Electric hot water heated load calculations: Calling the watts and amps gurus

[lkruper]

We have a six gallon propane on our Forest River travel trailer and it works great. However, I'm thinking 20 gallon is the smallest we want for a real shower. I have well water and septic so the only conservation will be solar/battery watts/generator use.

I understand. One thing you mentioned but I did not understand. You did not want to use propane because the garage would not be used all the time. What is your thought process on that?

[J.P.M.]

Since you ask, one thing I see missing: Most system sizing usually starts by specifying a load, that is, how much H2O you want/expect to use and how cold the H2O is when the heating process starts and how hot you want to get it. Also, some idea of tank and maybe line losses. Hard to design something until you know what it needs to do.

[lkruper]

Thanks for the reply. I'm thinking the water heater would be on max for two hours a day. I would probably fire up the generator whenever the water heater is on. The Conext inverter will charge the batteries even with a 120v generator using only one leg. Not sure what the charging amps would be using the Yamaha.

South Central Montana with tons of good summer sun. Most of the property use will be Summer. Occasional Spring and Fall use.

The Yamaha generator puts out 16.7 amps at 120v. Does that mean it puts out 2,004 watts per hour? Manual says it's rated AC output is 2.0 KVA.

I'm really not wanting to mess with propane since there will be long stretches of time when the garage is vacant.

I could also add two more panels pretty easy if I need more power. I really don't want to add any more battery.

What capacity do you need? Have you considered a 6 gallon rv hot water heater. They work with both electricity and propane. There is a conversion kit I have seen that adds electric heating to an existing propane tank and it is only 400 watts and works on 110v.

Another alternative is solar hot water. They are more efficient and if you have the sun that might work for you.

[sensij]

You are missing how many hours per day you would expect the element to be on.

Each hour would require 2000 Wh of energy (2 kWh). Your entire battery has only 208 * 48 = 10000 Wh, so you are at 20% discharge / hour, even before efficiency losses.

Powering it from those panels will help some, but not much, and whatever power you take from the panels will not be available to recharge the battery if you have other loads. You only have 1100 nameplate watts of PV, which may produce 1000 W at best during the peak part of the day, but will otherwise produce less than that (you are estimating 16 * 48 = 748 W), and I don't recall how many sun hours you expect to receive. Once you add in the efficiency losses, the system you've proposed may be inadequate to handle just the water heater even if no other loads exist at all.

[hammick]

I am installing a 48v solar system on my off grid garage with small living quarters. I know electric hot water heaters are energy hogs but I'm hoping to get away with it by only running it during solar production or if we need it at night by starting the generator. It will be a 20 gallon Kenmore 240v with a 2,000 watt element. By my calculations it will draw 8.33 amps. Am I missing anything?

System info.
Four 275w panels (estimated to produce 16a of current at 48v).
Conext 4048 inverter
Conext XW 60 MTTP CC
Eight Insterstate 6v GC2 batteries (208ah)
Yamaha EF2400is generator.


My next biggest load after the water heater is a 230v well pump that draws 8.4amps We will have a larger pressure tank so the pump doesn't run as often. We would turn off the hot water heater before showering so the pump and hot water heater don't cycle at the same time.