Battery Sizing Calculator

LiFePO4 vs lead-acid: compare physical capacity, weight, cycle life, and 20-year total cost of ownership side by side.

Your requirements

Usable capacity needed?

kWh

Cycles per year?

cycles/yr

LiFePO4 cost?

$/kWh

Lead-acid cost?

$/kWh

Side-by-side comparison

LiFePO4 vs Lead-Acid for 15 kWh usable

Metric	LiFePO4	Lead-Acid
DoD limit	80%	50%
Physical capacity needed	18.8 kWh	30.0 kWh
Weight (approx.)	131 kg	900 kg
Cycle life	4,000 cycles	500 cycles
Round-trip efficiency	95–98%	80–85%
Upfront cost	$7,500	$4,500
Years until replacement	13 yrs	1 yrs
Replacements in 20 yrs	0×	19×
20-year total cost	$7,500	$90,000

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How to Use This Calculator

Enter usable capacity needed

The usable capacity is how much energy you actually need to draw from your battery — not the rated capacity you buy. For example, if you need 10 kWh of usable storage, a lead-acid battery (50% DoD limit) requires a 20 kWh physical bank. A LiFePO4 battery (80% DoD) only needs a 12.5 kWh physical bank. The calculator shows both physical capacity requirements side-by-side.

Set cycles per year

Cycles per year drives the lifespan comparison. Daily cycling (365 cycles/year) is typical for off-grid or time-of-use arbitrage. Emergency backup-only use might be 50–200 cycles/year. The cycle count determines when each chemistry reaches end-of-life and needs replacement — the biggest factor in 20-year total cost of ownership.

Adjust costs per kWh

The default costs reflect 2026 US market prices. LiFePO4 runs $300–$600/kWh depending on brand and format (12V drop-in vs 48V rack-mount). Lead-acid runs $100–$200/kWh for quality AGM. Adjust these to your actual quotes for an accurate comparison.

The Formula

Physical capacity = Usable kWh ÷ DoD (LiFePO4: DoD = 80%, Lead-acid: DoD = 50%) Weight = Physical kWh × kg/kWh (LiFePO4: ~7 kg/kWh, Lead-acid: ~30 kg/kWh) Upfront cost = Physical kWh × Cost per kWh Years until replacement = Cycle life ÷ Cycles per year (LiFePO4: 4,000 cycles, Lead-acid: 500 cycles) 20-year TCO = Upfront cost × (replacements + 1)

The 20-year total cost of ownership (TCO) reveals the true cost difference. Lead-acid's lower upfront cost is often offset by 4–8 replacements over 20 years of daily cycling. LiFePO4's 4,000-cycle life means 0–1 replacements in most residential applications.

Example

Home storage — 15 kWh usable, 300 cycles/year

A homeowner needs 15 kWh of usable battery storage for daily solar self-consumption, cycling 300 times per year. They compare LiFePO4 at $400/kWh vs AGM lead-acid at $150/kWh.

Usable capacity needed15 kWh

Cycles per year300

LiFePO4 Result

Physical capacity (80% DoD)18.75 kWh

Weight~131 kg

Upfront cost$7,500

Lifespan at 300 cycles/yr13 years

20-year total cost$15,000 (2 banks)

Lead-Acid Result

Physical capacity (50% DoD)30 kWh

Weight~900 kg

Upfront cost$4,500

Lifespan at 300 cycles/yr1.7 years

20-year total cost$52,500 (12 banks)

Despite costing $3,000 less upfront, lead-acid costs 3.5× more over 20 years of daily use — and weighs 7× more. For occasional backup use at 50 cycles/year, lead-acid lasts 10 years and becomes cost-competitive.

FAQ

When does lead-acid make more sense than LiFePO4? ▼

Lead-acid wins when: (1) you cycle rarely (backup use, under 100 cycles/year), where its lower upfront cost isn't erased by replacements; (2) you're in a very cold climate — LiFePO4 doesn't charge well below 0°C/32°F without a built-in heater; (3) you need the absolute cheapest upfront cost and have a short time horizon; (4) you're building a first system to learn and don't want to commit to lithium pricing. For daily cycling applications, LiFePO4 is almost always cheaper over 5+ years.

What is round-trip efficiency and how does it affect costs? ▼

Round-trip efficiency is the ratio of energy out to energy in. LiFePO4 at 95–98% means you recover 95–98 kWh for every 100 kWh you put in. Lead-acid at 80–85% loses 15–20 kWh per 100 kWh cycled. For a system cycling 10 kWh/day, lead-acid wastes an extra 1–2 kWh/day — roughly $50–$100/year at $0.15/kWh. Over 20 years, that's an extra $1,000–$2,000 in solar energy wasted or grid power purchased to make up the difference.

What's the weight difference and why does it matter? ▼

LiFePO4 weighs roughly 7 kg/kWh of physical capacity. Lead-acid weighs roughly 30 kg/kWh — about 4× heavier per kWh, and lead-acid needs 2× more physical capacity for the same usable storage. Result: for 15 kWh usable storage, LiFePO4 weighs ~131 kg vs ~900 kg for lead-acid. This matters for structural load on floors, transport logistics, and RV/boat installations where weight is critical.

Are there other lithium chemistries besides LiFePO4? ▼

Yes. NMC (nickel manganese cobalt) is used in Tesla Powerwalls and EV batteries — higher energy density (lighter) but less thermally stable. NCA (nickel cobalt aluminum) is similar. LiFePO4 has lower energy density but excellent thermal stability (much safer, no thermal runaway), 4,000+ cycle life, and a flat discharge curve. For stationary solar storage, LiFePO4 is the dominant choice in 2026 for its safety, longevity, and improving price-per-kWh.

How do I calculate total cost of ownership (TCO)? ▼

TCO = initial purchase cost × (number of replacements + 1). Replacements are determined by lifespan: years = cycle life ÷ cycles per year. For a 20-year analysis, count how many times you replace the bank. Also factor in: installation labor for each replacement, disposal costs (lead-acid batteries require regulated disposal), and the opportunity cost of the wasted energy due to lower efficiency. This calculator handles the hardware purchase cost; add 10–20% for installation and disposal on each replacement.