Replacing or expanding a Siemens solar module
Most “dead” old panels are connection or diode faults. Two readings and a short routine tell you which, and how to bring modern panels in without breaking the controller.
Key takeaways
- Measure Voc and Isc in full sun before you condemn a module; the pattern of the two readings points to the fault.
- A 25-year-old module at 80 to 90 % of its rating is normal. The warranty floor is 80 % of the minimum rating, about 73 % of the label power for an SM55.
- Never mix old and new modules in one series string. Give modern panels their own controller input.
- Check the controller’s input rating against the string’s Voc at your coldest temperature, not at 25 °C.
An old Siemens module that has stopped producing is not always dead, and a system built around one is not always worth keeping. Most failures people blame on the panel turn out to be a connection, a diode or the controller. This guide gives you a ten-minute test, a table for reading the results, and the rules for bringing modern panels into an old system.
Test before you condemn
- Identify the model. Use the identifier and open its page for the datasheet values.
- Measure Voc across the disconnected output in bright sun, with a multimeter on DC volts. A 36-cell module in a 12 V version should read near 21 V.
- Measure Isc with a DC ammeter or clamp, again at full sun and with the module disconnected from everything else. Compare it with the datasheet value scaled to the sunlight you have (800 W/m² is about 80 % of the rated Isc).
- Open the junction box. Corroded terminals, loose wires and failed bypass diodes are common on old frames and are repairable.
What your readings mean
| You measure | Likely cause | What to do |
|---|---|---|
| Voc and Isc both close to the datasheet | The module is healthy | Look at cables, connectors, the controller and the battery |
| Voc low by a third or more (a 36-cell module reads about 14 V instead of 21 V) | A group of cells is out: a failed solder bond, a cracked cell, or a bypass diode (where fitted) conducting when it should not | Inspect the junction box; a failed diode can be replaced, a cracked-cell module usually cannot be repaired |
| Voc normal, Isc well below the scaled value | Partial shading, dirt or film, a browned laminate, or one weak cell | Clean it, check for shade at the time of the test, and compare with a neighbouring module under the same sun |
| Voc and Isc both near zero | An open circuit: a broken ribbon or a loose connection in the box | Check terminals and wire crimps before replacing anything |
| Voc looks 10 % high on a cold morning | Normal: Voc rises about 0.34 % for every degree Celsius of cooling | Use the cold-weather Voc when you check a controller’s input rating |
These thresholds are rules of thumb, not Siemens specifications: about 10 % on Voc and 20 % on Isc covers normal scatter in sun and temperature. If readings fall outside them, test again at midday in clear sky before you draw conclusions.
What decline is normal?
The SM46, SM50-H, SM55, SM110, SP75, SR50 and SR100 datasheets carry a warranty tier guaranteeing power above 80 % of the minimum rating for 25 years (and 90 % for 10 years). The minimum is below the rated power: an SM55 has Pmin 50 W, so the 25-year floor is 40 W, 73 % of the 55 W label. A widely cited NREL review of about 2,000 published degradation measurements (Jordan and Kurtz, Progress in Photovoltaics, 2013) found a median loss of about 0.5 % a year; the mean was higher, near 0.8 %, because some systems degrade much faster. At those rates a 25-year-old module producing 80 to 90 % of its rated power is ordinary, not defective.
Matching voltage
Do not mix old and new modules in one series string: current is limited by the weakest module, and different Vmp values make a controller track a compromise. A modern panel is a different electrical animal from a 36-cell 12 V module:
| Siemens SM55 | Typical modern 60-cell panel | |
|---|---|---|
| Cells in series | 36 | 60 |
| Vmp | 17.4 V | about 31 V |
| Voc | 21.7 V | about 38 V |
| Isc | 3.45 A | about 9 to 10 A |
| Fits a 12 V PWM controller? | Yes | No: needs MPPT with a higher input rating |
If you add modern panels to an old system, give them their own controller or their own MPPT input, and set the battery profile for the battery, not the panel. The calculator shows cold-weather Voc for any string, and the controller guide covers the sizing rules.
Replacement ideas by module
| Legacy module | What to look for | Check |
|---|---|---|
| SM6, SM10, ST5, ST10, ST20, ST40 | A small 12 V panel with a blocking diode or a small controller | Maximum system voltage is 25 V (ST: confirm on the original): do not exceed it with series strings |
| SM20 | A small 12 V panel | Maximum system voltage 20 V; the cold-weather Voc already exceeds it (see the wiring guide) |
| SM46, SM50-H, SM55/SM50, SP75, SR50 | A 50 to 100 W 12 V-class panel, or two in series for a 24 V system | Vmp above 17 V so a 12 V battery can reach absorption; see the headroom row on the model page |
| SM110/SM100, SR100/SR90 | A 100 to 200 W panel on the same racking or a new rack | Compare mounting hole spacing: old frames rarely match modern ones |
A worked example: four SM55 on a camper
Four SM55 in parallel on a 12 V battery give 220 W at standard conditions, Imp 12.6 A and Isc 13.8 A, so the controller must be rated for at least 1.25 × 13.8 = 17.3 A. Suppose you measure each module in about 900 W/m² sun and read Voc 20.4 V and Isc 3.1 A. Voc is 94 % of 21.7 V and Isc is 100 % of the scaled value (3.45 A × 0.9 = 3.1 A): the module is healthy. If one module read Voc 21.5 V but Isc 1.8 A, it is the weak one. In parallel that module simply contributes less; in a series string it would cap the current of the whole string, which is why series strings should be matched.
Questions readers ask
Can I put one new panel in parallel with my old SM55 modules?
Electrically it works only if the new panel’s Vmp is close to the old modules’ (within about 1 V) and the controller handles the combined current. A modern 60-cell panel with a Vmp near 31 V does not qualify; use a separate controller for it.
My module looks yellow or brown. Is that the end of it?
Not necessarily, but browning of the laminate lowers Isc, so it shows up as a low short-circuit current with a normal open-circuit voltage. The datasheets describe cells laminated in EVA; compare your Isc with the scaled datasheet value to see how much you have lost.
Can I test a module indoors or in the shade?
Not usefully. Isc scales with irradiance, so a reading in the shade or under a lamp says little about the module. Test around midday in clear sky and note the conditions.
Where do I find the original datasheet values for my module?
Identify the model with the identifier tool, then open its page: the page lists Pmax, Pmin, Imp, Vmp, Isc, Voc, the temperature coefficients, size, weight and the warranty tiers.
Should I replace the modules or the whole system?
If the modules test healthy, a new controller and battery often cost less and restore more than new panels. If several modules read low and the frames are damaged, a new array on modern racking is usually the better investment.
Related guides
Wiring Siemens solar modules: series, parallel and bypass diodes
Wiring rules from the Siemens datasheets: maximum system voltage by model, series and parallel strings, cold-weather Voc, 6 V versions and bypass diodes.
Read more →Charge controllers for Siemens solar modules
How to choose a charge controller for Siemens modules: PWM or MPPT, why 36 cells suit 12 V batteries, cold-weather Voc limits and how to size the current.
Read more →How to identify a Siemens solar module
Identify an unmarked Siemens solar module from its Voc, Isc and frame size. Includes the SM, SP, SR, ST naming scheme and a 16-model matcher.
Read more →Enter your module and string and see cold-weather Voc and the controller current you need.