Solar Battery Sizing Calculator: How To Size Your Battery Bank (+ LiFePO4 Guide)

I sized my own battery bank the hard way — two years of reading forums, watching Will Prowse teardowns, and second-guessing every decision. The formula itself is simple (three numbers multiplied and divided). The hard part is choosing between LiFePO4 and lead-acid, picking the right voltage, and understanding that "10 kWh battery" does not mean 10 kWh of usable energy. This guide gives you the formula, the calculator, and the context to make the decision in an afternoon rather than a month.

How To Size A Solar Battery Bank (Quick Formula)

Battery kWh = Daily kWh usage × Days of autonomy ÷ Depth of Discharge

Quick Reference

Most homes need 10–20 kWh for basic overnight backup (grid-tied with battery). Full off-grid homes need 20–50 kWh depending on usage and location.

Solar Battery Sizing Calculator

Enter your daily energy usage, desired backup days, battery chemistry, and system voltage. The calculator outputs total battery capacity needed, number of common battery sizes, estimated cost, and a 25-year lifetime cost comparison between LiFePO4 and lead-acid.

Step-By-Step: How To Calculate Your Battery Bank Size

Step 1 — Determine Daily kWh Usage

For grid-tied backup: Look at your electric bill. Take your monthly kWh and divide by 30. If you only want to back up essential loads (fridge, lights, internet, phone charging, sump pump), your critical daily usage is typically 3–8 kWh — much less than your total daily usage.

For off-grid: Add up everything you plan to run:

A small off-grid cabin uses 3–6 kWh/day. An average off-grid home (without AC or electric heating) uses 8–15 kWh/day. A full off-grid home with mini-split AC and electric cooking uses 15–30 kWh/day.

Step 2 — Choose Days Of Autonomy

Step 3 — Account For Depth Of Discharge

LiFePO4: 80–90 % usable. A 10 kWh LiFePO4 battery gives you 8–9 kWh before the BMS cuts off to protect cell longevity. This is the standard DoD for sizing calculations.

Lead-acid (AGM or flooded): 50 % usable. Discharging lead-acid below 50 % dramatically shortens cycle life. A 10 kWh lead-acid battery gives you only 5 kWh of usable energy. This means you need twice the nominal capacity compared to LiFePO4.

Step 4 — Choose System Voltage

48V is the standard for home energy storage. Lower current means thinner cables, smaller fuses, smaller charge controllers, and less voltage drop. Every serious home battery system in 2026 uses 48V.

Step 5 — Calculate And Select Batteries

Example: Average home backup

• Daily usage: 10 kWh
• Autonomy: 2 days
• Chemistry: LiFePO4 (80 % DoD)
• Voltage: 48V

Battery kWh = 10 × 2 ÷ 0.80 = 25 kWh
Battery Ah  = 25,000 Wh ÷ 48V = 521 Ah

Battery selection: Five 48V 100Ah (5.12 kWh each = 25.6 kWh total) or three 48V 200Ah (10.24 kWh each = 30.7 kWh total, slight oversize).

What Is A LiFePO4 Battery? (And Why It Is Best For Solar)

LiFePO4 stands for lithium iron phosphate — a lithium battery chemistry that uses iron phosphate (FePO4) as the cathode material instead of the nickel-manganese-cobalt (NMC) or nickel-cobalt-aluminum (NCA) used in EV batteries and Tesla Powerwalls.

Why LiFePO4 Is The Best Chemistry For Solar Storage

1. Cycle life: 4,000–6,000 cycles at 80 % DoD. At one cycle per day, that is 11–16 years. Lead-acid AGM lasts 500–1,000 cycles (1.5–3 years at daily cycling).

2. Depth of discharge: 80–90 % vs 50 % for lead-acid. You get 60–80 % more usable energy from the same nominal capacity.

3. Safety: No thermal runaway. The iron phosphate cathode is thermally stable — it does not decompose or release oxygen at high temperatures. NMC lithium (Tesla Powerwall chemistry) can enter thermal runaway above 210 °C. LiFePO4 remains stable above 270 °C.

4. Flat discharge curve. LiFePO4 maintains nearly constant voltage from 90 % to 20 % state of charge (3.2–3.3 V per cell). This means consistent power output until the battery is nearly empty. Lead-acid voltage sags progressively as it discharges.

5. Zero maintenance. No watering, no equalization charges, no acid off-gassing, no corrosion. Install and forget.

6. Weight: 50–60 % lighter than lead-acid for the same usable energy. A 5.12 kWh LiFePO4 server rack battery weighs ~100 lbs. An equivalent lead-acid bank (10.24 kWh nominal for 5.12 kWh usable at 50 % DoD) weighs ~300 lbs.

The One Disadvantage: Upfront Cost

LiFePO4 costs $300–$500 per kWh vs $100–$250 per kWh for lead-acid. But when you factor in usable capacity (DoD), cycle life, and replacements over 25 years, LiFePO4 is the cheapest option per kWh delivered:

LiFePO4 is 3–5× cheaper per kWh delivered over its lifetime. The upfront premium pays for itself within 3–5 years of daily cycling.

Popular LiFePO4 Battery Sizes For Solar

Size Guide By Application

The Tesla Powerwall comparison: A Powerwall 3 stores 13.5 kWh at about $9,200 installed ($681/kWh). Three 48V 100Ah LiFePO4 server rack batteries store 15.4 kWh at about $4,500–$7,500 ($290–$490/kWh) plus a compatible hybrid inverter ($2,000–$4,000). The DIY route is 30–50 % cheaper but requires more setup and has no Tesla app integration. See How Many Amp-Hours Is A Tesla Powerwall for the detailed Powerwall specs.

How To Charge LiFePO4 Batteries With Solar

LiFePO4 requires a charge controller with a LiFePO4 charging profile — which most modern MPPT controllers support (Victron, EPever, Renogy, and others all include LiFePO4 presets).

LiFePO4 Charging Parameters (12V Battery)

Critical: Never charge LiFePO4 below 0 °C. Charging below freezing causes lithium plating on the anode, permanently damaging the cells. Most pre-built LiFePO4 batteries include a BMS with built-in low-temperature cutoff. Some (SOK, EG4) include a self-heating pad that warms the cells before charging resumes.

Controller selection: Use MPPT for any system over 200 W — it delivers 25–43 % more energy to the battery than PWM. See MPPT vs PWM Charge Controller for the full comparison and sizing calculator. For wiring panels to the controller and battery, see How To Connect Solar Panels To A Battery.

LiFePO4 Portable Power Stations

Portable power stations are all-in-one units with a built-in LiFePO4 battery, inverter, charge controller, and outlets. They are not the same as a home battery bank — they are smaller, self-contained, and designed for camping, emergencies, and portable use.

Popular LiFePO4 portable stations: EcoFlow Delta 2 Max (2 kWh), Bluetti AC200MAX (2 kWh), Jackery Explorer 2000 Plus (2 kWh), Goal Zero Yeti 3000X (3 kWh).

For home backup, a dedicated battery bank + inverter is more cost-effective and expandable. For portable use, a power station is convenient but expensive per kWh.

DIY LiFePO4 Battery Bank

Building from raw prismatic cells (the EVE LF280K 280Ah cell is the most popular) saves 30–50 % vs pre-built batteries. A 48V 280Ah DIY bank (14.3 kWh) costs roughly $1,500–$2,500 in parts vs $3,500–$5,000 pre-built.

What you need: 16 cells (for 48V nominal: 16 × 3.2V = 51.2V), a BMS (JBD, Daly, or JK, rated for your max current), bus bars, compression rods, a case or rack, and fuses.

Risks: No manufacturer warranty, assembly requires care (torque specs on bus bars, cell balancing before first use, BMS configuration), and LiFePO4 cells at full charge hold significant stored energy — short circuits can be dangerous. This is a project for people comfortable with electrical work, not a beginner build.

How Many Batteries For Off-Grid Solar?

Off-grid sizing tip: Size your battery bank for your worst month production, not the annual average. If your solar panels produce only 40 % of their annual average in December (typical at 42°N latitude), your battery bank must cover the gap. A backup generator for the worst 2–3 weeks of winter is often more cost-effective than doubling the battery bank. See Solar Panels And Snow — Winter Output for monthly production expectations.

Common Misreadings

1. "A 10 kWh battery gives me 10 kWh." Only with LiFePO4 at 80 % DoD — you get 8 kWh. With lead-acid at 50 % DoD, you get only 5 kWh. Always size based on usable capacity, not nominal.

2. "Lead-acid is cheaper than LiFePO4." Only upfront. Over 25 years, LiFePO4 costs $0.08–$0.13 per kWh delivered vs $0.38–$0.63 for lead-acid. LiFePO4 is 3–5× cheaper per lifetime kWh.

3. "I should size my battery to match my solar panels." Battery size depends on your usage and backup needs, not your panel size. A 10 kW solar system might pair with a 10 kWh battery (grid backup) or a 50 kWh battery (full off-grid) depending on the application.

4. "I can charge LiFePO4 below freezing." No. Charging below 0 °C causes permanent lithium plating damage. The BMS should block it, but verify your battery has a low-temperature cutoff before relying on it in cold climates.

5. "Car batteries work for solar." Car batteries are starting batteries, not deep-cycle. One deep discharge can permanently damage them. Use deep-cycle LiFePO4, AGM, or flooded lead-acid designed for repeated cycling.

Bottom Line

LiFePO4 is the best battery chemistry for solar in 2026. It costs more upfront but delivers 3–5× more lifetime energy than lead-acid, with zero maintenance and no risk of thermal runaway. Size your bank with the formula: daily kWh × days of autonomy ÷ 0.8 (DoD). Use 48V for any home system. Budget $300–$500 per kWh for pre-built LiFePO4, or $150–$300 per kWh for a DIY build from raw cells.

Keep Reading

• MPPT vs PWM Charge Controller (+ Sizing Calculator)
• How To Connect Solar Panels To A Battery
• Solar Panel Charge Time Calculator
• What Size Solar Panel To Charge A 100Ah Battery
• How Long To Charge A 12V Battery With A 100W Panel
• How Many Amp-Hours Is A Tesla Powerwall
• How To Wire Solar Panels — Series vs Parallel
• String Inverter vs Microinverter vs Power Optimizer
• How Many Solar Panels To Power A House
• Solar Panels And Snow — Winter Battery Sizing
• Are Solar Panels Worth It?