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

 

[begreen]

I'm starting to notice on the EV6 forums, car owners asking how to V2L power to their house that are without power due to Helene.

 

[EbS-P]

I'm starting to notice on the EV6 forums, car owners asking how to V2L power to their house that are without power due to Helene.

And honestly at this point most people shouldn’t cobble together some VL2 load solution. Sure it can be done if one plans ahead. We had plenty of warning how bad it would be. We’re fine here. Cranking out 3-4kw of solar battery at 100% (I changed my setting last night to use my battery only for backup). Really Tesla could many utilize and extra set of conductors and relays and go HVDC direct from car to Powerwall. This would allow DC charging and not require any onboard exporting inverter on the vehicle. If you wired the charger directly from the PW you could choose AC or DC power the the wall using one set of conductors. It does seem silly to take DC invert it then rectify it again to charge your batteries. At any rate 50 kWh for home backup is good for emergencies. No AC that could be conserved to last a few days.

 

[begreen]

Yes, ideally this should be done with planning and advanced testing. Most people however are just hoping to run a refrig and maybe a freezer to avoid food loss. This can be done with extension cords in a pinch.

 

[EbS-P]

Yes, ideally this should be done with planning and advanced testing. Most people however are just hoping to run a refrig and maybe a freezer to avoid food loss. This can be done with extension cords in a pinch.

Agreed, but after returning after a storm the spoiled food was not a concern. Do we have a roof? Did we flood? Any dangerous trees? We had enough dry good to eat for a week but it would’ve gotten old.

 

[EbS-P]

Virtual power plant VPP application was approved. I’m now making 52$ a month. That’s the equivalent cost of 470 kWh or free home EV charging and then some.

This is my current thought. I want to get some cycles on the battery. While it’s new. Useful or not. I do it know. I’m set to discharge to 50 of my 13.5kwh daily. The VPP can draw down to 20% 35 times a year.

is it lower carbon emissions to store my green power on site and use the rely on the battery more sending less power to the grid, or send more clean power to the grid during the day and relying only on the battery as a backup. Generally look at the data the dirtiest time for the grid peaks at 6pm but only get 10% cleaner as the evening goes on.

At mile level I don’t think it matters one bit every bit of the power I’m generating replaces dirty fuel. Does it matter if I send to the grid at time of production or store it?

 

[gthomas785]

I assume charging the battery comes with some % of loss? So charging the battery less often is better from that perspective.

 

[stoveliker]

I though the round trip efficiency is about 90%, so any cycle looses a decent amount of kWh (which would have to be offset against transport losses when feeding it to the grid).

 

[gthomas785]

I though the round trip efficiency is about 90%, so any cycle looses a decent amount of kWh (which would have to be offset against transport losses when feeding it to the grid).

So the nice thing about being grid tied is when you export power you are mostly exporting to your closest neighbors unless they also have solar panels. If you share a transformer with some neighbors, you are actually decreasing the current through the transformer so the total system efficiency goes up.

 

[EbS-P]

So the nice thing about being grid tied is when you export power you are mostly exporting to your closest neighbors unless they also have solar panels. If you share a transformer with some neighbors, you are actually decreasing the current through the transformer so the total system efficiency goes up.

Carol won’t appreciate it though

 

[DBoon]

I want to discus the best way to use my new whole house battery backup.

There are really three considerations for this:

1. How do I run my whole house battery backup for maximum lifetime of the battery?
2. How do I run my whole house battery backup for best return on investment (given the utility rate structure)?
3. How do I run my whole house battery backup for most positive environmental impact?

For 1, you'd really want to minimize deep discharge cycles (minimize going below 50%, and ideally going below 60%) and also minimize charging above 80 or 85% of capacity. I recall reading that LiFeP batteries are a little less susceptible to degradation due to deep discharges. I think real data is going to be tough to come by re: this - I'm basing this on rules of thumb that I have heard in relation to other batteries, and (as an engineer by training), I hate rules of thumb, but its all we seem to have for this right now.

For 2, you'd have to analyze this one yourself.

For 3, the Independent System Operator (ISO) might publish some graphs of power prices at times of day, or even show what the generation mix is at any given moment in time. The NY ISO publishes data https://www.nyiso.com/real-time-dashboard and I've pretty much deduced that for best environmental impact I want to:

In Summer - push my demand down (and give all my solar to the grid) during the day and operate heavy loads at night.​

In Spring and Fall - run any heavy loads during days when the sun is shining on my solar PV. If it is sunny and windy, then power prices might dip into negative territory.​

In Winter - run any heavy loads during daylight hours when it is warmer and (hopefully) sunnier.​

All times of year - avoid any heavy loads during the morning and afternoon peaks (i.e., the head and tail of the notorious duck curve of electricity demand)​

​

Note that I am 100% net-metered, so your strategies might differ from mine.

 

[EbS-P]

my responses

1– you can only set the minimum discharge. It changes to 100% every time. I agree with reducing number of cycles. I do think calendar age will have capacity impacts.

2-I’m net metered per billing cycle. .11 $/kwh down .03 up. So if I run credit at the end of the month but didn’t use my battery I could have charged and discharged my battery 10kwh a day for 30 days saving the cost difference of 300kwh. Or 25$ a month. Take off 10% for round trop
Effect if 90%. Only if I have sent that excess 300kwh to the grid. That hardly seems worth the extra cycles but that’s $3000 over ten years. Reduces ROI by a not small amount. But I think there will be lots of months where I don’t run excess. I could just always charge the car at home. Poof there 300-400 kWh a month.

3- duke I think has enough solar that it’s probably cleaner to run all heavy loads during peak sun hours. Nuclear plant is close. So my thinking is. Run big loads off solar. And use the battery to get past peak demand. Coast on nuclear all night.


My solar company didn’t offer any insight but I didn’t ask. I really think their response would. Sign up for virtual power plant and only use the battery for backup. It’s just simple. And I’m probably overthinking it.

 

[gthomas785]

my responses

1– you can only set the minimum discharge. It changes to 100% every time. I agree with reducing number of cycles. I do think calendar age will have capacity impacts.

2-I’m net metered per billing cycle. .11 $/kwh down .03 up. So if I run credit at the end of the month but didn’t use my battery I could have charged and discharged my battery 10kwh a day for 30 days saving the cost difference of 300kwh. Or 25$ a month. Take off 10% for round trop
Effect if 90%. Only if I have sent that excess 300kwh to the grid. That hardly seems worth the extra cycles but that’s $3000 over ten years. Reduces ROI by a not small amount. But I think there will be lots of months where I don’t run excess. I could just always charge the car at home. Poof there 300-400 kWh a month.

3- duke I think has enough solar that it’s probably cleaner to run all heavy loads during peak sun hours. Nuclear plant is close. So my thinking is. Run big loads off solar. And use the battery to get past peak demand. Coast on nuclear all night.


My solar company didn’t offer any insight but I didn’t ask. I really think their response would. Sign up for virtual power plant and only use the battery for backup. It’s just simple. And I’m probably overthinking it.

As for #3 just remember if you're running peak loads during the day to use clean energy then that clean energy is not available to power someone else's load so the net environmental impact is nothing. Timing your heavy loads off or running on battery during peak demand is the only thing that might have a slight environmental benefit. Reducing or eliminating energy use will have a much larger impact.

 

[EbS-P]

As for #3 just remember if you're running peak loads during the day to use clean energy then that clean energy is not available to power someone else's load so the net environmental impact is nothing. Timing your heavy loads off or running on battery during peak demand is the only thing that might have a slight environmental benefit. Reducing or eliminating energy use will have a much larger impact.

Best case scenario for October my battery is charged by 1pm. These are really clean months for our grid overall. Lots of sun no one is heating yet.

I think your argument is true. (I have made it to myself) If that’s the case. It does matter when in charge or discharge because it’s all the same. But All my Green kilowatts are being consumed. I argue if if the grid is greener when I store them and dirtier when discharge the net effect is less CO2 than if I didn’t use my battery. All my free power gets consumed the. I consume dirty power during peak.

Not being home the house can run completely off grid. Except the power wall lets the grid supply any loads less than 300 watts so I can’t ever be 100% self sufficient unless I leave a few more lights on.

 

[DBoon]

As for #3 just remember if you're running peak loads during the day to use clean energy then that clean energy is not available to power someone else's load so the net environmental impact is nothing.

This would not be true if there are excess renewables or other clean power on the grid during that time period, and they were furloughing wind or cutting back hydro because I am putting a lot of excess solar on the grid. And this is exactly the situation in upstate NY for sunny (and especially both sunny and windy) days in the spring and fall, and warmish winter days.

I think the really key thing from an environmental standpoint is just don't use power (or limit power usage) between the morning and afternoon peaks (these actual times vary by time of year, but generally 5-10am and 4-8pm). This is a pretty big thing to do to keep dirtier power off the grid and pretty easy to do if you have a modest size battery-backup. If you want to keep it really simple, don't worry about anything else.

 

[EbS-P]

Here is typical day with some AC use. Then a day with no AC. Both of the days am set to use 80% of the 13.5kwh battery. Gray is grid power, yellow solar, and green is battery. Cooking uses quite a bit of the battery. If we are not home the battery can run the house for about 2 days.