Getting older ASIC miners and using them with cheap electricity?

Hey there, I’m pretty new to all this, so bear with me if my question sounds a bit naive. Here’s the deal: I recently set up a solar panel system here in Germany. Electricity costs around 25 cents per kilowatt-hour, but if I send any power back to the grid, I only get less than 7 cents per kilowatt-hour. So honestly, it feels like a total waste. On a good day, I usually have a surplus of around 10 to 30 kWh. That got me thinking about mining. Initially, I tried out my gaming rig with an RTX 3090, managing about 98 MH/s, but I’m worried that could fry the graphics card pretty fast. So I’m considering picking up some older Antminers on eBay for cheap and running them only when I have extra power. Since solar output can be pretty inconsistent, this could be a solid plan. Plus, I can set them up in my basement where the heat and noise won’t bother anyone. Now for my questions: Can I control the miners remotely to turn them on/off based on how much power I have? (Batteries are way too pricey.) Do older miners still mine Bitcoin effectively? (I think I heard about some algorithm changes that might affect them.) Is there a straightforward way to figure out if a miner can generate more than the 7 cents per kilowatt-hour I’d get for feeding energy back to the grid? Thanks a lot! Ron

Using some custom firmware you can "pause" mining, but the miner will still consume some power for the fans, LEDs and what not, the best solution IMO would be using a wifi-MCB which you can control using an app on your phone, it should be available in Germany, if not, get those little "WiFi Wireless Smart Switch", however, these are not rated for high power draw, so you will need to connect them to a relay/contactor that controls the circuit breaker/s. So the app sends the signal to the switch, the switch sends the signal to the relay/contactor, the contactor kills the MCB > the miners are shut down. No such thing as algo change, bitcoin mining has always been sha256, the difficulty grows higher and so the amount of btc the same asic miner can generate becomes less as time passes. Yes, use www.whattomine.com, an S9 at 14th makes 4.2$ as of today's prices, and it requires 1.25kw, so in theory, every hour it makes 17 cents, as opposed to you selling the 1.25khw for 7*1.25 which is 8.7 cents, but then you need to take into account the cost of the mining gear. The main problem I see here is that using only surpluses of your solar system means you can't use a single miner, the 10 or 30 khw you get are spread throughout the day, so if you use 1 Antminer S9 and want to drain those 30Khw, they need to last for 24hrs, which of course, won't happen. So what you really need to do is not only count the amount of extra power you generate but the average daily duration, if the surplus of 30kWh lasts for only 3 hours, it means you will have to consume 10kw every hour, which means you will need 8 S9s and not just one. of course, you can always just use what you can and sell the rest to the grid, but then, to ROI on that single machine it will take forever and you could be better off just sending that power to the grid. So you need to do the math,  I believe you can get an S9 for $500, so you might run the numbers based on just that and see if it makes sense.

Thanks for this detailed answer. For shutting down/starting up the miners: If I use Off-the-shelf Miners without any changes and start/stop them by just cutting off the power. Will they restart automatically? And I am afraid they will overheat, wehn cooling is shut down without any extra time. In fact the Solar Manager offers to control switches to switch when enough surplus is expected. But doing the math is very disappointing. I was on another mining calculator last night and if the ROI is about 1-3 EUR a day (which seems to be realistic when using older hardware in the range up to 500EUR) it takes of course very long to return the price of the Hardware. And you are totally right, that the miner would only run for 6 (winter) to 10 hours a day, which will reduce the Return even more. Given that the difficulty rises it may happen that the hardware is getting less profitable within the next 2 years witout having repaid. So I think I have to reconsider this. In a first step I will try to put all energy in a electric car (which I get in a few days). Then I will learn how much energy is left in reality and than I have to redo the math. Thanks a lot again Ronald

There is no shut down button on these miners, the normal way to shut them down is by cutting the power, so it makes no difference if you unplug the power cord manually or kill the breaker. Once the power is back they will go back to mining without any manual steps needed, now wether the continuous off and on will reduce the lifespan of these gears or not is a another different topic.

OK, this is an important info to me. In this case I would not need extra electronics besides the (cheap) remote controlled main plug.

Yes, a controlled plug is a good solution. A relatively old S9 is quite profitable with solar power, though amount generated is not so high. Just out of curiosity, how do you imagine controlling turining on/off the miner?  Some output of the inverter or measuring voltage or so?

7$c (assuming you meant USD cent) is not that cheap, but not that bad either. I did read somewhere on Bitmain's site that albeit plug on/off is ok, but not advisable to do so a lot. So for the very same issue we wrote a custom software to shutdown/restart Bitmain miners automatically (basically you just automate the web interface). S17 and S19 in our case, we don't have S9s. This also assumes the firmware has the sleep option. The key thing to make the current rate available for the application somehow. If this is done the machines shut on/off based on software defined rules (like delays to avoid on/off for spike, etc.). This way it's 100% automated very reliable too as this tool has master/slave configuration so you run it on two simple PC and if the first fails for any reason the second takes over the responsibility.

I've also heard often that frequent stops/starts are not good for the miner HW in the long term.  Mostly due to the temperature changes between operating and rest state, thermal dilatation can eventually damage the chips.  If that's the case, it doesn't really matter if stopping is clean or just power-stop.

I don't have anything against this "theory", in fact, I do tend to believe that it makes a lot of sense, however, I have a few S9s that have been mining like a beast for about 4 years, those gears are turned off and on at least once a day, sometimes 3-4 times a day, and they are treated like shit, up to this point, zero issues. On the other hand, I had some S17 series gears treated very well, not much off/on, very good cooling setups, and they died in no time. So really it all boils down to the quality of the miner, if the gear is already bad as is, I'd imagine that turning it off and on more often could speed up the failure process, but if the gear is rock solid, I highly doubt it will kill it before it becomes unprofitable to mine anyway.

okay if you own the 3090 gpu rig simply learn to clock it lower. a single card rig burns about 350 watts 24/7 or 8 kwatts a day. buying any asic gear would be kind of dumb if you still own the gpu rig. just an opinion not written in stone.

Unfortunately there really aren't any "cheap" miners left on EBay.  When the price of BTC went over $20,000 the price of old miners doubled and then doubled again when BTC hit $40,000 That said I run Antminer T15s and have a reliable supply of vast amounts of new and used miners.  Message me if you're interested in purchasing. Regarding controlling your miners from a distance I use Teamviewer.  It's a free desktop application (free for personal use that is).  They also have a nifty Android (and I imagine ios) app.  As long as you have a PC running on the same network your miners are you can log into that PC remotely using your phone and control your miners remotely as if you were sitting right in front of your PC.  This allows you to take miners in and out of lower power mode or sleep mode.  You can monitor how hot they're getting, update firmware, pretty much whatever you want remotely, securely and for free! To calculate the cost of running old miners vs new I use asicminervalue.com this site lists ever miner ever released and how profitable it currently is.  You can even modify variables on their site like how much you're paying for electricity locally and in what currency! Hope this helps you make your decision.  At 7c / kwh you can easily still make profit on older miners like Antminer S9s, T15s or S15s.