What are we even doing here?
This site teaches you how to hook some photo voltaic (PV) solar panels to the bottom element of your water heater. Cheap solar power with energy storage. I did this DIY and cut my household electric use in half.
FAQ
What is the payback period?
This depends a lot on how expensive your power is and how much hot water you use. In my case, it was costing me a bit over a buck a day to heat water. I was able to build a PV system for less then $2000. Even with cheap power in Colorado my payback was around 5 years. If you are using expensive California power, your payback could be closer to 2 years.
Will this kill me?
The main dangers are falling off the roof while installing the panels, electrocution, and fire. HVDC is hazardous, probably more so then AC. If you do not have much electrical experience it would be good to find an experienced person to help with the wiring part. That said, this is a very simple circuit we are building. Be sure to install all the wires in conduit to reduce fire risk. Bond the solar rails and negative side of the array to a ground rod in case of lightning. Turn the switch off and check for voltage before messing with the wires (or work at night).
How much hot water do I use?
According Estimating Daily Domestic HotWater Use in North American Homes people use about 64 liters/person/day on average in a four person household. This seems high to me, our house with 2 adults and 2 kids and we rarely use more then 150liters/day. I have noticed that I am more aware of how much hot water I am using after installing this system. On sunny days I use more and on cloudy days I use less.
How much energy can I store in my water heater?
If you assume the water starts off warm, heating 50 gallons from 40°C to 85°C, stores 9.89kWh. If you assume you are pulling water from the cold ground, heating 50 gallons from 5°C(ground temp) to 85°C stores 17.5kWh. A Tesla Powerwall, stores about 13.5kWh. For both the water heater and the Powerwall, you can expect an efficiency of about 90%. The powerwall cost around $9,300, and you should be able to DIY this system for around $2,000.
What is impedance matching and why does it matter?
Maximum Power Transfer theorem says we want to match the load impedance to the source impedance. To get the full capacity of the panels, it necessary to choose a water heater element that matches the source impedance(Vmp/Immp) of the solar panel array. You can also use DC solar optimizers or a MPPT thermostat, they will match the impedance for you (within reason). If you have a lot of partial shading, the optimizers MIGHT be worth it. In many cases, carefully selecting the resistance of your heater element is good enough. There is a calculator on the simulator page to help you with this.
What DC solar optimizers did you try?
After the first year, I installed 5 TS4-A-O solar optimizers. These cost about $40 each, and there is also a central controller setup you can get for $300 that will log the performance of each panel to the internet. I did not not get the central controller. After another year, I tested the panels with a DC clamp meter and found that two of the optimizers were producing zero amps. I replaced these two bad ones. I am not sure what is wrong with the two that failed, and it is not worth the money to buy the central controller to find out. The manufacturer calls running without the central controller a "blind install" and does not officially support it, possibly because they want the monthly revenue from their cloud service. On the whole, I am not sure the optimizers were worth it. See also
What is MPPT?
MPPT is Maximum Power Point Tracking, a special case of impedance matching. To get the most out of your solar panels, you need MPPT. You can do the MPPT for the whole array in your thermostat or you can do it with DC solar optimizers at the panel level. The downside of using MPPT is that it adds a lot of cost and electronics that can fail. It is possible to run without using MPPT if you carefully match the source impedance of your panels to your heater element. Without MPPT you will only get around 2/3 of the power that MPPT can make. If your solar panels are really cheap and/or you live in an area with few cloudy days, not using MPPT can make sense. There is an "Estimate Non-MPPT Power" checkbox above the graph that will show if you select your panel by name/pn.
What about permits?
Few or no permits are required since there is no connection to the grid. This is a major benefit! In my county, the permits would double the install cost. Off grid also means there are no delays waiting for approvals to turn on your system. Mounting the panels on a shed instead of your house roof can avoid the need for permits, this was the case in my locale. You should still follow the local electrical and building codes even if you don't get a permit though.
What about wind loads?
The 100mph wind load on a 45deg tilted 4'x8' panel is 578lbf. The pullout rating for a 3/8 lag screw into a 2x4 is 400lbf. So use at least 4 lag screws per panel and make sure you are screwing into a rafter, not just the sheathing. If your panel is mounted close to the roof you will have less wind load. My panels stick out past the edges of the shed about a foot. The panels have survived winds that knocked over two back yard fences for what that is worth.
Is this better then solar thermal?
This system has no moving parts and is cheaper and easier to install. It reuses your existing electric water heater. PV panels gain efficiency on really cold days when thermal panels have trouble. PV does require more panel area to collect the same amount of heat as thermal. If you want to heat the whole house solar thermal would be better.
Is this better then a heat pump water heater?
Heat pump water heaters are neat, but they have some downsides. They make noise, and you have to clean the air filter regularly. Some of them beep when they need the filter cleaned (and they always manage to start beeping at 3am). They cost more and are less reliable then the old fashioned electric water heater. They steal heat from the indoor air and dump it to the water, which is great in summer but less great in winter. If you already have a heat pump water heater, you may be able to hook up the bottom/backup element to solar, but I have not tried this myself.
What kind of shutoff switch do I need?
You need to mount the solar disconnect switch outside in a readily accessible place near the panels. This is so firefighters/electricians/plumbers/etc can turn your panels off while they are working. Search for "Solar Disconnect Switch" or "Solar Circuit Breaker Waterproof Isolator". You should also mount a second shutoff switch indoors near the water heater to make working on it safer.
What about fuses/breakers?
Solar panels are inherently current limited. When shorted they will never make more then the rated Isc, which is only about 10% more then the current they will make when running at MPP. Therefore, unlike with city power, a fuse is of limited utility with DC solar. The only situation I can think of where a fuse would help you is if you somehow managed to cross some city power AC wires with your solar DC wires and shorted out the city power. A fuse will not help with lighting, that is what your grounding system is for. Many systems have a fuse anyway and some codes require it because reasons? I started out using a fuse but then the fuse holder melted and so now I do not have one.
Why use the bottom element?
This way you can leave the top element hooked up to city power for when there is a long stretch of limited sunshine. If the bottom element is keeping the water hot enough, the top element will never turn on. In my case, the top element is only used in winter. The thermal breaker on the upper heating element will trip if the water gets really hot in the summer. You may have to take the cover off and reset the breaker come winter.
What is a thermostatic mixing valve (TMV)?
A mixing valve mixes some cold water in with the hot to maintain a set output temperature. In the summer,the water will get hotter then is safe so you definitely need a thermostatic mixing valve to prevent scalds. The extra-hot water in the tank increases the amount of energy you can store. I have made it as long as three cloudy days before I needed to use city power to heat water. Install the TMV at the output from the water heater where the hot water is piped to the rest of the house. Adding this valve is important for safety. An example is the Webstone TMV which costs $115 and handles hot water up to 85C.
Do I need a thermostat?
In most cases you will need a thermostat. If you want to use an array larger then 1kW or so, or you have a water heater with a high UEF (good insulation) you should use a thermostat. They make thermostats with MPPT built in so you do not have to worry much about impedance matching your heating element. The AC thermostat built into the water heater will not last long if you try to use it on DC power. DC arcs a lot worse then AC since there is no zero-crossing to extinguish the arc. If your array is pretty small you can make do with only a mixing valve and no thermostat. The standby losses of the water heater increase proportionally with the hot water temperature. This helps to bring the system into equilibrium.
What are standby losses?
The standby loss is how much heat energy leaks out through the water tank insulation and the attached pipes. It varies widely, from 20W for a new well-insulated tank to 200W for an old badly insulated tank. The higher your tank's UEF rating, the lower the standby losses. If your tank has a UEF rating above 0.90, you will want to use a thermostat so your tank will not overheat when you go on summer vacation.
Really you don't use thermostat?
My system has been running for three years with no thermostat. BUT I have a crappy old water heater with bad insulation (high standby losses), and I have trees that shade the solar panels for about 1/2 of the day. So my 1.7kW becomes more like 0.8kW. If ever replace the water heater, or the trees fall down, I will get a thermostat.
Ain't you seen that MythBusters exploding water heater episode?
Yes I have seen it, great episode. The T&P valve is there to prevent overpressure, and the MythBusters replaced that valve with a steel plug. Don't do that.
What is a Temperature and Pressure (T&P) Relief valve?
The T&P valve is a safety device that will let some hot water out if your water heater gets too hot or has too high pressure. T&P valves used on residential water heaters are designed to open at 150 psi or 98C. When water is heated it expands. In a 50-gallon water heater, water being heated to the thermostat setting expands by approximately 1/2 gallon. In most houses the extra pressure will push back into the city water supply but if you have a really tight backflow preventer installed on the water main feeding the house there is nowhere for the pressure to go. If you find your T&P is popping you may need an expansion tank. Make sure you test your T&P valve yearly by pulling the lever and listening for water flow.
What is the best angle to mount the solar panels?
If possible, face the panels due south and angle them so they are a bit steeper then your latitude in degrees. Steeper will collect more power in winter when the days are short and the sun is low. Steeper also sheds snow/leaves better. Flat mounted panels can be used but you will have to clean them frequently. If getting to the ideal angle is going to cause a lot of mounting headaches, don't worry about it too much. My system is about 15 degrees flatter then ideal. You can use the simulator to look at the seasonal affect that mounting angle has.
What kind of panels should I get?
I recommend getting some used panels from somewhere close to you on eBay, facebook marketplace, craigslist, etc. You want the largest panels you can fit as this will result in less mounting hardware and connector losses. As an example currently on eBay you can get panels as big as 655W for $250, they are 2.4m x 1.3m, so about the size of a sheet of plywood. Just three of those 655W panels would do the job.
What is the difference between commercial and residential panels?
Commercial panels are bigger and often have shorter wires. Residential panels are small enough to be handled by one person and have longer wires to make the install a bit easier. Either one will work, I got some used commercial panels because they were the cheapest option.
What is the difference between commercial and residential water heaters?
On a commercial heater, the Maximum Temperature setting of the thermostats is higher. Water heater thermostats also have a thermal safety breaker that will pop and disable the heater element if it gets too hot. The thermal breaker limit is higher on a commercial heater vs a residential one. This is why you often see residential max temperature spec at 150F and commercial at 170F. The insulation and everything else is pretty much the same on commercial and residential. The thermal breaker is a little button on the thermostat, you can push it back in to reset it after things cool down. You will not melt the insulation by exceeding the max temperature spec of your water heater. You would have to boil off all the water first to do that.
How big an array do I need?
I have 1.7kW on a standard 50 gallon water heater and it works great. In the summer this is almost too much, the water gets up to around 70C. In the winter I still need a bit of city power from time to time. I suggest you start with 1-2kW and if that does not meet your needs you can add more panels. If you plug your numbers into the simulator and you have multiple months with over 100% solar power generation then your array is probably too big.
What about trees or other shading?
Deciduous trees are not that bad because they block light in the summer when there is too much, and allow more through in the winter when there is not enough. If you have some evergreen trees that will block the sun for half the day, you can add more panels to compensate. That said, the most important thing for solar to work, is of course sunshine. If you do not have a spot you can mount the panels where they will be in direct sunlight for at least half the day, then this system will not work work for you.
What problems have you run into?
Squirrels chewed some of the wires and I had to replace them. A cheap ebay inline fuse holder melted. 2/5 DC solar optimizers failed. I am not sure the optimizers were worth it, they only get you 20-30% more power. The panels are not quite steep enough to shed snow by themselves, sometimes I have to clean them manually. I noticed some of the lag screws holding the solar rails down loosened up. Might want to give the system a yearly once over to check for lose bolts and chewed wires.
There is an error/bug/falsehood on this website!
Please open a ticket on the github.
Can you install this for me?
If you live on the front range and are want to pay me to install this for you, email: [email protected]
I have a question not answered here.
Please open a ticket on the github or email [email protected]
What element and panels did you use?
My heating element is Zoro #G1831724, 240V 2000W 28.8Ω. On the shed are SunPower SPR-P17-345-COM panels in 5S 1P.
How does the simulator work?
A python backend first fetches historical weather data for you location from the NSRDB. It then feeds the 30min interval weather data, solar panel data, and your water heater data to a simulator based on pvWatts. From this it generates the fancy graphs and spreadsheets. Thank you to the the NREL for making all this possible.