Solar heated water 38°C

read manual:
http://www.rheem.com.au/Assets/1294/1/Rheemsolarlolineelectric511series.pdf
Usually electric element heats about 20 gallons per hour.
It appears the element is controlled by thermostat and automatically kicks ON when tank falls below thermostat set point..
This will be handy since element will continually heat the circulating water when outside temperatures turn cool... of course it will also be wasteful if thermostat is set high.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The capacitor(s) may be open. or resistor has failed. It is also possible heat selector is fused to the one setting.

Usually toilets do not require hot water.
Is the plumbing divided into hot and cold water pipes?
If toilets are hooked to hot water, how many flushes and how many gallons per flush per day.
What is the expected consumption per each worker?
What is the latitude from equator, and expected solar input. Further north, and hazy atmosphere reduce input. Beijing has smog I believe.
How often is the solar panel going to cleaned of smog and dirt for maximum collection?
If the area has hard water, then the entire system is prone to sediment build-up and failure unless water is artificially softened.
Solar water heater temperatures can exceed 180 degrees at times. This temperature will damage storage tank that is not made for that temperature.
Most solar water heater systems have pressure regulating valve and expansion tank to accommodate occasional high temperature and pressure.

A good fellow to contact might be at following website:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Solar collectors are usually connected to a circulating pump, and then the heated water is stored in a tank.
The pump circulates water continually from solar collectors through a storage tank.
This type of system would use ordinary tank-type water heater for storage.
Most of these systems also utilize a back-up heating element inside the tank.

More complicated systems circulate special liquid through spiral tubes inside the water heater.
This type of heater would be an indirect water heater.
Indirect heaters are used for solar collectors, and used with boilers, or any outside heat source.
http://www.bockwaterheaters.com/Products/products_coil_tank_water_heaters.html
http://www.hotwater.com/products/residential/cirrex_solar.html
http://waterheatertimer.org/images/solar-water-heaters-on-roof_lbiRv_5784111-500x282.jpg

it could be the differential in pressure between the cold water from your booster pump and the hot water from your solar water heater . the cold water which feeds your solar heater is probably being reduced in pressure by a pressure reducing valve . so what you need is equal pressure on both your hot and cold water system you could install a pressure reducing valve on the pipe thart feeds your cold taps etc valve set at the same pressure as the one on the solar water heater that would give you a balanced hot and cold water system

A valid point but unfortunately your machine is programmed to take both and will register this as a fault.
This is probably not what you wanted to hear but please dont shoot the messenger.

can't see any reason why you can't use solar heated water as long as the pressure is sufficient for the machine. this is important

Yes. There is a part that allows you to connect the cold water supply to both the hot and cold inlet hoses. It has worked successfully on our 6100 for 8 years or so. I think it was part of the original kit rather than a subsequent purchase - it's a Y-shaped yoke with an inlet connection and two outlets that accept the two hoses feeding the inlets of the machine. From memory i tink the situation is mentioned in the user manual somewhere.