How to calculate Powerwall usage

[Docfxit]

I am considering purchasing Tesla solar and trying to figure out how many powerwalls I should get.
Currently I'm looking at a 11.6kW solar system with two powerwalls.
Everything in my house draws 1,000watts continuously 24hrs a day.
If I get 6 hours of PV per day and I hope the powerwalls charge to 100% how do I calculate the amount that will be left in the powerwalls after they drain for 18hrs?

 

[power.saver]

1000watts x 18hrs = 18kWh

Each PW has 13.5kWh energy. So one will not make it to 18hrs.
Two would make 18hrs with 9kWh left over, or 33% remaining (9/27)
Three would make 18hrs with 22.5kWh left over, or 55% remaining (22.5/40.5)

 

[ptdusn]

We have a 9.45kw system with a peak power of 6.8kW and during summer our 2 PWs charge just enough to last past our peak time to around 9:30pm. 20/20 hindsight wished we would have added a 3rd PW.

 

[Docfxit]

Thank you...
That helped me understand that my spreadsheet I created is no where close to what I am going to get.
I would need to break out in the column kWh used per month the number of kWh I use in the 18hrs the PV is not producing.
I am thinking most of the cost will be in the summer when the A/C is running all the time. I really have no idea how much the A/C runs at night.

Any thoughts would be appreciated.

 

[ucmndd]

It’s difficult to answer your question because it’s not clear what your objective is.

Is it to be as close to self-powered as possible? Is it maximizing return on investment? Is it ensuring you can ride out long outages?

With 11.6kw of solar, you really need at least two powerwalls - that’s too much solar for one. Two is really minimum for most loads and use cases. FWIW I have two powerwalls and a similar amount of solar in inland NorCal and while everything is adequate, I wish I had three.

 

[Docfxit]

It’s difficult to answer your question because it’s not clear what your objective is.

Is it to be as close to self-powered as possible? Is it maximizing return on investment? Is it ensuring you can ride out long outages?

With 11.6kw of solar, you really need at least two powerwalls - that’s too much solar for one. Two is really minimum for most loads and use cases. FWIW I have two powerwalls and a similar amount of solar in inland NorCal and while everything is adequate, I wish I had three.

Thank you for the reply...
I'd like to get my electric bill as close to zero as possible. My maximizing return on investment is out the door. I think my pay back period is about 12.78yrs. (until SCE charges more) My usage 24hrs a day is 1000w continuous. So that is going to use up most of my (proposed) two powerwalls already. I seam to keep raising the cost of this solar install.

It would be nice if they would start shipping the Powerwall 3. I hate to place my order now and two days later they come out with it.

 

[ptdusn]

Thank you for the reply...
I'd like to get my electric bill as close to zero as possible. My maximizing return on investment is out the door. I think my pay back period is about 12.78yrs. (until SCE charges more) My usage 24hrs a day is 1000w continuous. So that is going to use up most of my (proposed) two powerwalls already. I seam to keep raising the cost of this solar install.

It would be nice if they would start shipping the Powerwall 3. I hate to place my order now and two days later they come out with it.

We have our A/C set at 75 degrees and right now at 0330 Monday morning the outside temp is 70 degrees. From Midnight to now our house has used 4kW with the A/C kicking on every hour or so. We have a 16 SEER 5 ton unit that's 2 years old, (Bryant/Carrier).

 

[GregBallantyne]

I got my system commissioned and operating on Saturday, so I am into my second full day of operation. I developed a fairly sophisticated estimate tool for performance and production, with monthly granularity based on some daily stuff, including data for perfect capacity, a shade factor, a cloud factor, etc... all applied to power usage historical data. My annual use is 20k to 22k kwh.

Since start of operation, I've been "nerding out" on it - only left the house for an hour (I'm retired). With regard to your Powerwall question, my observation is that adding a 3rd Powerwall to my original contract with two Powerwalls was a really good choice, with regard to achieving the goals I set going in.
My first full day of operation the system generated 49.8 kwh, with peak output reading 13.4 kw. I've got a 17 kw system, but 15.2 kw inversion capacity. There were some clouds, a full sun day probably would have yielded around 60kwh. I ran the house, charged my car to 80% (didn't require much), and the Powerwalls to 95%.

By morning, the Powerwall charge was 59%. Overnight useage is very low, but HVAC was running from time to time. Today is a rainy, cloudy day. In spite of that, I've been self powering right along. Producing, but so far less than 3 kw. I expect that I'll keep on self powering until tomorrow, when there is a partly cloudy day forecast.

I'm fairly certain I would be receiving grid power at some point today had I stayed with 2 Powerwalls, and would have put power on the grid yesterday. Nothing wrong with that, especially since full net metering in my service area is in place, but one of my goals was as high a degree of self sufficiency as possible (with $$ limit or course, which got stretched for the 3rd Powerwall) along with extended power outage powering. This system looks like it will meet those goals very well, especially this time of year. I am putting in a propane fireplace to extend my outage duration ability in winter months, since running my heat pump for heating will be a larger demand than HVAC I this time of year. Also, I'm expecting to fall short annually by somewhere between 7k and 2k kwh, depending on actual performance and some useage reduction projects I'll implement this year. Extra storage would help a little with that, but it's really generation compared to useage is where the shortfall lives. I would need many Powerwalls to save the extra summer power, and still be short.... So Cal of course will be somewhat different - I think you'll get better production for your capacity than is available here, for example.

Let me add that whatever your projections are now, the actual operating experience will be a real learning opportunity. Your initial take will get adjusted a lot. In my case, it's getting adjusted in a positive way. My battery drain overnight pleasantly surprised me. But my heat pump is 2 ton....

 

[Docfxit]

We have our A/C set at 75 degrees and right now at 0330 Monday morning the outside temp is 70 degrees. From Midnight to now our house has used 4kW with the A/C kicking on every hour or so. We have a 16 SEER 5 ton unit that's 2 years old, (Bryant/Carrier).

It sounds like I'll burn through two powerwalls very quickly. I have two A/C units. The heater on one uses 1745w the other uses 1644watts. Many days during winter it could get to 32-40degrees outside. I keep it at 68degrees inside the house. Most of the time I only use one of the heaters at night. The third powerwall will cost me about $6,878 after incentives. I thought I was spending way too much already. I'm hoping I have enough panels to support three powerwalls. It would be a shame to have enough panels and not have enough powerwalls to cover my needs. Maybe they'll give me a 1 year trial and let me adjust the install based on my results. -

 

[Docfxit]

GregBallantyne: Thank you for the input...
Congratulations on PTO. You have gone through a lot and waited way too long to get it running. That must feel great to finally have it producing as you hoped. I think everyone on here should come over to you and through you a party. Of course we wouldn't want to spike your electricity demand to through your plans off. With your short up time, it sounds like you are doing really great.

 

[Matt-FL]

My suggestions if you are planning an install:

1) Get an emporia or sense into your breaker panel. It will give you good detail on how much energy you use during the day and at night on average.
2) If you can, put a few cameras on your chimney that can view your roof during the day. This will allow you to see sunny/shady parts during the day, and how much the obstructions will affect. You can also use this to show the installer how to configure the system (if Tesla, they may break up the strings to accommodate). I did this a month or so before install, and got a good idea of tree and chimney shading that I was able to share with the installers. They made some minor adjustments based on this detail.
3) If going with Tesla, insist they install adequate inverters; I know this is a contested subject, but don't let them sell you on the '125%' overage capabilities. You wouldn't want your car engine running at 125% capacity all the time- insist the designer provide 100% or less on the inverters. I bought a 20.4kw system from Tesla, and they only installed 2 inverters. On the best day, I'll see almost 17kw in bursts. If I had a third inverter, I'd see much closer to my full capacity
4) Figure out how many PowerWalls you will need, then add at least one more. You will never look back and say 'I have too many powerwalls' - well, except for the guy that put seven in his townhouse

5) Ask about putting equipment in your garage (if that is an option for you). I think having at least your powerwalls in the garage in a semi-controlled environment will achieve a longer lifespan. I worked with Tesla designers on this, ended up costing me about $1200 to have them installed in the garage, and every penny worth it (IMHO).
6) When your installers arrive, discuss the design with them. Mine attempted to adjust the design based on ease-of-install, but that most likely will not be the best long-term design for your install.
7) Happy crew = better install - have a cooler for the installers with beverages (not beer, we don't want that kind of install). I also ordered pizza and bbq for their lunch. They were very appreciative and I think took extra time to ensure everything was as close to perfect as they could get.


My roof cameras that I installed. I plan to install a sun camera (180 degree sky-facing camera) so I can view how cloudy the day is.

my 2 cents, take it or leave it in the share bin.

 

[zƬesla]

@Matt-FL - Is that a leaf, dirt, or scratch on the top right panel of the second photo?

 

[zƬesla]

Also, curious as to the guards on the sides/bottoms but not the tops. Is that typical for an install? Seems like a good way for things to accumulate back there or provide entry to critters.

 

[Matt-FL]

Also, curious as to the guards on the sides/bottoms but not the tops. Is that typical for an install? Seems like a good way for things to accumulate back there or provide entry to critters.

That is a leaf imprint, I haven't had a chance to get up there and see if I can clean it off.

That's the typical Tesla install, they are not 'guards', they are only for aesthetics. Critters could get under them front or back. However, during the day, the heat makes them leave. I have seen some people have issues with birds though, I haven't had that on my house though.

 

[GregBallantyne]

GregBallantyne: Thank you for the input...
Congratulations on PTO. You have gone through a lot and waited way too long to get it running. That must feel great to finally have it producing as you hoped. I think everyone on here should come over to you and through you a party. Of course we wouldn't want to spike your electricity demand to through your plans off. With your short up time, it sounds like you are doing really great.

Yes, finally - but I've read a few accounts from others that make me glad their experience wasn't my own. But it's great to be operating, and meeting expectations or better. One of my electrical usage issues is two refrigerators and three freezers, but that makes for more beer space. By design....but it's a long way to travel for a cold beer!

 

[tabushah]

Hey there, I guess am replying a little late to you, but after a lot of brainstorming we have created an awsome testa powerwall calculator. It uses Tesla Powerwall 2 and it let's you select home appliances from the system like TV, Fridge, Lights and others or you , EV car charger and others and you can get estimates of how much battery they would require and how many kilowatts will remain after you choose appliances. If you wanna check it our you can go and visit the calculator: Tesla Powerwall Energy Storage Calculator | YellowLite
I have also added a screenshot of the tool just for preview.

 

[Docfxit]

Hey there, I guess am replying a little late to you, but after a lot of brainstorming we have created an awsome testa powerwall calculator. It uses Tesla Powerwall 2 and it let's you select home appliances from the system like TV, Fridge, Lights and others or you , EV car charger and others and you can get estimates of how much battery they would require and how many kilowatts will remain after you choose appliances. If you wanna check it our you can go and visit the calculator: Tesla Powerwall Energy Storage Calculator | YellowLite
I have also added a screenshot of the tool just for preview.

How can I download the calculator?

 

[jjrandorin]

How can I download the calculator?

The link that they provided is to a solar companies website, which they are obviously affiliated with, with the term "we created..."

You can take a look at it on their website if you want, but it doesnt look that much like a calculator to me (I looked at it myself) as a sales suggestion tool.

 

[tabushah]

How can I download the calculator?

Hey, there is not the option to download it, you can see the calculations on the page (which i thought might help you). I get that there is a limitation of adding custom values in that calculator like you mentioned and compare them to how many hours they are using the energy. I myself is new to the solar energy and in the learning phase. And I mentioned we because I wanted to be transparent.