Most solar storage apps provide a simple headline figure labeled State of Health (SOH), displayed as a percentage like 94% or 88%. While homeowners frequently glance past it, the SOH report is the definitive diagnostic record of your battery’s internal health, capacity loss, and remaining operating life. Understanding how to interpret the data behind that percentage allows you to spot failing cells, track real degradation, and file successful warranty claims before coverage expires.
Quick Answer
A Battery State of Health (SOH) report compares your pack’s current usable capacity, internal resistance, and cell balance against its factory-new baseline. Read it by cross-referencing the headline SOH percentage with lifetime throughput (MWh), cycle counts, and delta cell voltages under active load. A sudden drop below warranty thresholds indicates a failing module.
What the Signal Means: The Anatomy of an SOH Metric
Your State of Charge (SoC) is your real-time fuel gauge (0% to 100% full right now).
Your State of Health (SOH) is the physical size of that fuel tank compared to the day it left the factory floor:
$$
\text{SOH (\%)} = \left( \frac{\text{Current Maximum Usable Capacity (Ah or kWh)}}{\text{Original Nameplate Usable Capacity (Ah or kWh)}} \right) \times 100
$$
FACTORY NEW (Day 1)
┌────────────────────────────────────────────────────────┐
│ Usable Capacity: 10.0 kWh │ SOH: 100%
└────────────────────────────────────────────────────────┘
YEAR 5 (Normal Linear Aging)
┌──────────────────────────────────────────┬─────────────┐
│ Remaining Usable Capacity: 8.5 kWh │ Lost: 1.5kWh│ SOH: 85%
└──────────────────────────────────────────┴─────────────┘
YEAR 10 (Approaching End-of-Life Warranty Threshold)
┌────────────────────────────┬───────────────────────────┐
│ Remaining Usable: 7.0 kWh │ Lost Capacity: 3.0 kWh │ SOH: 70%
└────────────────────────────┴───────────────────────────┘
A battery showing an SOH of 85% with an SoC of 100% means the battery is charged to the absolute maximum physical limit of its current chemical ability, but it can only store 85% of the energy it could when brand new.
For clarity on the operational differences between these two metrics, read Why SOH and SoC Are Often Confused.
What Can Cause It: Deciphering the Key Metrics in the Report
Advanced installer interfaces (like Tesla Pros, Enphase Enlighten Manager, SolarEdge SetApp, or Victron VRM) provide diagnostic reports containing several interrelated values:
┌─────────────────────────────────────────────────────────────┐
│ TYPICAL SOH REPORT FIELDS │
├──────────────────────────────┬──────────────────────────────┤
│ 1. Current SOH Percentage │ Overall chemical status │
│ 2. Lifetime Energy Throughput│ Total MWh processed │
│ 3. Equivalent Full Cycles │ Total normalized cycles │
│ 4. Cell Voltage Delta (ΔV) │ Balance between worst/best │
│ 5. Internal Resistance (mΩ) │ Impedance of cell groups │
│ 6. Historical Min/Max Temps │ Thermal operating history │
└──────────────────────────────┴──────────────────────────────┘
1. Lifetime Energy Throughput (MWh) and Full Cycles
This shows the cumulative electrical energy that has passed through the battery terminals over its operational life.
Manufacturers track this to evaluate warranty terms. If your warranty covers 10 years or 28 MWh of throughput, this number tells you exactly where you stand against that contractual limit.
2. Cell Voltage Delta (Balance Metric)
The report will typically list individual cell voltages or summarize them as a Delta ($\Delta V$)—the difference between the highest-voltage cell and the lowest-voltage cell in the series string.
- Healthy Delta: Under an idle or light resting state, a healthy lithium pack typically shows a delta between 0.005V and 0.025V (5mV to 25mV).
- Degraded or Imbalanced Delta: A delta exceeding 0.080V to 0.100V (80mV to 100mV) during normal operation indicates that individual cells are falling out of alignment.
To learn how uneven cells limit overall storage performance, see Why One Weak Cell Limits the Entire Battery’s Capacity.
3. Internal Cell Resistance / Impedance (m$\Omega$)
As cells break down, their internal resistance rises. High-end diagnostic reports list impedance measurements for individual modules in milliohms. A module showing significantly higher internal resistance than adjacent packs will run hotter, drop voltage faster under heavy house loads, and degrade prematurely.
How to Tell the Difference: Normal Degradation vs. Actionable Failure
Reviewing your SOH history over months and years lets you distinguish between predictable chemical aging and an actionable hardware defect:
| Metric Pattern | Normal Chemical Degradation | Actionable Defect / Hardware Fault |
|---|---|---|
| SOH Trajectory | Smooth, gradual decline of roughly 2% to 4% per year after an initial first-year settlement. | Sudden cliff: SOH drops 8% to 15% over a span of days or weeks following a system fault. |
| Throughput Correlation | SOH decline directly mirrors increasing cycle counts and lifetime megawatt-hour throughput. | SOH drops precipitously despite very low cycle counts and minimal energy throughput. |
| Cell Delta Under Load | Cell voltages drop uniformly when an air conditioner or EV charger activates ($\Delta V$ remains low). | Delta spikes to 150mV+ under load because one weak cell collapses while others hold steady. |
| Temperature Logs | Historical operating temperatures consistently sit within recommended manufacturer windows. | Logs reveal severe historical over-temperature excursions that explain accelerated cell death. |
If your system experienced a sudden drop in its health score following an inverter crash or grid disturbance, see Why Your Battery SOH Dropped Suddenly After a Fault.
What the Data Can—and Can’t—Tell You
While the SOH report is a powerful diagnostic tool, it has notable limitations:
- SOH is an Algorithmic Calculation, Not a Direct Physical Measurement: No sensor inside the battery directly measures remaining chemical health. The BMS calculates SOH by tracking coulomb efficiency during full recharge cycles and comparing real-world voltage responses against internal look-up tables.
- Firmware Updates Can Shift the Number Overnight: Manufacturers frequently revise their capacity estimation algorithms. A firmware update can alter the reported SOH by 2% to 5% instantly without any actual physical change occurring within the lithium cells. For details on this phenomenon, read Can Firmware Updates Artificially Increase Usable Capacity?.
- Partial Cycling Obscures Accuracy: If a battery cycles between 40% and 75% for months without completing a full top-balancing charge to 100%, the SOH algorithm begins guessing. The reported health metric can drift downward artificially until a complete, continuous calibration cycle is performed.
What To Do With This Information
To manage your system using the data found in your SOH report:
- Establish a Baseline Calibration:
If your SOH metric appears suspiciously low or erratic, perform a manual capacity sync. Charge the battery fully to 100% until the absorption current drops below the BMS cutoff threshold, allow it to rest, discharge it steadily down to minimum reserve, and recharge it back to 100%. This updates the BMS mathematical model. Follow the steps in How to Perform a “Top-Balance” to Restore Capacity. - Log Your Metrics Annually:
Once a year, take a timestamped screenshot or export a CSV log of your SOH, lifetime throughput, and cell deltas. Having a historical paper trail makes filing an end-of-warranty capacity claim straightforward. - Escalate When Warranties Are Breached:
If your SOH report confirms your usable capacity has fallen below your contract’s guaranteed retention level (commonly 70% within 10 years) while your total throughput remains below the stated cap:- Download the complete diagnostic raw event and health log from your web portal.
- Verify that your installation meets the environmental guidelines defined in the warranty terms (e.g., historical temperatures stayed within spec).
- Submit the log directly to your solar installer or the equipment manufacturer to initiate a formal warranty evaluation.
- Download the complete diagnostic raw event and health log from your web portal.
- Verify that your installation meets the environmental guidelines defined in the warranty terms (e.g., historical temperatures stayed within spec).
- Submit the log directly to your solar installer or the equipment manufacturer to initiate a formal warranty evaluation.
For a comprehensive guide to understanding capacity guarantees and fine print, refer to Why Your Capacity Warranty Isn’t What You Think It Is.