If it is a properly installed solar system, you should have a tempering valve between the tank hot water outlet and the pipe in the wall. That can fail, or be misadjusted so it does not allow the proper temperature water to the faucet. There are also several other issues with solar water heaters that can only be checked by being there. They are a "unique" beast and not every plumber is qualified to service or work on them.
: we have solar panels and since the control panel stopped working (we have now repaired it)our hot water taps run cold after a few seconds, even though our water tank is hot. the bath tap often splutters before letting out hot water, then the water runs cold. suggestions please. many thanks