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

[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.

[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.

[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.

[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]

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?

[thastinger]

You have a system pretty close to mine, I'd be looking for another way to heat water as any resistive electrical element and 1000W of panels don't go well together. You've got a system designed for 2.5Kwh/day by conventional design parameters, some days you can harvest 5Kwh from the panels but those will be more rare than not and the "extra" would only run that water heater for an hour and 10 minutes.

[LucMan]

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.

You will need approximately 3000 wh ac to heat 20 gallons of water from 55 to 115 degrees.
You more immediate problem is you water pump, the startup amp draw on your pump is over 30 amps and I don't think that your inverter or your generator will be able to supply the in rush current required to start the pump. I had a 3500/5000 watt generator and it struggled to start my 3/4 hp pump rated at 8 RLA.
I switched to a 5500/7000 generator and the pump starts normally.

[hammick]

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.

Just guessing but I think no more than 20 gallons of hot water a day. Tank will be very close to shower. Not sure of temperature of water as I'm in the building process right now and well pump is not installed yet. Pump will be at 119' in well. 5000' elevation in South Central Montana.

We can usually get our six gallon propane DSI water heater in the RV hot enough for a shower in 10 to 15 minutes so I'm hoping a 20 gallon in 45 minutes or less.

[J.P.M.]

Just guessing but I think no more than 20 gallons of hot water a day. Tank will be very close to shower. Not sure of temperature of water as I'm in the building process right now and well pump is not installed yet. Pump will be at 119' in well. 5000' elevation in South Central Montana.

We can usually get our six gallon propane DSI water heater in the RV hot enough for a shower in 10 to 15 minutes so I'm hoping a 20 gallon in 45 minutes or less.

1st VERY SWAG on ground H2O temp.:~~ 90 - lat. in temperte zones. In warm weather, preheat H2O in amb. air if possible. Some experimentation is usually necessary/helpful.

More SWAG: ~~3 to 4 kWh of elec. for 20 gal. Add 10% or so for losses to time/environment.

[hammick]

You will need approximately 3000 wh ac to heat 20 gallons of water from 55 to 115 degrees.
You more immediate problem is you water pump, the startup amp draw on your pump is over 30 amps and I don't think that your inverter or your generator will be able to supply the in rush current required to start the pump. I had a 3500/5000 watt generator and it struggled to start my 3/4 hp pump rated at 8 RLA.
I switched to a 5500/7000 generator and the pump starts normally.

It is a Grundfos pump with soft start so I'm hopeful the inverter will handle it.

The Generator recommendation for my pump is 50% over P1 power input or minimum 10% over P1 power input. P1 power input of the pump is 1.65kw.

The Conext SW4048 has a surge rating of 7000 watts for five seconds at 77 degrees farenheit.

[inetdog]

It is a Grundfos pump with soft start so I'm hopeful the inverter will handle it.

It should. Nice thing about inverter pumps and inverter mini-splits. (Although the mini-split still requires agile thermostat handling to keep the temp differential from going too high!)

Getting back to water heating, the on-demand heater has the advantage of "limitless" time if not flow rate and no wasted hot water left over to cool.
But the camp size tankless seem to have some reliability and flame loss problems.

[thastinger]

I would have to buy or lease a propane tank. Propane is expensive, especially in remote ares. I'm not sure if a 20 gallon propane tank would last two weeks. A refill is about $20. My property is about nine miles off the highway and is remote. We travel mostly by ATV once we are on the property. Not going to haul a 20 gallon propane tank on the ATV.

I'm open to people's opinions on propane for hot water though. I have looked at the various tankless models that would work with a 20 gallon propane tank and none of them look promising.

My thoughts on a tradition electric hot water heater (assuming solar will power it during the day and my generator at night) is that it's fairly cheap, I can install myself, easy to turn on when needed and water in tank stays warm for a long time during summer temps. Also super easy to winterize.

You're going to have to embrace the lifestyle then as you can't live a modern life on 1Kw of panels. How about 2 black 55 gallon olive barrels which are elevated and gravity feed sun heated water as a shower?
FYI, a new 100LB propane tank is 120 bucks from tractor supply, good for 12 years, cost 100 bucks to fill it at a propane filling station.

[thastinger]

All right you have convinced me to look into propane. I'll probably want to have a fridge at some point other than in the travel trailer and propane can run that also. Maybe also a gas range. Can a 100lb tank be transported full on it's side in the bed of a pickup? Can a full tank be stored inside an un-insulated garage?

Thanks for the good info!

I store 2 100lb tanks, one in a metal shed, the other in a garage, both are not climate controlled. I transported mine standing up in the bed as they have pickup tubes which could snap off if transported horizontal. 40lb tanks might be an option if it is just you to move them around, the 100s are kinda heavy when filled.

[Wy_White_Wolf]

Forget the PV for heating water. Thermal is easier, cheaper, and more efficient. I have a weekend cabin in central Montana (Bull mountains) and we use a couple cheap solar showers for our hot water and only have problems on real cloudy days. Then we just heat what is needed on the stove.

The heater may only draw 8.33 amps at 240volts. But that is 41.66 amps at 48volts. When Peukerts Law gets takes effect you don't have enough battery to keep the voltage stable for more than an hour.

WWW

[Naptown]

All right you have convinced me to look into propane. I'll probably want to have a fridge at some point other than in the travel trailer and propane can run that also. Maybe also a gas range. Can a 100lb tank be transported full on it's side in the bed of a pickup? Can a full tank be stored inside an un-insulated garage?

Thanks for the good info!

Never ever store propane inside.
Is theft really a problem?
If you insist on storing indoors you should build a vapor proof wall to separate the rest of the structure from the tanks and place some vents high and at floor level to the exterior.
Also no access from inside to tanks put in an exterior door.