
For a 3,000W inverter:
| Feature | 12V System | 48V System |
|---|---|---|
| Current draw | 250A+ | ~60A |
| Wire needed | 4/0 AWG (massive) | 4-2 AWG (manageable) |
| Cable cost | High | Low |
| Installation | Difficult | Easy |
FAQ & Expert Tips
Expert Tips
"Oversize the Bank, Undersize the Solar"
It's easier to add panels later than to mix old and new batteries. Buy the biggest battery bank you can afford upfront.
The "Pre-Charge" Resistor
When connecting a large inverter to LiFePO4 batteries, use a pre-charge resistor (or battery with this built-in like EG4/SOK) to avoid the spark that can fry BMS units.
Buy for the Cold
If you live north of the Mason-Dixon line, buy self-heating batteries. External heating pads are inefficient compared to internal BMS-controlled heating.
Myths vs Reality
Myth: "You can mix old and new lithium batteries easily."
Reality: While safer than mixing lead-acid, mixing a 3-year-old degraded battery with a brand new one will limit the new one's performance.
Myth: "LiFePO4 is dangerous like EV batteries."
Reality: LiFePO4 is chemically much more stable than the NMC chemistry in cars/phones. They are extremely difficult to ignite.