When DIY solar builders and off-grid enthusiasts talk about repurposing EV cells, the Tesla Model S battery size is the gold standard for second-life energy storage. Depending on the production year, the main pack comes in 85 kWh, 90 kWh, and 100 kWh total capacities. However, you rarely buy a whole pack for a home system; you buy individual modules. A classic 85 kWh pack consists of 16 identical modules, each offering roughly 5.3 kWh of usable energy. When configured correctly, these 18650-based modules provide exceptional energy density, high continuous discharge rates, and a proven track record.
This guide breaks down the exact module specifications, system architecture, and sizing math required to turn salvaged Tesla Model S modules into a reliable 48V home battery bank.
Anatomy of the Pack: Module Specs and System Block
Before wiring anything, you need to understand what is inside the aluminum casing. The most common module on the salvage market is from the 85 kWh pack (part number 1014114-00-E). Internally, it is wired in a 6s74p configuration using Panasonic NCR18650B cells. This means 6 cells in series (for voltage) and 74 parallel strings (for capacity).
| Parameter | Value | Notes |
|---|---|---|
| Configuration | 6S74P | 444 total 18650 cells per module |
| Nominal Voltage | 22.2V | Based on 3.7V per cell |
| Max Charge Voltage | 25.2V | 4.2V per cell absolute max |
| Capacity | ~250 Ah | 74p × ~3.4 Ah |
| Total Energy | ~5.3 kWh | 22.2V × 250Ah |
| Weight | 55 lbs (25 kg) | Includes aluminum casing and cooling tubes |
The Source-to-Load System Block
To integrate these 22.2V nominal modules into a standard home solar setup, we step them up to a 48V nominal architecture. Here is the exact signal and power flow for a robust DIY system:
- Source: Solar PV Array (e.g., 16x 400W panels) feeding into an MPPT Charge Controller (e.g., Victron SmartSolar 250/100).
- Storage: 8x Tesla 85kWh modules wired in 2P4S (2 parallel strings of 4 series modules) to achieve ~44.4V nominal and ~500Ah capacity (~22 kWh total).
- Management: A 16-cell BMS (like the Orion BMS 2) monitoring individual series groups, controlling a heavy-duty contactor to prevent over-discharge.
- Inversion: 48V DC bus feeding a 5000W Hybrid Inverter/Charger (e.g., Victron Quattro or Growatt SPF 5000ES).
- Load: Inverter outputs 120/240V split-phase AC to a critical loads subpanel.
Sizing Math: Inverter Loads and Efficiency Factors
When sizing a battery bank for an inverter load, old-school solar guides will immediately bring up Peukert’s Law. Peukert's law describes how the usable capacity of a battery drops as the discharge current increases. For lead-acid batteries, the Peukert exponent ($k$) is around 1.3, meaning a heavy load drastically shrinks your available Ah.
Here is the reality for lithium-ion: The internal resistance of Tesla 18650 cells is so low that the Peukert exponent is effectively $1.05$. Capacity loss at high draw is negligible compared to lead-acid. Instead of Peukert derating, we calculate usable capacity using Round-Trip Efficiency (RTE), inverter conversion losses, and voltage sag.
Worked Sizing Example
Let’s size the DC current draw for a continuous 3000W load on our 48V (nominal 44.4V) Tesla bank.
- Target AC Load: 3000W
- Inverter Efficiency: 93% (0.93)
- Battery Nominal Voltage: 44.4V (4 modules in series × 22.2V)
Formula: DC Amps = AC Watts / (DC Voltage × Inverter Efficiency)
DC Amps = 3000 / (44.4 × 0.93) = 3000 / 41.29 = 72.6 Amps
Because we wired the bank in 2P (two parallel strings of 4 modules), this 72.6A draw is split between the two strings. Each string sees roughly 36.3A. Given that a single module has a 250Ah capacity, a 36.3A draw represents a C-rate of 0.14C (36.3 / 250). This is well within the safe continuous discharge limit for these cells, ensuring minimal voltage sag and maximum cycle life.
| Inverter Size | Max DC Draw (Approx) | Minimum Wire Size (THHN) | BMS / Contactor Rating |
|---|---|---|---|
| 3000W | 75A | 4 AWG | 100A |
| 5000W | 125A | 1 AWG | 150A |
| 8000W | 200A | 2/0 AWG | 250A |
Series vs. Parallel Wiring and Charge/Discharge Limits
Understanding the consequence of series and parallel wiring is non-negotiable when building a high-voltage or high-capacity pack.
- Series Wiring (S): Connects the positive of one module to the negative of the next. Consequence: Voltages add together, but the Amp-hour (Ah) capacity remains the same. Four 22.2V, 250Ah modules in series yield 88.8V at 250Ah.
- Parallel Wiring (P): Connects positives to positives and negatives to negatives. Consequence: Amp-hour capacities add together, but voltage remains the same. Two 22.2V, 250Ah modules in parallel yield 22.2V at 500Ah.
Charge and Discharge Limits (C-Rates and DoD)
Tesla modules are engineered for the extreme demands of EV acceleration, meaning they can technically handle a 1C to 3C discharge rate (250A to 750A per module) for short bursts. However, in a stationary solar application, heat dissipation is poorer than in a car with active liquid cooling loops.
For off-grid solar, limit your continuous discharge to 0.5C maximum (125A per module), and aim for a daily operating rate of 0.2C.
Regarding Depth of Discharge (DoD), never treat lithium-ion like lead-acid. While you can technically discharge to 0%, doing so accelerates degradation. Program your BMS low-voltage cutoff to 3.0V per cell (18.0V per module) and your inverter low-battery cutoff slightly higher. Keep your daily DoD between 10% and 90% State of Charge (SoC) to push cycle life past 4,000 cycles.
Salvaged Tesla modules lack the factory-integrated liquid cooling and proprietary thermal management software. If a cell internally shorts due to dendrite growth or physical damage, it can trigger thermal runaway.
Mandatory Safety Steps:
1. Never install DIY lithium banks in living spaces or near primary egress routes. Use a detached garage or fire-rated battery enclosure.
2. Keep a large ABC dry chemical or Class D fire extinguisher nearby. Water will not extinguish a lithium metal oxide fire; it only cools adjacent cells.
3. Ensure your BMS has secondary, independent high-voltage and high-temperature cutoffs that physically drop the main contactor if the primary logic fails.
4. Never leave a newly assembled, untested pack charging unattended.
Tesla Model S Battery Size FAQ
How many kWh is a single Tesla Model S battery module?
A single module from the widely available 85 kWh Tesla Model S pack contains approximately 5.3 kWh of total energy. The newer 100 kWh packs use slightly different module configurations (often 5.7 kWh per module), but the 85 kWh 6s74p modules remain the most documented and supported in the DIY community. If you are building a standard 48V system using 8 of these modules (configured 2P4S), your total gross bank size will be roughly 21.2 kWh, yielding about 17-19 kWh of usable daily capacity when respecting 80-90% DoD limits.
Can I mix different Tesla Model S battery sizes or ages in parallel?
Absolutely not. This is the most common mistake that destroys DIY battery banks. You must never parallel mismatched cells, modules, or strings with different internal resistances, capacities, or degradation levels. If you parallel a newer 5.3 kWh module with an older, degraded 4.5 kWh module, the lower-resistance module will take the brunt of the discharge current and the bulk of the charging current. This leads to severe over-stressing, localized heating, and eventual venting of the weaker module. Always build parallel strings using modules from the same pack, with matched open-circuit voltages (within 0.1V) before connecting the busbars.
What charge and discharge limits apply to salvaged modules?
For longevity in a stationary storage environment, set your BMS charge voltage limit to 4.1V per cell (24.6V per module) rather than the absolute EV maximum of 4.2V. This slight reduction in top-end capacity drastically reduces electrolyte oxidation and extends calendar life. On the discharge side, set the hard cutoff at 3.0V per cell (18.0V per module). For current limits, restrict continuous charging to 0.2C (50A per module) and continuous discharging to 0.5C (125A per module) to keep internal cell temperatures below 35°C without active liquid cooling.
How do I size the inverter and charger for a 48V Tesla module bank?
Your inverter and MPPT charge controller must be sized based on the maximum continuous DC current your battery bank can safely deliver, not just the AC load. For a 48V bank built from Tesla modules (2P4S configuration), the bank can safely output 200A+ continuously. However, to prevent voltage sag from tripping the BMS, a 5000W 48V inverter is the practical maximum for this specific bank size. Pair it with an MPPT charge controller capable of outputting at least 80A to 100A (like the Victron SmartSolar 250/100) to ensure you can recharge the 22 kWh bank in a single day of good solar production. Always use an inverter with a programmable low-voltage disconnect set slightly above your BMS cutoff to prevent the inverter from fighting the BMS during heavy loads.
