When enthusiasts and off-grid builders search for the Model S battery size, they are usually looking at two very different numbers. At the vehicle level, Tesla produced Model S packs in 60 kWh, 85 kWh, 90 kWh, and 100 kWh total capacities. But for the DIY solar and power storage community, the entire pack is irrelevant. The real focus is on the individual 5.2 kWh module—the building block of the 85/90 kWh packs. Salvaged from wrecked EVs, these 18650-based modules offer massive energy density at a fraction of the cost of new server-rack batteries.

However, repurposing EV modules for stationary solar storage requires a fundamental shift in how you manage charge rates, thermal limits, and system architecture. Unlike plug-and-play LiFePO4 rack batteries, raw Model S modules demand custom busbars, external battery management systems (BMS), and strict adherence to lithium fire-safety protocols.

Tesla Model S Module Specs and Sizing Math

Before wiring anything, you need the exact bench specifications. The standard 5.2 kWh module from an 85/90 kWh Model S pack is configured internally as 6S74P (6 series groups of 74 parallel 18650 NCA cells). This yields a nominal voltage that sits perfectly in the middle of a 24V system, but requires two in series for a standard 48V inverter setup.

Table 1: Tesla Model S 5.2 kWh Module Specifications (85/90 kWh Pack Era)
Parameter Value Notes / Tolerances
Internal Configuration 6S74P Panasonic NCR18650B (NCA Chemistry)
Nominal Voltage 22.8V Based on 3.8V per cell nominal
Operating Voltage Range 18.0V - 25.2V 3.0V low cutoff to 4.2V max per cell
Rated Capacity 232 Ah Measured at 0.2C discharge rate
Total Energy ~5.29 kWh Usable energy depends on DoD limits
Weight 55 lbs (25 kg) Requires heavy-duty racking
Dimensions (L x W x H) 26.5' x 11.5' x 3.2' Approximate; varies slightly by revision

Calculating Usable Capacity and Sizing Math

When sizing your battery bank, you cannot use the raw 5.29 kWh figure. You must apply Depth of Discharge (DoD) limits and system efficiency factors. Furthermore, we must address Peukert's Law. While Peukert's exponent heavily penalizes lead-acid batteries at high discharge rates (k ≈ 1.3), the lithium-ion NCA cells in these modules exhibit a near-ideal Peukert exponent (k ≈ 1.05). This means you actually get nearly the rated Ah even at high C-rates, though heat generation becomes your primary limiting factor.

The Sizing Formula:
Usable Energy = (Module kWh × Number of Modules) × DoD × Inverter Efficiency

Worked Example: You are building a 48V system using 4 modules (2 in series, 2 parallel strings).
Total Raw Capacity = 5.29 kWh × 4 = 21.16 kWh.
Apply 85% DoD (to maximize NCA cycle life) = 17.98 kWh.
Apply 93% Inverter Efficiency = 16.72 kWh of usable AC energy.

System Architecture: Source to Load and Inverter Sizing

Raw EV modules do not have built-in communication protocols that off-the-shelf solar charge controllers understand. You must build a complete system block from source to load, inserting an external BMS to act as the brain.

System Block Flow:
Solar PV Array → MPPT Charge Controller (e.g., Victron SmartSolar 250/100) → DC Disconnect → External BMS (e.g., Orion-2 or Electrodacus) → Model S Modules → DC Disconnect → Hybrid Inverter (e.g., Schneider XW Pro) → AC Main Panel.

Inverter and Charger Sizing for the Stated Load

Your inverter and charge controller must be sized not just for the load, but to keep the battery within its safe C-rate limits. A standard 48V system built from two Model S modules in series (45.6V nominal) has a total capacity of 232 Ah.

  • Continuous Discharge Limit (0.5C): 116A. At 45.6V, this equates to roughly 5,200W of continuous DC power.
  • Inverter Sizing: A 4000W to 5000W continuous 48V inverter is the sweet spot. It will pull roughly 90A to 110A at full rated load, keeping you safely under the 0.5C continuous threshold. If you need a 6000W+ inverter, you must add a third parallel string of modules to keep the per-module amperage down.
  • Charger/MPPT Sizing: Limit charge current to 0.3C for longevity (approx. 70A). A 100A MPPT charge controller is ideal, provided you configure the software to cap the output current to 70A.

Series vs. Parallel Wiring and Charge/Discharge Limits

Understanding how to manipulate voltage and amperage through wiring topology is critical when mixing EV modules with standard 48V off-grid inverters.

Series vs. Parallel Consequences

  • Series Wiring: Voltages add, Amp-hours remain the same. Wiring two 22.8V nominal modules in series yields 45.6V nominal (perfect for a 48V inverter) at 232 Ah. Never wire more than two in series; three in series yields ~68V, which will instantly destroy most standard 48V inverters and MPPT controllers.
  • Parallel Wiring: Amp-hours add, Voltage remains the same. Wiring two 45.6V series-strings in parallel yields 45.6V at 464 Ah (~21 kWh).
⚠️ CRITICAL LITHIUM FIRE-SAFETY WARNING: Never parallel mismatched modules. If you parallel a module with 70% State of Health (SoH) alongside a module with 95% SoH, the healthier module will force massive equalization currents into the degraded one, bypassing the BMS limits. This can cause localized heating, venting of NCA cells, and catastrophic thermal runaway. Only parallel modules that have been top-balanced, capacity-tested, and matched within 2% of each other. Furthermore, NCA chemistry is significantly more volatile than LiFePO4. Modules must be kept in compression, housed in a fire-rated enclosure, and equipped with individual cell-level thermal fuses.

Charge and Discharge Limits (C-Rates and DoD)

While a Tesla vehicle's thermal management system allows these cells to sustain 2C or 3C discharge rates during hard acceleration, a stationary DIY powerwall lacks active liquid cooling. You must rely on passive air cooling and strict C-rate limits.

  • Continuous Discharge: 0.5C (116A per module). Sustaining higher than this without active cooling will cause the internal cell temperatures to creep past 40°C, accelerating degradation.
  • Peak Discharge: 1.0C (232A) for a maximum of 30 seconds (e.g., to cover inverter surge loads when starting a well pump or air compressor).
  • Charge Rate: 0.3C (70A) is optimal. Pushing 1C charge rates into stationary NCA modules without liquid cooling plates will cause lithium plating on the anodes, permanently destroying capacity and creating internal short-circuit risks.
  • Depth of Discharge (DoD): Set your BMS low-voltage cutoff to 3.2V per cell (19.2V per module) and high-voltage cutoff to 4.1V per cell (24.6V per module). Avoiding the absolute extremes (3.0V and 4.2V) will triple your cycle life, easily pushing past 3,000 cycles.

Decision Tree: Are Salvaged Model S Modules Right for Your Build?

Repurposing EV batteries is not a simple plug-and-play endeavor. Use the decision matrix below to determine if the Model S battery size and format align with your project constraints, or if you should pivot to commercial LiFePO4 server-rack batteries.

Table 2: Salvaged Model S NCA Modules vs. New LiFePO4 Server Rack Batteries
Criteria Salvaged Tesla Model S Modules (NCA) New LiFePO4 Server Rack (e.g., SOK, EG4)
Cost per kWh (Usable) ~$120 - $160 (Requires DIY BMS, busbars, fuses) ~$220 - $280 (Plug-and-play, BMS included)
Energy Density / Footprint Extremely high; compact footprint for high kWh Moderate; requires standard 19-inch server racks
Thermal Runaway Risk High (NCA chemistry); requires strict safety protocols Very Low (LFP chemistry); inherently stable
Integration Effort High; requires custom wiring, external BMS, compression Low; RS485/CANbus communication with hybrid inverters
Best Use Case Space-constrained off-grid cabins, high-DIY skill builders Residential solar backup, code-compliant garage installs

If you choose the Model S route, your first purchase after the modules should be a high-quality BMS capable of handling the specific voltage taps of a 6S configuration, such as the OpenInverter community's recommended BMS boards. Properly managed, the energy density and raw power delivery of these salvaged modules remain unmatched in the DIY space, provided you respect the chemistry and the math.

For deeper insights into managing lithium-ion degradation and safety in stationary applications, refer to the lifecycle data published by Battery University and the NREL battery testing databases. Always verify your local AHJ (Authority Having Jurisdiction) requirements regarding indoor installation of salvaged NCA lithium batteries, as many residential fire codes strictly limit their use to detached, fire-rated outbuildings.