Using solar hot water with a washing machine
Washing machines and dishwashers can heat up their own water but the heating element tends to be between 2000 and 3000 watts. You would have to have a very big off grid solar system with a large inverter to use the actual dishwasher or washing machine heating element.
The simple answer is to plumb these appliances into your hot water system and always wash your clothes on the cold setting so that it never attempts to heat the water up.
If you are in an off grid situation it is probably best to wash your dishes by hand. Most dishwashers have a heating element and unless you have a big lithium ion system the solar inverter will shut the system down after a short time of having a draw of 2000 watts unless the sun is actually shining at the time. If you are connected to the grid then connecting your dishwasher directly to the hot system would be a good idea. Note: The time of writing this is 2021. In a few years it will be much more common to have large capacity batteries which will be OK with a dishwasher.
On the other hand clothes washing machines are perfect for solar power. As long as you have an inverter of over 700 watts they will work fine. Most detergents nowadays are designed for cold washes. However from time to time it is nice to be able to do a hot wash. The most important thing with clothes washing is that the water that enters the washing machine should not be too hot for the type of clothes that you are washing. For this reason you need to install a mixer tap or mixer valve.
The photo at the top of this page is the mixer tap on our washing machine. Mixer taps have 2 inlets. Hot and cold. They have 1 outlet. The water in the outlet should be limited to the maximum temperature set on the control dial. I personally would not trust the mixer tap to have an accurate temperature setting and I would use an external thermometer to make sure the temperature is correct.
Note that using this method you will end up rinsing your clothes with warm water. If this is a problem you could switch the hot inlet off after a certain amount of time.
Warning: I still don’t know how it happened but my mixer tap has some sort of back flow issue and it resulted in cold water being mixed with my general hot water system. I solved this by putting a tap to stop the hot water inflow but beware if this happens to you, it took me a month to work it out.
Remember that even if you run hot water into the inlet of the washing machine you should choose a cold wash setting on the machine. (you don’t want the internal water heater to start)
One last idea: Why not use the used water from the washing machine for irrigation. Pipe the water onto some trees or into a container to be used for watering.
This is a link to a mixing tap on Amazon.
If you go to a plumbing shop in Spain ask for “Válvula Mezclador termostático or “Válvula de mezcla caliente y fría de 3 vía”.