EV Battery Second-Life ROI Calculator

Introduction

Second-life EV batteries—packs removed from vehicles after years of driving—often still retain meaningful usable capacity. Many packs are retired from automotive service when they no longer meet range expectations, not because they are unusable. In stationary applications (home storage, small commercial storage, off-grid cabins, or backup power), a repurposed pack can still deliver value by shifting energy from low-value times to high-value times, increasing solar self-consumption, or reducing generator runtime.

This calculator estimates three practical outputs from a simplified financial model: annual savings, payback period, and lifetime value. You can adjust the assumptions to match your situation—battery capacity, total acquisition and retrofit cost, round-trip efficiency, remaining cycle life, the value of each delivered kilowatt-hour, and typical daily throughput. The goal is not to replace a full engineering study; it is to provide a fast, transparent ROI estimate you can use to compare scenarios before committing time and money.

How to use

1. Battery Capacity (kWh): Enter the usable capacity you expect to access in stationary service. If the pack is rated at 40 kWh but you plan to limit depth-of-discharge for longevity, enter the smaller usable number.
2. Acquisition & Retrofit Cost ($): Include the purchase price of the pack plus integration costs (inverter, BMS, wiring, enclosure, labor, permits, and any required safety equipment).
3. Round-Trip Efficiency (%): The fraction of energy you get back after charging and discharging. For example, 90% means 10% losses.
4. Remaining Cycles: The number of additional full-equivalent cycles you expect before the battery reaches an end-of-life threshold for your use (often defined by capacity fade, internal resistance, or safety constraints).
5. Value per kWh ($): The economic value of each delivered kWh from the battery. This is commonly the difference between peak and off-peak rates (time-of-use arbitrage), the retail rate avoided by using stored solar at night, or an avoided generator cost.
6. Daily Throughput (kWh): How many kWh you expect to move through the battery per day on average. This can be less than capacity (partial cycling) or more than capacity (multiple cycles per day). The lifetime calculation in this tool caps throughput at capacity to avoid overstating lifetime value.
7. Click Compute ROI to display results. Use Copy Result to share the output.

Formula and assumptions

The model treats savings as proportional to energy delivered from the battery. Let:

• C = battery capacity (kWh)
• cost = acquisition & retrofit cost ($)
• η = round-trip efficiency (decimal, e.g., 0.90)
• cycles = remaining full-equivalent cycles
• p = value per delivered kWh ($/kWh)
• n = daily throughput (kWh/day)

Daily savings are computed as: $Sd=n\times \eta \times p$ and annual savings are S_a = S_d × 365.

Payback period is estimated as: $P=\frac{\mathrm{cost}}{S}$ (years). If annual savings are very small, payback becomes very large.

Lifetime value is estimated from remaining cycles and a conservative cap on daily throughput: $V=\min (n,C)\times \eta \times p\times \mathrm{cycles}$ This reflects total value from the remaining full-equivalent cycles, assuming each cycle delivers roughly the capped throughput.

Important: the calculator uses value per kWh as a single blended number. In real projects, value varies by hour, season, tariff structure, demand charges, export compensation, and outage frequency. Treat the result as a planning estimate.

Worked example

Imagine you acquire a 40 kWh second-life pack and spend $5,000 total on the pack plus retrofit. You expect 90% round-trip efficiency and 2,000 remaining full-equivalent cycles. You plan to move about 20 kWh/day through the battery (partial cycling), and you estimate the delivered energy is worth $0.20/kWh (for example, a mix of time-of-use arbitrage and solar self-consumption).

• Daily savings: 20 × 0.90 × $0.20 = $3.60/day
• Annual savings: $3.60 × 365 = $1,314/year
• Payback: $5,000 ÷ $1,314 ≈ 3.8 years
• Lifetime value: min(20, 40) × 0.90 × $0.20 × 2,000 = $7,200

In this scenario, the battery pays back before the expected cycle life is exhausted, and the lifetime value exceeds the upfront cost. If your true value per kWh is lower (for example, $0.10/kWh), payback roughly doubles. If your throughput is lower because you only cycle on sunny days, annual savings drop accordingly.

Limitations and practical considerations

This tool intentionally simplifies several real-world factors. Use it as a screening calculator, then validate with project-specific data.

• Degradation is not modeled dynamically. Real batteries lose capacity and efficiency over time, and degradation depends on temperature, depth-of-discharge, C-rate, and calendar aging.
• Throughput is averaged. Actual cycling varies by season, household behavior, solar production, and tariff periods.
• Value per kWh is a simplification. Time-of-use spreads, export rates, demand charges, and outage costs can change the economics substantially.
• Soft costs and compliance are not included. Permits, inspections, insurance requirements, fire code compliance, and professional installation can materially affect total cost.
• Safety and suitability matter. Second-life packs can pose hazards if damaged or improperly integrated. Always use appropriate battery management, fusing, enclosures, and qualified installers.

If you want to compare common use cases, the scenario table below provides representative value per kWh assumptions. These are examples only—your local rates and avoided costs may be higher or lower.