Home battery backup usage case discussion

[EbS-P]

I want to discus the best way to use my new whole house battery backup. It’s a Tesla power wall 3 with 13,5 kWh of storage. Net metering makes these really a standby power system when the grid goes down. Its default is to charge from solar but it will charge from the grid but I’m not sure if I can force that.

I can set the minimum state state of charge under normal grid up conditions. Right now I just picked 70%. That means in the evening once the PVs are not producing enough the battery will discharge all the way down to my set 70% before using grid power.

With net metering this is rather pointless to save me money. I get net metered then sell my overage back at $0.03/kwh. What I don’t know is if my surplus carries over month to month or the pay out at the end of the month and I start each month with now surplus. (I should have asked but didn’t really care what the answer was because it’s not like has a choice in utility providers and I don’t think the extra incentive money is going to be around for ever).

So the question is if the battery has a 10 year warranty (70% capacity at 10 years with unlimited cycles. And another warranty use case 70% at 10 years and 38 Mwh throughput. That works out to be 280 full cycles per year for 10 years ) where should I set my discharge limit. Really low meaning the battery will have more cycles or at 100% meaning the battery is only used for backup power? How much greener is using more of the battery on a daily basis. Is storing my solar electricity and using a full batteries worth of electricity each day any greener than just sending my excess generation to the grid.

here is a pretty graph showing the battery usage in green, solar in orange and grid in grey.

 

[stoveliker]

I don't know.

Though it depends on what your grid has. If you're offsetting coal by sending it to the grid, it's good.
If you're offsetting hydro, put it in your battery and use more yourself to avoid needless transport of power (=losses!)

But some utilities have a once a year kWh bank reset, i.e. the accounting happens once a year.
If that is the case for you too, normally that would be one year from when you started. But you want to reset that to the first month in the year that your usage breaks even after the winter.

 

[bogieb]

In NH, you get one chance to cash in per year - March (about the time of year you should be producing what you use). HOWEVER, ours rolls over into perpetuity if you don't cash in. I have to actively tell them I want to cash in or it just rolls over to the next year.

I've heard some states you use it or lose it at the end of the "year" (being March or whatever month they chose, or maybe your start date anniversary). Some states, don't roll over to the next year, but will pay out if you ask for it. Pretty sure there are no utilities that pay out monthly (I could be wrong, but I'd bet on it - at least here in the USA).

It is important to figure that out in order to not lose money that is due to you, or at least not spend money you don't need to. It is also important information for you to make an informed decision about how to use your battery.

A cycle on a battery is all the way to zero - it is not how many times it is charged (at least it is for lifepo4 battery - I'd assume it is the same for Tesla). Many people set a 20% lower limit so they always have a buffer and then make sure they are fully charged before expected storms or power outages. Maybe figure out how much you would use in a typical night, and set the lower limit at that point - then it can recharge during the day.

My feeling is it would be greener to use your battery storage at night:

• The damage has already been done for the equipment you have on hand - so basically no more emissions are happening. The battery will naturally degrade overtime regardless of whether you "cycle" it or not. It may actually degrade faster if you don't use it (IDK)
• When you use grid power, there are additional emissions happening at the power plant (okay, a drop int the bucket for one household - but it adds up)
• There are additional emissions for obtaining any fuel source for the power plant to run that overnight
• There are additional emission for all phases of upgrades to the grid to support more energy flow (unless youare right next to the power plant.

 

[EbS-P]

any excess production is credited monthly. It’s not good. It’s better than the “residential solar choice” rider which forces time of use billing and credits net production for each TOU period. We will be forced on to it 15 years. I’m smart I’ve read the rider 3 times. I still don’t comprehend it completely.

It’s not like I have a choice. Best simple explanation I’ve found is below.

Everything North Carolina Residents Can Expect From Duke Energy’s New Solar Net Metering Policy

Duke Energy's net metering proposal has officially been passed by the NCUC, meaning big changes are coming this summer for solar homeowners.

www.solarreviews.com

To me the battery usage question (how much to discharge daily) depends on how long the battery will last and how to balance the financial side with battery longevity. Factor in how important lowering carbon footprint matters some. LFP cells used in the PW3 probably have enough data for a decent assessment at this point but I haven’t found it. Im impressed that my model X battery at 8 years old and about 50k miles with was probably parked in hot Florida with high SOC and it’s at 80% rated capacity.

I need to think about the bill cycle net metering more. This is a huge negative for the heating season for those utilizing heat pumps. I this means I will keep the wood stoves cranking.

Here is the rider. Net metering bridge rider.

https://www.duke-energy.com/-/media/pdfs/for-your-home/rates/dep-nc/leaf-no-669-rider-nmb-ry1.pdf?rev=632ff216660b4a078ba6fd7f4ff5e195