I posted about my offgrid solar set up and commented in response to a query from @siggy47 about how/if solar mining is viable for the home/hobbyist btc miner - my initial response was using solar for small scale btc mining is difficult to make economic because to run 24/7 you need a significant battery capacity and that is expensive to a point of being almost certainly uneconomic. However thinking about this and there is one narrow (and arguably circuitous!) window of opportunity that may be open to some- it is an indirect way of using solar to mine but it could work...depending on your circumstances.

So if you have an electric hot water cylinder type of home water heating and you convert that to being powered by solar you could then use the mains power you would have consumed for your water heating for mining- at arguably minimal cost- the cost of installing a PV solar water heating system can be DIY about USD$1000...and if you are reducing your power bill by 30-40% that 'liberates' a substantial amount of mains power for use in BTC mining. So its not direct solar BTC mining but it is potentially more economically viable than reliance on expensive batteries...and it does allow full 24/7 use of your mining rig/s.

Precondition- You need to currently rely upon a mains electricity powered hot water heating/storage cylinder system- this is common here in NZ but in other places I am not so sure it is as common. Here in NZ most houses have a 40-60 gallon hot water tank that is heated using an immersion heater element powered by mains power at 230 volts. It is estimated that in the average NZ household about 40% of the electricity cost is water heating. Many years ago I realised that with PV panels now(then) fairly cheap it is viable to power your hot water cylinder with solar power- directly. You feed the DC current from the PV panel/s into an immersion heater element with a suitably matched resistance. For many years I used an immersion heater element with a resistance of 14 ohms powered by three x 200watt PV panels wired in series. When wired in series the PVs provided at peak 100 volts x 6 amps=600 watts. In summer that was plenty- over winter it was not enough so I used mains power 1-2 hours a day to back it up.

Using Ohms law you can calculate the ideal resistance for the heater element.

Whatever your PV configuration you can plug in the voltage and watts to the above online calculator to get the ideal resistance for the heater element to run on your PV configuration. In NZ DC current circuits under 110 volts are subject to less regulatory restrictions and anyway keeping voltage below 110 volts is probably a good idea.

Most PV panels give their claimed outputs but what you want to know is the voltage under load which is usually 10-20% below the stated open circuit voltage. The PV panels I used were designed for charging 24 batteries and gave under load about 33 volts.

So if you can calculate the resistance required for your heater element how to obtain a heater element of that resistance? You may be able to find one that is already available or you can get one made to your specs. I got a small batch made in China to my specifications with two heater elements built into the one screw in unit - one for the solar heating and one for mains backup.

Now if you have a solar PV array and a suitable heater element for the water cylinder you need to connect the circuit using wire of adequate capacity and take care you use good quality triple gated switches as DC current is much more prone to sparking than AC. For this reason it is also not practical to use a thermostat to limit/control the solar heating circuit. You just need to think of the solar water heating as a form of sun powered wetback like you have on woodstoves. As long as your hot water cylinder is big enough you dont need a thermostat because every day the sun goes down.

You can also always turn off the solar circuit if too much hot water results or just run off the hot water excess.

If you fit a water tempering valve then excess hot water is less of a problem. I never got around to that and so would just run the washing machine on hot cycle to use the excess hot water.

Matching the power of the PVs to the size of the cylinder will also limit the risk of the water boiling over- up to 1000 watts / 300 liters cylinder capacity would be my rough guide. Using 600watts in a 180 litre cylinder it never boiled over but did get very hot on some long summer days.

Whether you choose to use the electricity saved for BTC mining or just to save on power bills is the option...but to use it for BTC mining you could now argue you are mining with solar (indirectly) and using hot water as a battery!

Note lets say this can all be done and you save 4kw/day on average in power that would have been used to heat your hot water- this is still only enough to power one Nano3S (140wx24=3.36kw/day) with a little margin to spare...