Electric Water Heating

[smadjuster]

Hi - I'm looking for a controller/inverter that can be pole-mounted outdoors near my modules. I am thinking that two Janko 470w modules would do the trick. I want it to heat water in my electric water heater with a 4500w element. I was looking at the CyboEnergy 1200H inverter, but IMO it is prohibitively expensive for a 1200w inverter. I believe I can power the water heater on single-phase as long as there is amperage. But any ideas on a controller/inverter that can do the job without having to switch out my heating element? Thanks in advance.

 

[Ampster]

Have you calculated the kWhs which might be needed to heat a 30 gallon tank of water? There are formulas about Whs needed to heat one gallon of water one degree. Off the top of my head you will probably need more panels and a bigger inverter.

 

[smadjuster]

Have you calculated the kWhs which might be needed to heat a 30 gallon tank of water? There are formulas about Whs needed to heat one gallon of water one degree. Off the top of my head you will probably need more panels and a bigger inverter.

Well, the CyboEnergy inverter will heat a 50 gallon tank with a 4500w element and it is a 1200w inverter. May take awhile to heat, but I'm not usually in that much of a hurry for hot water.

 

[Ampster]

May take awhile to heat, but I'm not usually in that much of a hurry for hot water.

That works if you have a few months to heat the water to 100 degrees or more. According to this formula it will take almost ten kwhs to heat fifty gallons one degree. If conditions are ideal, you might get 3 kWhs per day out of those panels so assuming ambient water temperature of 50 degrees and a desired temperature of 110 degrees. That is 600 kWhs or two hundred days until the first shower. The good news is if you only use ten gallons psr shower the energy to keep the tank at 110 degrees would be less and you can shower more often. Have you looked at some of the heat pump water heaters that use 120 volts? They are much more expensive but four or five times more efficient because they move heat instead of creating it with resistance elements.
EDIT: A subsequent post found that it might only take 8 kWhs to initially heat the water. I was off by a large factor. I queried for a gallon of water by one minute but got a different result. My mistake.

 

[Bkord]

You will definitely need more pv

 

[smadjuster]

You will definitely need more pv

More PV is not a problem.

 

[smadjuster]

That works if you have a few months to heat the water to 100 degrees or more. According to this formula it will take almost ten kwhs to heat fifty gallons one degree. If conditions are ideal, you might get 3 kWhs per day out of those panels so assuming ambient water temperature of 50 degrees and a desired temperature of 110 degrees. That is 600 kWhs or two hundred days until the first shower. The good news is if you only use ten gallons psr shower the energy to keep the tank at 110 degrees would be less and you can shower more often. Have you looked at some of the heat pump water heaters that use 120 volts? They are much more expensive but four or five times more efficient because they move heat instead of creating it with resistance elements.

Here is what the CyboEnergy website says about their inverter specifically designed for electric water heaters:

A simple 1.2KW off-grid solar power system includes one CyboInverter H model and four 250W to 300W solar panels. In most areas, it can generate about 5KWh to 8KWh usable solar energy daily for electric water heaters with heating elements ranging from 1000W to 5000W.

I have not problem adding modules and microinverters is that is a workable solution. But at over $1k for this CyboEnergy thing I can buy four microinverters and I can add modules, no problem.

 

[smadjuster]

Here is what the CyboEnergy website says about their inverter specifically designed for electric water heaters:

A simple 1.2KW off-grid solar power system includes one CyboInverter H model and four 250W to 300W solar panels. In most areas, it can generate about 5KWh to 8KWh usable solar energy daily for electric water heaters with heating elements ranging from 1000W to 5000W.

I have not problem adding modules and microinverters is that is a workable solution. But at over $1k for this CyboEnergy thing I can buy four microinverters and I can add modules, no problem.

I want to keep things as simple as possible. Is it possible to stack a couple of microinverters together and add more panels to get what I need? Or do I have to buy a CyboEnergy thing? I have tons of sun and am off-grid.

 

[AntronX]

You don't need an inverter or charge controller for this. Just get one more Jinko 470w panel and have 3 of them in series directly connected to 4500w heater element via >150Vdc rated SSR controlled by water heater thermostat (mechanical thermostat cannot break 150Vdc without arching and burning up). The impedance of 4500w heater is 12.8 ohms. The impedance of each Jinko panel is 43.3V/10A=4.33ohm, multiply by 3 panels in series and you get 12.9 ohms, spot on impedance of your 4500w heater. In full sun (at 25C panel temp) you will get 130V 10A = 1300W of heat. In partial sun the panels will operate in below Vmpp but still will do maximum current in that light intensity. For this to work optimally all 3 panels must be shade free at noon (+/-2 hours).

 

[Ampster]

Here is what the CyboEnergy website says about their inverter specifically designed for electric water heaters:

A simple 1.2KW off-grid solar power system includes one CyboInverter H model and four 250W to 300W solar panels. In most areas, it can generate about 5KWh to 8KWh usable solar energy daily for electric water heaters with heating elements ranging from 1000W to 5000W.

That is more optimistic than my estimate. Even on a perfect day at the longest day of the year 1200 Watts of solar panels would have a hard time generating more than 6 kWhs. If you want to check my numbers go to PVWatts and look at the daily numbers throughout the year. Winter will most likely be one quarter of summer production. I think CyboEnergy is using a bit of puffery to sell their system.

I have not problem adding modules and microinverters is that is a workable solution. But at over $1k for this CyboEnergy thing I can buy four microinverters and I can add modules, no problem.

Yes, for a small system micros scale up well. Furthermore you can add them incrementally as your needs increase. Also most of them run on 240 volts so that makes them more efficient.

 

[Ampster]

I want to keep things as simple as possible. Is it possible to stack a couple of microinverters together and add more panels to get what I need? Or do I have to buy a CyboEnergy thing? I have tons of sun and am off-grid.

You can't use micros off grid. They are designed for grid tied. The suggestion above by @AntronX might be a good one for heating water off grid.

 

[smadjuster]

You don't need an inverter or charge controller for this. Just get one more Jinko 470w panel and have 3 of them in series directly connected to 4500w heater element via >150Vdc rated SSR controlled by water heater thermostat (mechanical thermostat cannot break 150Vdc without arching and burning up). The impedance of 4500w heater is 12.8 ohms. The impedance of each Jinko panel is 43.3V/10A=4.33ohm, multiply by 3 panels in series and you get 12.9 ohms, spot on impedance of your 4500w heater. In full sun (at 25C panel temp) you will get 130V 10A = 1300W of heat. In partial sun the panels will operate in below Vmpp but still will do maximum current in that light intensity. For this to work optimally all 3 panels must be shade free at noon (+/-2 hours).

So, switch to a 4500w DC Element? Sounds like a plan, but my modules are down the hill, at least 150 feet away from the water heater, which is why I am considering microinverter/AC. Larger wire will do, I guess. Where would I find a thermostat for that, or can I use the existing one in the w/h?

 

[smadjuster]

You can't use micros off grid. They are designed for grid tied. The suggestion above by @AntronX might be a good one for heating water off grid.

The CyboEnergy inverter is a microinverter. They can be used for off-grid, although their primary application is for grid-tied systems.

 

[Ampster]

The CyboEnergy inverter is a microinverter.

Interesting product. I just found a YouTube video in which Engineer775 used them. It was not clear how he installed them. He mentioned the bottom element which would be fine for maintaining the water temperature but perhaps he also hooked up the top element to the other systems in that installation to get the initial temperature up to the right temperature.

 

[AntronX]

So, switch to a 4500w DC Element?

No, your existing 4500w element will work on DC just fine. You just need to disconnect it from the thermostat switch and wire the heater via a solid state relay to solar panels. You can still use the thermostat to turn the SSR on and off. You cannot wire solar panels through the thermostat since it will burn up when turning off under DC load. Since you will have more solar than needed at times it may be a good idea to also install thermostatic mixing valve to keep water temp at safe level while allowing your water heater to store more heat.

 

[smadjuster]

Micro as in small? Do you have any specs?

Attached.

 

[smadjuster]

No, your existing 4500w element will work on DC just fine. You just need to disconnect it from the thermostat switch and wire the heater via a solid state relay to solar panels. You can still use the thermostat to turn the SSR on and off. You cannot wire solar panels through the thermostat since it will burn up when turning off under DC load. Since you will have more solar than needed at times it may be a good idea to also install thermostatic mixing valve to keep water temp at safe level while allowing your water heater to store more heat.

That mixing valve is a good idea. My tank will do double-duty as hot water storage from the cookstove boiler during the winter, too, that will be important. Appreciated!

 

[smadjuster]

Interesting product. I just found a YouTube video in which Engineer775 used them. It was not clear how he installed them. He mentioned the bottom element which would be fine for maintaining the water temperature but perhaps he also hooked up the top element to the other systems in that installation to get the initial temperature up to the right temperature.

Yeah, that's how I found it, through Engineer 775. I believe the top element in his application is grid-tied.

 

[Ampster]

This calculator shows it would take ~13kwh to heat 50 gallons of water from 40F to 120F with an efficiency of 80%

That is much better than my math. Possibly only a day or two for the first tank then? I will check and see what I did wrong and correct or edit my post.

 

[Ampster]

Yeah, that's how I found it, through Engineer 775. I believe the top element in his application is grid-tied.

Okay that makes more sense that three panels would maintain the temperature after the top element heated the water. Do you have another source of 240 volts?