I am in the process of researching electric tankless water heaters for our remodel as I want the space our traditional tank heater requires. I stumbled across your post looking for people posting their research on which size electric they have. The tankless sites have spreadsheet style charts showing gpm flow plus temperature rise.

You may have a third option of installing a new electric tankless that will produce a greater temperature rise at your greatest demand (gpm). However the electric heaters have huge energy demands when they turn on so you may not have enough capacity in your electrical panel to upgrade.

the ones I am debating between for our two adult one bathroom home are either the 96 amp size or the 120 amp size. For the same situation you have-solar preheat most of the time but during our California winter storms we can be 7-10 days with no solar to speak of and then the water passing through the solar storage tank is not much warmer than it is coming out of the ground, maybe 50 degrees.

I assume you are being reasonably careful about not doing two hot water demand things at once? It is easy for our household to not shower and do dishes/laundry, for example. But with a baby & visitors maybe coordinating running the dishwasher/washing machine when you also need hot water to bathe baby or yourselves gets too complicated?

Another option that crosses my mind is to change the solar storage tank to one that could be turned on (gas or electric) when you absolutely need to have warmer water coming into the on demand heater. You could set it to the lowest possible temperature setting and then switch it off when the solar is working to preheat the water. Or possibly get one that can be set to preheat the water on a timer so it runs just once a day, just enough to preheat the water to the level that the tankless can keep up because it needs to raise the temperature only 30 degrees instead of 50-60 degrees.

Sorting out what exactly is the issue may help you decide what to do. Do you not have enough temperature rise only when you are trying to do two hot water demand activities at once? If this issue arises only when you have guests you may be able to suffer through the inconvenience of not running the dishwasher/washing machine when someone is showering?

I am curious as to what size heater you have and how long you have had it and have you had maintenance issues yet (do you have hard or soft water and do you pretreat the water if it is hard?) and anything else you would share with someone who hasn't taken the plunge!

Thanks for the input. I should have been more clear, I'm using solar for electricity not water heating.
And yes, I did consider a new electric tankless. The next size up gives me another 0.4GPM - probably enough but I really don't want to risk being in the same spot again. And, as you mentioned, I can only go one size up as the next size beyond requires a 300A entrance (only have 200A). And you're right again, I don't have enough temp rise when using more than one hot water demand at the same time (also don't have enough if i try to use HOT for clothes washing (very rare) or try to fill the bathtub using the bath spigot instead of shower head). When it was just the two of us we could easily work around these issues, but that is just not possible any more.
Here in NJ, the water inlet gets to about 40 in the winter, so a bit more of a rise is needed compared to CA.
I'm currently using a Stiebel Eltron Tempra 24 (100Amps). Have been using it for just over 3 years. As to the maintenance, have not had to do anything! Our water here is relatively neutral - not hard or soft. Only water treatment is a whole house filter to remove chlorine and other contaminants (using a Pentek RFC20-BB filter). Based on what you've mentioned, I'd say go for it! Sounds like a good fit for your usage. My only suggestion is go one size higher than what you think you need.

this morning after my earlier post I found someone somewhere mentioning putting an uninsulated water storage tank in the house to allow water coming in from the outside to warm up before moving along to the water heater. In this post it mentioned warming up to room temperature which I think is unrealistic. But if your water warmed up to summer inlet temperatures maybe you could continue to be green and save money and be happily using hot water without rationing? I am sure the water flowing out of our solar storage tank is warmer than what flowed in just from hanging out. And that tank is insulated (actually it is an 80 gallon electric tank with no heating elements).

I have just begun looking at the relatively new technology water conditioner/anti scale solution devices to pre treat our hot water. There was quite a buzz on this forum a couple of years ago when a couple of people posted questions re these devices. They do sound a lot like a magic wand to us hard water people!

When I called Stiebel Eltron they said if heating elements or other components needed replacing I would have to remove the device and mail it in for servicing. Which sort of put the kabosh on that brand for me. But they have a great rep as far as I can see and your three trouble free years sounds good! But then you don't have hard water issues either.

Thanks for the input on choosing one size more than I think I need! We are going to have to upgrade our panel to 200 apm service from 125. I need to sit down and figure out what exactly we are removing and adding and see how much amperage we will have for our electric heater. And how much we will have available in the end. I do feel encouraged that you were happy with your 100 amp heater until your needs increased.

What is missing from your analysis is numbers. Unless the solar panels were free, you are paying a lot to heat your water. The least expensive way to heat water is with a gas water heater. If you have a gas or oil boiler, it might be less expensive to use that than a separate water heater. The next less expensive is electricity from the grid unless you have great solar incentives. If you are getting great solar incentives, 70% or so in credits or refunds, enough to get your investment back before 20 years, solar thermal or an electric tank would be the next best bets, in that order. Note that solar thermal is a lot more economical than using photovoltaics to heat water with electrical resistance heat. If your credits or refunds are not that great and you are using electricity to heat your water, you bought too many solar panels and should have planned on using gas to heat your water.

Unless you can take out your costly electric heater and sell it to some other sucker for a high price, you are kind of stuck. Gas tankless is not an efficient way to heat water either, from a financial standpoint. The equipment just costs too much and you will be facing the same problem as with electricity. Your service might be too small and you will have to install a big pipe to the water heater even if the service is big enough.

I assume that you are net-metered and you have a balance that you have to pay each month or a credit that can carry over. You best bet is to buy a gas, tank water heater (unless you have a boiler that you can use). If you have a lot of surplus electricity, use the instant water heater. If you are at a point where you have to pay for electricity, use the gas heater. In times of high demand, use both. If you generally have electricity excess, plumb the demand water heater in first. If not, plumb in the gas first. Have a plumber set them up so they can be used in series or independently.

Thanks for the comments. Yes, I received excellent solar incentives. My ROI will be 4 years - already halfway there. I know it sounds crazy but with the fed, state, and utility incentives, my out of pocket for a $50,000 PV system was $4800!
Yes, I do have net metering and it rolls over month to month. Typically, I have 6 months of surplus and 6 months where I need from the grid. Average electric cost is $25/month.
At this point, I'm going to get an estimate for the gas tankless to see what upgrades are necessary. I'll then subtract the state and fed incentives to see where I'm at. I like your idea of being able to switch between using tankless and tank...In the summer, when I typically have the most excess electricity and I don't have GPM issues with the tankless, I can use tankless only. In the winter I can use a combo of tankless first and then gas tank.

100a * 240v = 24 kwts. That's a lot of panels. And a fair-sized inverter.

If it were me, I think I would use some solar to heat some of the water directly.

I've seen the bit about passive water heating to bring the temp up to room temp using an uninsulated tank. The best thing about it is that it is relatively cheap.

I think the idea of using either a gas or electric tank heater prior to the tankless is good. When company comes you can turn it on and within 30-min to an hour you'll have all the hot water you need. Afterwards you can either turn it off or turn it down to it's lowest setting.

My setup has 32 panels across 2 inverters. Each panel capable of 195w. Keep in mind, when using the tankless it pulls from the grid and solar (if daytime). Using net metering, it all balances out - sometimes i take from the grid other times i push out to it. But, the solar PV was not added to have electric hot water. Its the opposite, I put in electric hot water because I knew I was going to add solar PV. Unfortunately, I do not have space to add solar water heat - it was more important for me to use the space for electricity generation.

So, you say put the tank before the tankless. I see the benefit of that approach too. I read somewhere there was a concern with the water being too hot at the tankless inlet. I don't see mention of max water temp for my tankless, but it could be something that could make that approach more difficult.
Also the passive tank is interesting too, will look more in to that. Thanks.

Jozzy - Did you ever pull the trigger on this? If so, would you mind describing your setup and any advice to offer? I'm considering a similar setup, my H.E. gas water heater needs replacing and I'm thinking about a tankless electric + tank as well. I also have solar PV system.

Thanks,

Jesse

Better check the amperage required by tankless electric that fits your needs. With a solar pre-heat, it will certainly be less of a rise needed, but you've got to analyze the coldest possible incoming temp in order to determine the max rise needed. That's usually a significant amount of your panel capacity.

hey Jesse... no, I never did do it. actually ended up moving a few months later to a different house.. today I'm using a high efficiency boiler to heat the house and provide indirect hot water that is stored in a tank.. and just finished installing solar pv here too...maybe time to think of this again :)