Cost of investment VS return gain

[MrModules]

Hello, im looking for some feedback if its cost effective purchasing 2x batteries (them alone) compared to if i just paid my electricity bill.
A few points are:
- I dont have batteries
- no fuses/breakers
- no charge controller
- I pay ~$0.2 per Kw/h
- I do have spare solar pannels

My friend and i have dont some looking around for batteries from physical store (you can walk into) As we think this will be better if anything was to go wrong with the batteries, thered be more of a store than a online held desk. The overal goal is to run my PC on these batteries to slowly cut back our power bill as well as begin a hobby.

For the picture below we have 2 batteries costing $2500 ($1250 each) 12.8V 200Ah upping to 25.6v (24v) 200Ah
Fulling draining the battery each day says we get ~2000 cycles coming to $0.51 Per cycle = $1,020 or with help of a solar pannel and charge controller while limmiting myself to ~50% drain i get ~5000 cycles which = $2,550 that covers the batteries. Would this be worth as a 'hobby' side of things? The charge controller i need (from what victron says) is another $700-800.



My Pc runs at ~300Watts with 2 screens included, i have never seen it over. With that the running time we calculated is between 7-8 hours without solar and up to 13-17 hours with solar providing the solar pannels can charge and run my pc at the same time. I do use my computer for those hours each day.

 

[Symbioquine]

Just working with some ballpark figures...

If we (incorrectly) supposed that those batteries and solar system could power your computer 24x7, then it would take a little less than 5 years to pay back the $2,500 battery cost;

((2500 [$] / 0.2 [$/KwH]) / 0.3 [KwH/Hour])) = 41667 [Hour] = 4.75 [Year]

But, as you've calculated, the batteries could actually only last around 16 hours per day if fully discharged.

(24 [V] * 200 [Ah]) / 300 [W] = 16 [H]

If you only get 16 hours then the payback time increases according to the inverse of the fraction of the day you ran off solar;

4.75 [Year] / (16 [H] / 24 [H]) = 7.125 [Year]

Of course that assumes batteries' performance is staying the same and the solar system is supplying ample power.

• Often manufacturers will provide a chart showing the capacity degradation by cycle count so you should check that to derate the capacity over time. Doing so should be more accurate than just assuming the battery will stop working completely at 2K or 5K cycles.
• Use a calculator to figure out how your solar system will perform: https://re.jrc.ec.europa.eu/pvg_tools/en/#PVP

 

[Symbioquine]

Some examples of capacity degradation charts for different batteries; (don't just use these ones, find the actual charts for the batteries you're considering)

Renogy 200AH AGM:



Eve LF280K LFP:



To turn those graphs back into numbers, you want a tool called: WebPlotDigitizer

 

[MrModules]

Found this one for a 250Ah 12v and slightly cheaper because its on sale. 1600 at 100% and rated for 15years of service. Its confusing me that Lifepo can use 100% but it seems to suck compared to just using a AGM that has the same 30-50% requirement. just that lifepo's are lighter and more expensive. i dont want to end up spending $10k to just run my pc with 30% DoD when a 13Kwh tesla power wall is $12k and comes basically all done and will draw from the whole house.


A 12v 50Ah has ~2200 at 100% DoD and up to 7000 at 30% from a blurry graph, but isnt there a limit to how many batteries you can stack? depending on brands. I'd need two sets of four to get the same setup. Seems like smaller are more stable than larger ones.

 

[Symbioquine]

Without information about which batteries you're looking at, it's hard to refute what you're saying specifically, but it doesn't sound right to me. Maybe provide links to the batteries you're comparing?

 

[MrModules]

https://www.batterycentralbrisbane.com.au/batteries/solar-and-accessories/off-grid-solar/q-lfp12200/ <link for the store that im looking to buy
https://maxonbatteries.com.au/shop/q-lfp12200/ <and the site they are from i guess? its easier to read from this link

 

[Jordi]

Hello, im looking for some feedback if its cost effective purchasing 2x batteries (them alone) compared to if i just paid my electricity bill.
A few points are:
- I dont have batteries
- no fuses/breakers
- no charge controller
- I pay ~$0.2 per Kw/h
- I do have spare solar pannels

My friend and i have dont some looking around for batteries from physical store (you can walk into) As we think this will be better if anything was to go wrong with the batteries, thered be more of a store than a online held desk. The overal goal is to run my PC on these batteries to slowly cut back our power bill as well as begin a hobby.

For the picture below we have 2 batteries costing $2500 ($1250 each) 12.8V 200Ah upping to 25.6v (24v) 200Ah
Fulling draining the battery each day says we get ~2000 cycles coming to $0.51 Per cycle = $1,020 or with help of a solar pannel and charge controller while limmiting myself to ~50% drain i get ~5000 cycles which = $2,550 that covers the batteries. Would this be worth as a 'hobby' side of things? The charge controller i need (from what victron says) is another $700-800.

View attachment 81985

My Pc runs at ~300Watts with 2 screens included, i have never seen it over. With that the running time we calculated is between 7-8 hours without solar and up to 13-17 hours with solar providing the solar pannels can charge and run my pc at the same time. I do use my computer for those hours each day.

After I check that the dimensioning of the installation is correct; which in your case seems quite fair, I usually try to oversimplify the ROI calculations. The only issue I see is that in consequent sunny days, If you miss consumption for one single day, you will quickly start dumping free kWh to the grid.

Considering that:
- You don't get paid for the electricity sent to the grid but that you will be able to use 90% of the electricity produced thanks to the battery.
- In your location you have an average of 5 hours of sun per day (365 days per year).
- The battery offers more cycles than the payback time.

990w = (5h x 990w x 365 days) = 1086,75kWh per year; from which 1626 kWh are used and the rest dumped. That is at 0,2 USD per kWh; 325 USD per year, so 7,7 year payback.

If you were able to count how much production you are able to use nowadays without battery, then you could isolate the true returns from a battery. So, say that without battery you are able to use 20% of the production and the rest is dumped to the net; then your battery effectively brings you 70% capacity usage; 253 USD per year, 9,9 years payback.

So, after you break even the battery, you will have a few years to profit from it. Plus I hope after the 5000 cycles, you will still be able to use it at lower capacity.

 

[MrModules]

Should of said im AUD, though i guess calculations are the same, our sun hours are higher though, they say roughly 8 hours i think we had in our notes give or take depending on cloudy days. Its not connected to the grid so its plain dumped surplus on what would be made.
solar > charge controller > 2 batteries > inverter > pc. as the primary goal.
if i buy the other batteries (the 250Ah ones) its slightly cheaper with more capacity extending day use and/or battery logevity.
https://maxonbatteries.com.au/shop/q-lfp12200/ < original
https://www.aussiebatteries.com.au/atlas-250ah-lithium-deep-cycle-battery-online <currently on sale
the maxon qiamtium ones dont have a degration chart for DoD over time, even emailing the company proved they didnt have one.

 

[Rednecktek]

For the picture below we have 2 batteries costing $2500 ($1250 each) 12.8V 200Ah upping to 25.6v (24v) 200Ah

First off, unless those batteries are made of Gold and Unobtanium, you're getting ripped off. Those batteries are a version of Lead Acid I assume which means your 200Ah bank is really only 100Ah after 50% DoD. If you're trying to save money and maximize your ROI, you can get 20% more capacity for $400 if you use the Wally World Deep Cycle 120Ah batteries for $100ea so that's 24 batteries @ 120Ah each X 50% DoD /2 for the 24v conversion. That's 720Ah or 17.3KwH worth of battery and still enough to take the wife out to dinner as an apology for spending all that money on batteries.

If you're ordering online rather than wanting something you can buy in stores, any of the 200Ah Chins/Ampertime/Bob'sBattery/whatever are a much better deal.

The charge controller i need (from what victron says) is another $700-800.

Second off, since you're doing this as a "hobby" system, unless you really, REALLY want the bluetooth and internet and online monitoring 24x7 and washing the car and vacuuming the floors and emptying the cat box, don't get a Victron. There are many MANY other manufacturers out there with perfectly suitable charge controllers for about half the price that will turn solar panel power into battery power.

 

[MrModules]

First off, unless those batteries are made of Gold and Unobtanium, you're getting ripped off. Those batteries are a version of Lead Acid I assume which means your 200Ah bank is really only 100Ah after 50% DoD. If you're trying to save money and maximize your ROI, you can get 20% more capacity for $400 if you use the Wally World Deep Cycle 120Ah batteries for $100ea so that's 24 batteries @ 120Ah each X 50% DoD /2 for the 24v conversion. That's 720Ah or 17.3KwH worth of battery and still enough to take the wife out to dinner as an apology for spending all that money on batteries.

That may work if i was american. Australia likes to put up the prices.

 

[Rednecktek]

That may work if i was american. Australia likes to put up the prices.

Look up your local stores for "Leisure Batteries" and see what's available. I think that's the term you need.

 

[Jordi]

Should of said im AUD, though i guess calculations are the same, our sun hours are higher though, they say roughly 8 hours i think we had in our notes give or take depending on cloudy days. Its not connected to the grid so its plain dumped surplus on what would be made.
solar > charge controller > 2 batteries > inverter > pc. as the primary goal.
if i buy the other batteries (the 250Ah ones) its slightly cheaper with more capacity extending day use and/or battery logevity.
https://maxonbatteries.com.au/shop/q-lfp12200/ < original
https://www.aussiebatteries.com.au/atlas-250ah-lithium-deep-cycle-battery-online <currently on sale
the maxon qiamtium ones dont have a degration chart for DoD over time, even emailing the company proved they didnt have one.

Adding the inverter costs and so on, you will not break even I am afraid. The price per kWh should be more on the 0,3dollar side and the equipment costs somewhat cheaper.

Assuming 5 hours of sun per day is a way to say some days 8 hours, some days no sun, thus average 5 hours per day, 365 days per year. The 5 hours include the indirect or passive production from a cloudy day which also account for a significant amount all together. (eg. 10 hours of cloudy weather = 1 hour of sun).

 

[MrModules]

I may wait a few years before trying again, or when my bill changes. scrounging around for deals it looks like i lose out a little (even though the setup will last years) or i might save for $3k for a decent test build. over the years of it running, hopefully ill pickup a better job. $3k over atleast 4 years doesnt sad bad does it?