Electric Water Heating

[wattmatters]

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

Only if they are on automated multi axis trackers.

For a fixed PV array then you will be doing well if you can get over 4 kWh/day per kW of PV, on average, over a year. Less output in winter, more in Summer.

Depending on how well insulated the tank is, you'll need ~ 2kWh/day just to cover heat losses. That doesn't leave much spare for consumption energy and it's Winter production you'll need to consider, not Summer.

I want it to heat water in my electric water heater with a 4500w element.

If this water is for personal use (bathroom, kitchen, showers) or you expect to be exposed to it in some other way then you absolutely do not want to under egg the power supply.

Keeping water lukewarm for long periods comes with significant risk of legionella bacteria growing. It is deadly. Absolutely you need to ensure water in storage is heated to 140°F on a regular basis (at least once every few days).

 

[smadjuster]

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?

I do not. I am currently talking with Vaughn, who can make a custom w/h tank for me. The tank they currently have is 80 gallons and has a 4 inch heat exchanger coil for hydronic (wood cookstove) heat built-into it. The tank includes one 4500w heating element for "backup" heat. I asked Vaughn whether they can supply an extra heating element it so I'l have two. Then I was thinking of doubling-up the microinverter application to feed both elements. Instead, I could double up on AntronX's DC idea.

I could make it easy and just hook the w/h up to my 12kw inverter, but do not want that kind of load (24/7) on my batteries. Would rather have a separate system to heat water and deal with it the reduced heating on cloudy days.

 

[Ampster]

I do not.

What does your existing 12 kW system put out?

 

[smadjuster]

Only if they are on automated multi axis trackers.

For a fixed PV array then you will be doing well if you can get over 4 kWh/day per kW of PV, on average, over a year. Less output in winter, more in Summer.

Depending on how well insulated the tank is, you'll need ~ 2kWh/day just to cover heat losses. That doesn't leave much spare for consumption energy and it's Winter production you'll need to consider, not Summer.


If this water is for personal use (bathroom, kitchen, showers) or you expect to be exposed to it in some other way then you absolutely do not want to under egg the power supply.

Keeping water lukewarm for long periods comes with significant risk of legionella bacteria growing. It is deadly. Absolutely you need to ensure water in storage is heated to 140°F on a regular basis (at least once every few days).

Thanks - yeah I plan to double up on the heating elements and power. Where I live in S. Colorado we get about 333 days of full sun per year (5+ hrs/day). I have no trees/shade and ample space for modules. Thank you.

Only if they are on automated multi axis trackers.

For a fixed PV array then you will be doing well if you can get over 4 kWh/day per kW of PV, on average, over a year. Less output in winter, more in Summer.

Depending on how well insulated the tank is, you'll need ~ 2kWh/day just to cover heat losses. That doesn't leave much spare for consumption energy and it's Winter production you'll need to consider, not Summer.


If this water is for personal use (bathroom, kitchen, showers) or you expect to be exposed to it in some other way then you absolutely do not want to under egg the power supply.

Keeping water lukewarm for long periods comes with significant risk of legionella bacteria growing. It is deadly. Absolutely you need to ensure water in storage is heated to 140°F on a regular basis (at least once every few days).

Agreed

 

[smadjuster]

What does your existing 12 kW system put out?

Right now, very little. I have a 48v, 380 AH battery bank. But when our home is finished I plan to install a 48v 600ah battery bank, along with 24 470w Jankos. And now, probably 6 extra Jankos to heat water. I will take no chances on hurting my batteries.

 

[Ampster]

But when our home is finished

If you are in the planning stages and investing in capital assets, it still may be worth some inquiry into heat pump water heaters. I still think having a hydronic loop to your stove would work even with a HPWH. From a system perspective the cost of a HPWH integrated into you future system might be less initial cost than adding panels and inverters solely for heating water. And in the long run it may be more efficient since once the water is heated the energy to maintain it would be less and that energy could be used to charge your batteries. Many HPWH have timers so you can schedule them to run during the middle of the day when batteries are charged and there is still plenty of solar available.
It all depends on where you are standing.

 

[smadjuster]

If you are in the planning stages and investing in capital assets, it still may be worth some inquiry into heat pump water heaters. I still think having a hydronic loop to your stove would work even with a HPWH. From a system perspective the cost of a HPWH integrated into you future system might be less initial cost than adding panels and inverters solely for heating water. And in the long run it may be more efficient since once the water is heated the energy to maintain it would be less and that energy could be used to charge your batteries. Many HPWH have timers so you can schedule them to run during the middle of the day when batteries are charged and there is still plenty of solar available.
It all depends on where you are standing.

Thanks, Ampster. I'm not familiar with them. Can you point me toward some info on 'em? We are very remote, and so having systems that are simple, reliable and that I can repair/maintain is important to us, since we won't be able to get technicians to come out here and work on stuff. As it is, we can't get contractors to come out and work. Thanks

 

[Ampster]

We are very remote, and so having systems that are simple, reliable and that I can repair/maintain is important to us, since we won't be able to get technicians to come out here and work on stuff.

HPWH water heater brands like Rheem/Ruud are very reliable, but there are no user serviceable parts except or the usual heating elements for fast recovery modes. In your remote circumstances they may not be the best, especially since you are thinking about a wood burning hydronic backup.

 

[Ampster]

With a 4kW element it would take 3 hours.

That is consistent with my revised math in terms of kWhs. I was off by a factor of 70. However as also mentioned three panels would not produce 12 kWhs in a day so the initial heating would still take over a day with that configuration. It would only take three hours if a system could produce 4kWs all the time.

 

[smadjuster]

Thank you all very much for your help!

 

[Whats-n-Watts]

(No, your existing 4500w element will work on DC just fine.)

As long as it gets 93.75A @ 48v it will perform at 4500 watts.

Or 18.75A @ 240v

Much less PV needed for the dc element.

 

[jonas]

Deleted

 

[Ampster]

As long as it gets 93.75A @ 48v it will perform at 4500 watts.

Yes that is an important assumption. As pointed out, Ohms Law suggests that it won't "get" 93 Amps and will only pull 3.75 Amps. It should also be pointed out that long term the thermostat would probably need to be changed before the contacts arc and stay in the closed position.

 

[jonas]

New post with correct calculations but a 4500W 240V heating element will produce just 180W @ 48V

Current at 240V is 18.75A (=4500/240)
The resistance of the element is 12.8 ohms (=240/18.75)

At 48V the current is 3.75A (=48/12.8)

The power at 48V is 180W (=48*3.75)

 

[Whats-n-Watts]

New post with correct calculations but a 4500W 240V heating element will produce just 180W @ 48V

Current at 240V is 18.75A (=4500/240)
The resistance of the element is 12.8 ohms (=240/18.75)

At 48V the current is 3.75A (=48/12.8)

The power at 48V is 180W (=48*3.75)

Yes that was what I have read. I need to write that down.

 

[Whats-n-Watts]

Yes that is an important assumption. As pointed out, Ohms Law suggests that it won't "get" 93 Amps and will only pull 3.75 Amps. It should also be pointed out that long term the thermostat would probably need to be changed before the contacts arc and stay in the closed position.

There are dc thermostats available at 140 F as well as 174 F eliminating arching.

 

[time2roll]

+1 for the heat pump. Should get 2x or 3x the BTU of water heat per kWh. Usually better to move heat instead of creating heat.

 

[Whats-n-Watts]

+1 for the heat pump. Should get 2x or 3x the BTU of water heat per kWh. Usually better to move heat instead of creating heat.

That price tag though is a killer lol

 

[time2roll]

That price tag though is a killer lol

Compared to adding how much more solar and battery?

 

[Ampster]

That price tag though is a killer lol

Not as bad as the killer energy rates in California. The first heat pump water heater I put into my all electric home paid for itself in five years compare to the resistive element one I had before. It also could be remotely programmed so when I went on vacation I could turn it off.