Determining Payback & adding storage

[Stephen InGeorgia]

So, my power company has slashed their purchase price for my electricity down to $.04/kw. I'm grid tied and they meter on an instantaneous value, so I now buy for $.13 and they buy my excess for $.04. How do I go about thinking of adding storage and the cost benefit of it?
Setup: 13Kw of solar into two SMA SunnyBoy 5.0. Each output to a 200A service panel (yes I have 400A service) so I think they share the load since they are both behind the meter (maybe not?)
Throwing around the idea of adding 30Kw of storage through a SolArk 8K and telling it to do the following: charge battery and power service panel 1, then when battery fully charged, sell to grid. I'd leave one of the SB 5.0 as is and just replace the other inverter. EG4-LL rack at $11000 + $6400 SolArk - 30% tax = $12180 investment. This means a 12-ish year payback. Seems like a wash if I need to replace anything.
Last month I sold 1000Kw to the grid for a measly $40 because I don't have storage. Storing it is beneficial, but the payback on the investment seems long.
Thoughts?

 

[SeaGal]

a) build your own batteries to reduce investment cost ?️
b) move to Europe where electricity costs $.50+ per kWh to buy

 

[timselectric]

I believe that you can ad Sunny Island inverters to work with the Sunny Boy stuff and add storage to them. But I'm not familiar with that equipment.
@Hedges is the expert on that equipment.

 

[GregTR]

Napkin math here:
Your delta between buy and sell rate is $0.09/kWh. So you'd have to store 135MWh to break even on your investment, not accounting for price increase of electricity or the time value of your initial investment. For you to store 135MWh, assuming 85% efficiency, you'd need 160 MWh of solar generated. That's 160 months at 1MWh/month if that is indeed what you're exporting in an average month. That's 13.3 years.

I would not get the battery just for the payback's sake, but there are other benefits such as resiliency, backup, piece of mind that you may or may not be able to put a price on.

I'm 100% WFH so having battery backup is important. I'm also betting on price of electricity and inflation is going to go up in the long term making my ROI shorter.

For me, batteries also allowed to do a good bit of TOU arbitrage, I get free electricity at night but "pay" $0.29/kWh during the day. With batteries I never pay for electricity as it covers my sunset to 9PM and 7AM to sunrise time periods which makes electricity essentially free at my house.

 

[BentleyJ]

At 13 cents per kWh your electricity costs are very low but most likely will increase quite a bit over the years so your payback may end up being a little less then your 12yr projection. However, Its very difficult to justify battery storage based only on payback especially since it is likely whatever battery you choose will need some maintenance and maybe a BMS replaced or even a couple of cells so there are unknown costs.
Personally, I consider battery back up a luxury item and love the fact we don't have to worry about rolling black outs, flex alerts or the so called Public Safety Shut Downs. Its really no different than purchasing a fully loaded premium brand SUV or Electric car because you simply want it but don't really need it.

 

[houseofancients]

a) build your own batteries to reduce investment cost ?️
b) move to Europe where electricity costs $.50+ per kWh to buy

make that usd 0.95
and they will buy your solar power for 0,05 usd

we're getting tickled here in place where we dont want it

 

[wattmatters]

Throwing around the idea of adding 30Kw of storage through a SolArk 8K and telling it to do the following: charge battery and power service panel 1, then when battery fully charged, sell to grid. I'd leave one of the SB 5.0 as is and just replace the other inverter. EG4-LL rack at $11000 + $6400 SolArk - 30% tax = $12180 investment.

Let's assume you can get the equivalent of 3600 full charge/discharge cycles from your 30 kWh of storage. That's 108,000 kWh.

Costing $12,180, that's 11.3 c/kWh just to cycle energy through the battery.

Even if the extra PV generation capacity could not be exported, it's only 1.7c/kWh lower than the cost of your grid power. Over 108,000 kWh, that's a saving of just $1836 which would take well over a decade to realise.

If the extra PV capacity could be exported then given your import-export tariff differential is only 9 c/kWh you will be losing money to cycle energy through the battery.

Throwing around the idea of adding 30Kw of storage through a SolArk 8K and telling it to do the following: charge battery and power service panel 1, then when battery fully charged, sell to grid.

If the grid is paying you 4c/kWh then why would you do that instead of just exporting it directly from the PV during the day?
A battery just adds cost and loses some of the energy along the way.

I would not get the battery just for the payback's sake, but there are other benefits such as resiliency, backup, piece of mind that you may or may not be able to put a price on.

+1

And if you do this then you can more appropriately scope a system to deliver these benefits for fewer $.

 

[Stephen InGeorgia]

Napkin math here:
Your delta between buy and sell rate is $0.09/kWh. So you'd have to store 135MWh to break even on your investment, not accounting for price increase of electricity or the time value of your initial investment. For you to store 135MWh, assuming 85% efficiency, you'd need 160 MWh of solar generated. That's 160 months at 1MWh/month if that is indeed what you're exporting in an average month. That's 13.3 years.

I would not get the battery just for the payback's sake, but there are other benefits such as resiliency, backup, piece of mind that you may or may not be able to put a price on.

I'm 100% WFH so having battery backup is important. I'm also betting on price of electricity and inflation is going to go up in the long term making my ROI shorter.

For me, batteries also allowed to do a good bit of TOU arbitrage, I get free electricity at night but "pay" $0.29/kWh during the day. With batteries I never pay for electricity as it covers my sunset to 9PM and 7AM to sunrise time periods which makes electricity essentially free at my house.

Thats kinda where I'm at, such a long payback since we are inexpensively on the grid. Thanks for the input!

 

[Stephen InGeorgia]

Let's assume you can get the equivalent of 3600 full charge/discharge cycles from your 30 kWh of storage. That's 108,000 kWh.

Costing $12,180, that's 11.3 c/kWh just to cycle energy through the battery.

Even if the extra PV generation capacity could not be exported, it's only 1.7c/kWh lower than the cost of your grid power. Over 108,000 kWh, that's a saving of just $1836 which would take well over a decade to realise.

If the extra PV capacity could be exported then given your import-export tariff differential is only 9 c/kWh you will be losing money to cycle energy through the battery.


If the grid is paying you 4c/kWh then why would you do that instead of just exporting it directly from the PV during the day?
A battery just adds cost and loses some of the energy along the way.


+1

And if you do this then you can more appropriately scope a system to deliver these benefits for fewer $.

Thanks for the viewpoint. I got the SunnyBoys for the 2Kw secure output without the grid to keep the freezers frozen. A recently added battery backup to the well pump which is the only required 240V load. I think I've made my decision to stay my current course and maybe get a small battery backup for other things in the house.