Why were residential systems in the 1990s at such low voltages? Was this due to low PV module efficiencies? Or simply because the niche applications (rural telephone systems etc.) did not require a high voltage?
In the 1990s, most PV systems were battery based and battery based systems usually have lower voltages to match battery voltages. Our old system (which is still working) is a 12V battery charging system. It has a charge controller that is not an MPPT charge controller. This means that the battery “nominal” voltage has to match the PV “nominal” voltage.
Our 12V system is very typical of the 1990s and has 4 12V Arco Solar PV modules that are combined with only parallel connections (no series connections) to form a 12V system.
12V nominal modules have a Voc that is about 22V and a Vmp of about 17V at STC. This means that when it is warmer on the PV, the Vmp will drop down a few volts, which gets the module in the 14 to 15V range, which is close enough to the MPP of the module to make a good match with the 12V battery.
12V nominal batteries have a sitting charged voltage of about 12.6 volts and like to be charged in the 14 to 15V range, which is a good match for the 12V nominal module. If you have a voltmeter on your car, you will notice that the car is usually charging the battery between 14 and 15V. For small general aviation airplanes, they call the equipment 14V radios, etc., since that is really the voltage it will be operating.
Some other reasons that we did not use higher voltage systems is that PV was very expensive and we could not afford to have 12 modules in our systems a lot of the times.
Also, back in the day inverters were less efficient and reliable, so we were more likely to have RV equipment in our houses. Many people still have 12V and 24V refrigerators in their off-grid homes.
Grid-tied inverters were invented by Ward Bower at the Sandia Lab in NM in 1977. Grid-tied inverters did not become popular and mass produced until the German Feed-in-Tariff, which kicked into gear in the very late 90s. Grid-tied that didn’t require batteries were brought over from Germany and worked better with higher voltages, since we were connecting to 120Vac and 240Vac at that time. Nowadays 120Vac inverters are hard to find. It is more efficient to make power with a grid-tied inverter when we go from a higher dc voltage and drop it down to a lower ac voltage. Less voltage boosting inside of the inverter while the power is being sliced and diced by the small people working inside the inverter:)