Question about watt hours

[TrashPanda10k]

I've got a bank of 8 LiShen 78 amp 3.2 volt batteries that I'm setting up to be 24 volts. Overall, this will then be 624 amps. My question is, for my system, because I'm connecting them in series and pushing the voltage up to 24 volts, would I then use the equation 24 (volts) X 624 (amps) to find the available watts? (I'm ignoring hours in this equation) Did I set that up correctly?

 

[snoobler]

Welcome to the forum.

You can't "ignore hours" in this context.

Wh is a quantity of energy independent of volts.

Ah is a battery capacity at a given voltage.

If you are saying you have 78 Ah cells, when you take 8 of them and put them in series, you have a 78Ah 24V battery.

Expressed in Wh, that's 78Ah * 24V = 1872 Wh

 

[TrashPanda10k]

Gotcha! Thanks. That's why I put this in the beginners section...

 

[DThames]

Just curious but,
1. What will your new battery power?
2. Do you plan to include a BMS?

Welcome.

 

[TrashPanda10k]

To reply, 1, it'll be powering a small travel trailer. My main draw is a 2 gallon 120v hot water heater. Kill-a-watt says it consumes 550 watts every time I fill and heat the tank, which I estimate we will do 4 times over the course of a day. Otherwise, it's an occasional use microwave and lights and stuff.

2- for sure, I'll be using a BMS. I need to get most of the parts. At the moment, only the batteries have been bought.

 

[snoobler]

To reply, 1, it'll be powering a small travel trailer. My main draw is a 2 gallon 120v hot water heater. Kill-a-watt says it consumes 550 watts every time I fill and heat the tank, which I estimate we will do 4 times over the course of a day. Otherwise, it's an occasional use microwave and lights and stuff.

2- for sure, I'll be using a BMS. I need to get most of the parts. At the moment, only the batteries have been bought.

550W or Wh?

W, Watts is a measure of power - the rate of energy consumption. Your equipment must be able to supply this power to allow the items to operate.

Wh is the measure of the total amount of energy used, power * time.

It is important to keep these straight.

Your 78Ah cells may be limited to 78A of output. That means you're limited to 1872W. A 1000W microwave will use 1500-1600W, so you're getting close to that limit.

For the stated purposes, your battery seems undersized.

 

[TrashPanda10k]

Ah, yes, those are distinctly different units. Killawatt said it was 0.55 KWH.

 

[TrashPanda10k]

Also, these are my batteries: https://batteryhookup.com/products/new-4-pack-3-2v-78ah-lifepo4-lishen-prismatic

 

[snoobler]

.55 * 4 = 2.2kWh

This exceeds your battery capacity of 1.87kWh

 

[TrashPanda10k]

Welp, off to go buy more batteries then...

 

[DThames]

Welp, off to go buy more batteries then...

Or less hot water

 

[Supervstech]

.55 * 4 = 2.2kWh

This exceeds your battery capacity of 1.87kWh

He said the kill a watt showed .55KWh...

So... well within the battery’s ability... no?

Odd, the meter isn’t run across the entire day to evaluate total consumption.

 

[TrashPanda10k]

I just wanted to get an estimation of what each instant hot cycle would give. After it was hot, it went to zero watts used.
Here's the amazon link- it's actually called tankless, but has a 2.5 gallon tank:
https://www.amazon.com/Eemax-Electr...qid=1623519635&sprefix=eemax+,aps,258&sr=8-10

 

[TrashPanda10k]

Also, thanks everyone, I really appreciate your thoughts and opinions on my wandering into the world of solar power. There is quite a learning curve to all this stuff.

 

[snoobler]

He said the kill a watt showed .55KWh...

So... well within the battery’s ability... no?

Odd, the meter isn’t run across the entire day to evaluate total consumption.

.55kWh "every time I fill" Said 4 fills/day, 4* .55kWh = 2.2kWh; greater than battery cap.

 

[TrashPanda10k]

Y'all raise a good point. I was mainly focused on how much power it would take to just heat the water, not maintain it. It's still sitting in my utility sink, I'll let it run until tomorrow and get a better sense of a 24 hour cycle. Still, the consensus is- more batteries, and more solar as well. Since I'm mainly focused on summertime camping with this RV, could I get away with more solar and keep my 2kwh battery? I probably should just bite the bullet and buy more, but if I can get away with less, it would be nice. Fyi, the water heater in question is currently pulling about 1400 watts at 110 to heat from room temp to 140 degrees.

Also, thanks! I appreciate your input in my tinkering.

 

[Bubba1]

Maybe try a solar shower bag or a bucket of water in the sun. That's a lot of power for small system.

 

[DThames]

I just wanted to get an estimation of what each instant hot cycle would give. After it was hot, it went to zero watts used.
Here's the amazon link- it's actually called tankless, but has a 2.5 gallon tank:
https://www.amazon.com/Eemax-Electric-Water-Heater-Plug/dp/B007PH7QLW/ref=sr_1_10?crid=3GTZSBN9RV5FM&dchild=1&keywords=eemax+tankless+water+heater&qid=1623519635&sprefix=eemax+,aps,258&sr=8-10

I am curious about how one would use a water heater like this. Do you let it heat and then run more cold water in to flush/use the hot water?

 

[Hedges]

Still, the consensus is- more batteries, and more solar as well. Since I'm mainly focused on summertime camping with this RV, could I get away with more solar and keep my 2kwh battery? I probably should just bite the bullet and buy more, but if I can get away with less, it would be nice. Fyi, the water heater in question is currently pulling about 1400 watts at 110 to heat from room temp to 140 degrees.

Also, thanks! I appreciate your input in my tinkering.

How much solar can you fit?
Apparently your battery can do about 3 water heating cycles.

Fixed PV panel orientation gets maybe 5.5 hours sunshine. 2200 Wh/5.5 = 400W. At least 500W optimally oriented, or more PV less optimally oriented, would supply the water heating needs, recharging battery throughout the day.
You only need more battery if you want to get more than 3 showers in darkness.

1400W to deliver 550 Wh is 25 minutes.
If you replace the 1400W 120V element with a 1400W 240V one, then on 120V it would draw 350W and take not quite 2 hours to reheat.
You can pick the element according to the load and reheat time you want.

A SPDT switch or spring-wound timer could temporarily transfer inverter output to circuit with microwave, avoiding both at once.

 

[TrashPanda10k]

So, after some consideration, and much input from the community, I'm not going to use solar pv for this project. It might work, and if it did, it would take a lot of batteries and a lot of solar. Instead, I'm going to do the other option, which cost roughly 25% of my solar setup and just replace the propane hot water heater. Eh, that's why it's called tinkering. Sometimes it works, sometimes it doesn't...