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ROI on a DIY Powerwall - non existant?

petee_c

New Member
Joined
Feb 20, 2021
Messages
64
Location
just outside Waterloo, ON Canada
I came across this forum on my search for info on choosing/building a LiFePO4 battery for a kayak trolling motor build. The info here is awesome.

Then part of me was wondering about building a DIY Powerwall to use at home. I was trying to think of something to use it for, I wondered about moving my 75 gallon planted aquarium to an off grid scenario with charging from a solar setup

Between the lights, pumps and heaters for my aquarium, I estimate I would need about 2Kwh of energy per day. Which would mean I would need to use a 250Ah 12V battery or thereabouts to make it work.

As I was piecing stuff together on paper and in my head, I quickly came to the realization, that I would never see a ROI for this little project. At 2Kwh per day, my aquarium uses about $0.27/day of electricity from the grid. Say somewhere around $100/yr.

Price estimates in Cdn$$
280Ah LiFePO4 with shipping from China $550
400W solar panels , charger from China $500
BMS, inverter, cabling $250...

Total about $1300 if lucky, and I will need a way to augment the charging with grid power. I'm near Waterloo, ON, and I don't have very good southern exposure on my roof. 43.5N latitude

13 yrs before Return on Investment, and by then my battery probably has a 50/50 chance of being dead...
 
Return on Investment well that would be nice, but at this size think of more of a hobby , do you worry about ROI on the fish , tank and heaters ?

running off your power wall when the grid is down , now i find that priceless :)
True...

our grid hasn't been down for years, cause I bought a 3KW portable gas generator about 7 yrs ago for $300USD. 1 blackout since then, and it was probably less than 6hrs long....
 
Part of the Return is the joy it brings you⭐

That said, part of the return is the autonomy. It’s probably possible to reduce the cost further with component selection.
 
True...

our grid hasn't been down for years, cause I bought a 3KW portable gas generator about 7 yrs ago for $300USD. 1 blackout since then, and it was probably less than 6hrs long....
yup thats how it goes have not started the gen in 4 years , find my self looking at storms to see if there will be a power outage , so i can use my battery back up :) oh look it snowing out :)
 
I bought a small inverter generator just to not have to throw out everything in my refrigerator after an overnight power outage. Then I starting experimenting with what else it could run. That lead to not wanting to drag out the generator and not deal with running extension cords all over the house. And then built a battery backup to handle overnight outages until I decide to drag out the generator. Now I am looking at solar panels to keep the battery topped off. It is a bit of a hobby right now, I guess. And it's been fun to research and figure this stuff out (but not as much fun to run new circuits through the attic).

I can't say it is that much different from my car hobby. There is certainly no monetary return on investment, but I sure do enjoy working on them ... most of the time.
 
why wouldn't you move it to a circuit in your house that you could use it on daily? or is that not easy to do?
at this point mine is strictly a battery back up ,with no solar , but if i added solar all i would have to do if flick the breaker off and it would switch over to the furnace automatically

 
If you can leverage the power wall (ignoring solar) to charge on cheaper power during off peak and utilize the power during peak pricing, you can make a diy power wall ROI pretty easily.
Add solar charging to it and you can probably make it even faster.

A simple idea I’ve been considering is to simply run my computing systems from one. The load is relatively consistent so I can plan the loads. These days I really like the idea of a no-cost hot tub. Those things are energy suckers, but oh so nice in the winter!
 
I'm offgrid. my roi is the $5000 a pole the rec wanted to run electric to my house. Also 5acres offrid runs about $2500 here 5acres ongrid runs about $50,000
 
And then built a battery backup to handle overnight outages until I decide to drag out the generator. Now I am looking at solar panels to keep the battery topped off. It is a bit of a hobby right now, I guess. And it's been fun to research and figure this stuff out (but not as much fun to run new circuits through the attic).
I just finished adding solar PV to my battery backup system. Up to now they were being kept topped up by grid power. Now the solar keeps them ready, as well as being able to extend the backup duration of supply from the battery alone. Generator is still there for redundancy and if the outage extends for long periods.

Because the PV array output is way more than needed to keep the batteries topped up, I now use it to run the pool filter pump. Battery just acts as ballast but is hardly used.

So I figure whatever consumption I can move to my daytime off-grid power is being done at the amortised cost of the array infrastructure alone given the battery system's primary duty is for outage backup. The racking and panels were second hand so that helps. The pool pump uses about 1MWh/year, so that's a nice load to shift away from from my grid tied system. Most of that energy will be additional excess exported to the grid for extra bill credits and a smaller proportion will be grid import replacement (which is more valuable than grid export).

All that said, ROI for this DIY project was secondary but the ability to get some returns is nice. Like you I started out wanting a better grid outage backup system on a budget but also to learn new things and have a project to do that was at least useful.
 
Not long after posting in this thread I added a few solar panels. The system worked fine for a couple of power outages. Very happy with it.

Last month I doubled my battery capacity. With the refrigerator running off of the system all the time I wake up to between 60% and 70% of battery capacity. 2 days of rain/heavy clouds would run the system too low to back up much of anything.

I think I am going to need more solar panels!
 
I came across this forum on my search for info on choosing/building a LiFePO4 battery for a kayak trolling motor build. The info here is awesome.

Then part of me was wondering about building a DIY Powerwall to use at home. I was trying to think of something to use it for, I wondered about moving my 75 gallon planted aquarium to an off grid scenario with charging from a solar setup

Between the lights, pumps and heaters for my aquarium, I estimate I would need about 2Kwh of energy per day. Which would mean I would need to use a 250Ah 12V battery or thereabouts to make it work.

As I was piecing stuff together on paper and in my head, I quickly came to the realization, that I would never see a ROI for this little project. At 2Kwh per day, my aquarium uses about $0.27/day of electricity from the grid. Say somewhere around $100/yr.

Price estimates in Cdn$$
280Ah LiFePO4 with shipping from China $550
400W solar panels , charger from China $500
BMS, inverter, cabling $250...

Total about $1300 if lucky, and I will need a way to augment the charging with grid power. I'm near Waterloo, ON, and I don't have very good southern exposure on my roof. 43.5N latitude

13 yrs before Return on Investment, and by then my battery probably has a 50/50 chance of being dead...

Here in MD where it is regulated so any excess solar capacity is bought for the same price I pay storage becomes something just for emergency backup if the grid goes down.

So my focus is to generate enough solar in the few hours of sun to cover our 24 hour consumption so we are fully offsetting our power footprint.
 
Utility rates could be $0.05, $0.15, $0.50/kWh

All hardware for grid-tie PV (but not labor) costs about $1/W, amortize over 10 years $0.05/kWh.
DIY LiFePO4 battery can be $0.05/kWh if it lasts 10 years of 80% DoD cycles.
Inverter to work with battery may be more $$$, or maybe a good hybrid for grid-tie net metering (or zero export) will handle that.

Depending on your utility rates, PV can have 1 to 3 year payback.
Batteries, depends on the value of peak-shaving during hours of higher rates.
But if your utility rate is $0.05 to $0.10, then no savings.
 
DIY LiFePO4 battery can be $0.05/kWh if it lasts 10 years of 80% DoD cycles.
It can be but many applications will never get close to that level of capacity utilisation. I suspect most people design their battery system specifically to avoid such a high average daily capacity utilisation, or their actual use is somewhat less frequent than daily.

Aside from the differences between applicable utility rates (peak / off peak / export) and the upfront cost to build/install, battery financials very much depends on average capacity utilisation. Round trip efficiency also matters when doing the financial calculations.
 
Round trip efficiency also matters when doing the financial calculations.

Somewhat. Or a great deal if wholesale rates at utility scale. But for consumer, if $0.05/kWh is cost of PV, inefficiency charging/discharging batteries doesn't matter so much. We might be storing power when credit would have been $0.15, drawing when charge would have been $0.50. PV is cheap, batteries are expensive.

In my case, AGM could be $0.50/kWh if I got 650, 70% cycles in a decade. Grid backup, so I don't expect that many. I will cost me several $$/kWh for convenience. But its other function is a buffer, so I can run PV direct during the day when grid is down. My battery is way undersized (not 3 days or 1 day, more like 2 hours worth of storage.)
 
But for consumer, if $0.05/kWh is cost of PV, inefficiency charging/discharging batteries doesn't matter so much.
The amortised value of the solar PV output only sets a floor on its "value" but that does not mean it is only worth that much. It may have a more valuable immediate use. Or it may not - it will depend on individual use case.

e.g. if you can export the solar PV energy to the grid for 10c/kWh then that sets its floor value, not the 5c/kWh it costs to produce.

This is where round trip efficiency can have an impact.

If I get 10c/kWh for export of excess PV output and pay 25c/kWh for grid imports, then the benefit of storing the energy instead to use later is not the 15c/kWh difference in tariffs, but the difference accounting for the round trip losses. e.g. I forego 10c/kWh of export income but I only get 85-90% of that energy back later, meaning the imports replacement value is now less than 25c/kWh. So the overall benefit i reduced from 15c/kWh to around 12c/kWh. Now that may well be fine, but it does reduce the financial benefit by quite a chunk and that will push payback out by quite a bit.

But if you can't export excess solar PV or the export value is less than the amortised production cost, then storing it for later use may make sense.

Or it may still make more sense to find alternative uses for the energy, such as water heating, or pre cooling/heating a space. It helps to consider the relative difference in total cost/benefit for the different uses of the energy when assessing relative financial benefit.
 
Some of us bump up against our export limit. I originally aimed panels at afternoon sun. For me, adding panels in parallel aimed at the morning sun means I can give the utility 3 kWh at $0.15, earning a credit for most of the $0.50 they charge me late in afternoon. So I'm doing that rather than using batteries for peak-shaving (my batteries are only for backup, they aren't drawn from while grid is up.) My peak time of use had been Noon to 6:00 PM, now 4:00 to 9:00 PM. And of course I choose when to turn on discretionary loads.
 
It really depends on how much you pay for your batteries. And ow you use them.
I use 11kwh out of my 18.6kwh of batteries on average everyday. I’m using battery powered grid tie inverters. 11kwh is saving me right at $1 a day. I paid $1400 for the 18.6kwh. They are EV batteries all with roughly 25k miles on them. The ROI for my batteries is 4 years. Before I had batteries I was using the grid tie inverters without batteries. The solar put into these batteries would have otherwise been wasted if I didn’t have batteries. I could have sold to the grid but that would have cost me an atleast an additional 14k and then never seen an ROI on my investment.
I had to do some labor buying full EV packs (and like $6k initially invested) and selling some of them etc. to get my money back but ended up paying $1400 for 18.6kwh.
 
11kwh is saving me right at $1 a day.
Yeah the $1/day sounds about right.

Coincidentally that's about what I'd save if I added a grid tied battery system to my home. I built a battery simulation model to see how a battery would impact what our billing would be. It uses our 5-min energy interval data (PV production and total consumption data) for the past nearly 3 years. We have an 11kW grid tied stystem.

If I had a battery with the specs of a Powerwall 2 (13.8kWh, 10% reserve, 89% round trip efficiency and max charge/discharge of 5kW) then this is what our cumulative billing difference would be:

Screen Shot 2021-08-02 at 10.22.31 am.png

That's also with time of use billing which is advantageous to battery economics (discharging mostly into peak periods) and not accounting for any battery capacity degradation. After 978 days a battery of that spec would have saved us $971 or 99.3c/day for an ROI of under 3% or a payback in the vicinity of 40 years.

Just shows how here in Australia it's the solar PV which does all the heavy financial lifting. And it gets better because fully approved grid tied systems cost ~A$1/W installed (~US$0.70-$0.75), often much less, in some cases as low as A$0.50/W.

I paid $1400 for the 18.6kwh.
That'll help!

A Powerwall 2 installed in Australia is about A$14,500 (or it is where I am). You ain't getting that outlay back no matter how you use it on our grid.

DIY (to save a bucket on the battery system) + grid tie just isn't much of an option here. At least not something that is readily approved for connection. DIY + off-grid, sure.

Typical capacity utilisation for a dozen PW2 owners I surveyed was ~60%. My simulated PW2 over that period has a capacity utilisation of 65%.

13.8kWh * 60%/day = 8.3kWh/day
 
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