Battery Storage vs Solar Panels - What is your thoughts?

[MattMan119]

Seems to me the best goals/policy would be #1 - never have a full battery and #2 - never have an empty battery .. This would allow you to keep everything you make, and always have enough to never run out.

BUT, there is practical challenges to this .. Cost, Necessity, Available Space, City Codes .. I am really not interested in them today, but more in the perfect world if I had 1,000,000Kwh battery that was 50% charged and 50Kw of Panels, I would be set .. Now, how can I achieve this type of independence without going way way way overboard, and with real accuracy..

I assume if i can produce 10% more then I used on any given day, then I can bank the extra 10% and use it on a rainy day .. The question becomes how much do you need to in extra each day, and batteries to ensure both Goal #1 and Goal #2 can be met? If there a good way to calculate this, are their too many variables?

I would think - 10% extra power, and 7-10days of operating power would allow this to happen ... But that is a very simplistic view of the problem .. Would love to see what others think..

(Please do not point out its not cost effective .. that don't matter, there are 1000's of things we all do because of other reasons and not because it is cost effective)

 

[DougfromdaUP]

When I was trying to decide whether to buy more panels or batteries, I looked at my production and use history.

I made a table of all the days I fell short, then looked at the difference a panel or battery would make.

You also have to compare it to when your batteries are full and can’t store more.

Here in the frozen s#$&hole of Michigan’s upper peninsula panels were the clear winner.

I’m almost ready to do it again. I think batteries will win this time.

 

[MattMan119]

When I was trying to decide whether to buy more panels or batteries, I looked at my production and use history.

I made a table of all the days I fell short, then looked at the difference a panel or battery would make.

You also have to compare it to when your batteries are full and can’t store more.

Here in the frozen s#$&hole of Michigan’s upper peninsula panels were the clear winner.

I’m almost ready to do it again. I think batteries will win this time.

Would be nice if we had a way to know the lost potential .. A black hole to have that production go to so we could measure what it would have been ..

Do you just use averages to know how much potential you could have stored, and the shortage to know how much would bridge the gaps?

 

[Wes975]

Would be nice if we had a way to know the lost potential .. A black hole to have that production go to so we could measure what it would have been ..

Do you just use averages to know how much potential you could have stored, and the shortage to know how much would bridge the gaps?

In addition to DougfromdaUP's advice - I also use my weather station to help "judge" my lost potential. For us - more batteries. I'll put away roughly 15k to 30k each day before my batteries are topped off, and loose the rest of the free stuff the house don't use. Granted my days don't line up below - it does help show me what I loose and have the potential to keep.

 

[Backyard]

You can approach this several ways

More batteries to cover however many dark days you think you'll ever have in a row. And enough solar to charge you back up slowly plus whatever your loads are

You can do a battery bank only large enough to get you through the night and enough solar to charge and run your loads even with the darkest skies

A compromise between those two where you have a battery that can carry you through several days and a solar system sized for this. And maybe use grid power or a generator if conditions exceed the capabilities

A common rule of thumb is/was 80/20 for batteries where you don't run them down past 20% and don't charge them more then 80%. But newer battery technology like LFP and better BMS' are making this less necessary to get the most life out of your packs.

Another point to consider is when you are using the packs at then then their full capacity to extend their life you are going to need more batteries to make up the kwh. If you do the math for kwh over the life of the pack, you might find that you don't get much more kwh over say a smaller pack that is run harder but replaced sooner. The cost of an equivalent sized pack or even a larger one in 5 years or whatever you get from full cycling them will cost less so it might make sense to just run them at full capacity and replace them sooner for something what has more capacity, longer life or some other feature you want.

Example. Hypothetical figures. A NMC pack rated at 10kwh and doing 100% DOD cycles. Might make it 500 cycles. That's 5 Mwh and assuming one cycle a day will be 1.36 years.

Or do a 50% DOD or whatever number is the new hotness. That number only give you 50% of the available capacity. So you need a 20kwh pack. But you are cycling it 10kwh. You might get 1000 cycles. That is the same Mwh. Cost twice as much as the previous bank. Lasts twice as long. If the prices for the same cells doesn't change then the overall cost is the same. If the cost of the batteries goes down the the prior option would be cheaper in the long run. But you'll have to run the numbers to see what DOD gets for cycle life to whatever degradation you can tolerate. Most of the time the lower DOD will net more Kwh's but you'll need to do the math yourself for your costs and bank size to see if it makes sense

I for one am an believer in adding as much panels as one can fit and afford. Then a battery that will carry you through at least several days of dark skies. Then... Add more over the years as you can afford until you no longer have range anxiety.

 

[SolarScott]

Time of day usage. If all the clothes washing, drying, cooking, vacuum cleaning, water heating etc is done during good sun, much less battery is needed. If you come home from work and start doing all these things, big battery needed.

 

[JäseH]

In my region my consumption was heavily skewed towards cooling in the summer months. My consumption essentially doubles for 4 - 5 months of the year, as compared to winter. My objective was to cover my winter and summer usage, with storage for hours during summer where the panels couldn't supply enough power. In addition, we frequently have short-term outages, so I wanted something that could cover our backup power needs for a least a day. We also have free nights plans available here, so the plan was to charge the batteries and our cars at night.

I pulled my usage for peak consumption days and made a daytime chart, excerpted below. Based on the chart peaks and hourly consumption data, I estimated the system size needed to cover the bulk of my daytime usage and determined how much storage would be needed for days where panels would be insufficient. For example, on a really hot day, my hourly consumption might exceed12 kW. But that would only be for 3 - 4 hours, nearer midday when solar heating is at its peak. The rest of the day, my hourly usage would be well below that. Based on these estimates, I sized 10 kW solar and a 30 kWh battery, as this would have enough generation to cover most of the day, along with supplemental battery to cover loads most mornings, evenings, and hours where solar wasn't enough. I added another 15 kWh to reduce C loads for power hungry appliances (car charger, electric dryer, double ovens) and for grid out conditions. Ultimately I settled on 10 kW solar and 45 kWh of batteries.

I briefly evaluated going battery only, but I figured the utility might be unhappy with me doing 0 kWh in daytime while pulling 80 kWh nightly, so the panels were a hedge against that and daytime power outages. It's only been three weeks since I finally connected the batteries, but so far I'm very happy with the way everything has turned out.

 

[DougfromdaUP]

I wasn’t at all concerned with lost production. I just wanted to decrease the power I buy from the utility- less than 3% now.
I would work backwards from a day I bought power, looking at the output from my panels and calculating how much additional panels would add.
Informed guessing is involved with the battery side, since I’d go back to the last day I had 100% soc and guess if I could have filled another battery.

 

[wheelman55]

OP. Why #1? With higher end lithium batteries the bms keeps some room at the top and at the bottom. Our 7.3kw batteries have 6.6kw useable, governed by the bms.

And #2…you can go to zero charge on the bms. The system will recharge the battery as soon as power is restored…either via the grid or via the sun.

 

[MattMan119]

OP. Why #1? With higher end lithium batteries the bms keeps some room at the top and at the bottom. Our 7.3kw batteries have 6.6kw useable, governed by the bms.

If you batteries hit 100% then you have power that is waisted, if you are waistline power then their is a missed opportunity to store it and reduce the need for so many panels.. in the perfect world …

And #2…you can go to zero charge on the bms. The system will recharge the battery as soon as power is restored…either via the grid or via the sun.

We are talking about a system that has no downtime or grid need. so yes you can add generators, grid, reduced usage and certain times of day, but in the perfect world if you store everything, how many batteries do you need to never hit the top or the bottom of that battery.

 

[MattMan119]

I wasn’t at all concerned with lost production. I just wanted to decrease the power I buy from the utility- less than 3% now.
I would work backwards from a day I bought power, looking at the output from my panels and calculating how much additional panels would add.
Informed guessing is involved with the battery side, since I’d go back to the last day I had 100% soc and guess if I could have filled another battery.

Yeah this seem to be the mod of operation for most of us, made an educated guess (or uneducated) and then adjust .. like you said many of us have different goals so the methods used are different .. so your method is rocker back and forth from panels to batteries till you find the balance that achieves your goals.

 

[MattMan119]

Time of day usage. If all the clothes washing, drying, cooking, vacuum cleaning, water heating etc is done during good sun, much less battery is needed. If you come home from work and start doing all these things, big battery needed.

But they are done at these times because you do not have to battery storage to allow it to not matter what time of day.. that is the real question here how would/could you calculate the perfect balance where no waisted production would occur but at the same time there would be no downtime ..

 

[CaliSunHarvester]

1,000,000Kwh battery that was 50% charged and 50Kw of Panels

50kW panels is nothing special. Could be just 100 to 150 panels. Maybe $10k. I have 100 panels, smaller/ older, totalling 22k pv.

1 million kWh battery.. certainly much more $. Also a lot more dangerous IMHO.

 

[CaliSunHarvester]

Weather forecast. How much battery would I need for the coming week? 6 days @ 20kWh = 120kWh? (I have 41kWh only.)

My hope is that there is some solar output in spite of the rain.

At least the app displays the proper advertising at the bottom. Funny, how they always advertise what I purchased already.

 

[MattMan119]

50kW panels is nothing special. Could be just 100 to 150 panels. Maybe $10k. I have 100 panels, smaller/ older, totalling 22k pv.

1 million kWh battery.. certainly much more $. Also a lot more dangerous IMHO.

You missed the point ..

 

[SolarScott]

But they are done at these times because you do not have to battery storage to allow it to not matter what time of day.. that is the real question here how would/could you calculate the perfect balance where no waisted production would occur but at the same time there would be no downtime ..

Because there is no sense in spending thousands on a larger battery while I have the grid as my back up battery. Ii is much more cost effective to try to use the power when it is available. I have more than I ever need if it is sunny, but if I want to do these things late in the day or at night, then I need to be prepared to waste thousands on a much larger battery.

 

[MattMan119]

Because there is no sense in spending thousands on a larger battery while I have the grid as my back up battery. Ii is much more cost effective to try to use the power when it is available. I have more than I ever need if it is sunny, but if I want to do these things late in the day or at night, then I need to be prepared to waste thousands on a much larger battery.

Ok then move on to other thread.. This is not what we are talking about.. Please try and stay on subject..

 

[MattMan119]

Weather forecast. How much battery would I need for the coming week? 6 days @ 20kWh = 120kWh? (I have 41kWh only.)

My hope is that there is some solar output in spite of the rain.

At least the app displays the proper advertising at the bottom. Funny, how they always advertise what I purchased already.
View attachment 284058

Yes, this is true, I have noticed I can get 10-15% almost everyday regardless of the weather, but I live in Florida and have NO SNOW... I assume other locations would suffer complete block out..

 

[MattMan119]

In my region my consumption was heavily skewed towards cooling in the summer months. My consumption essentially doubles for 4 - 5 months of the year, as compared to winter. My objective was to cover my winter and summer usage, with storage for hours during summer where the panels couldn't supply enough power. In addition, we frequently have short-term outages, so I wanted something that could cover our backup power needs for a least a day. We also have free nights plans available here, so the plan was to charge the batteries and our cars at night.

I have summer usage very much the same, not to the extent of usage that you have.. I only running a few mini splits or ac.. But I see you're point. Amazing how much we come to depend on the power company.

I pulled my usage for peak consumption days and made a daytime chart, excerpted below. Based on the chart peaks and hourly consumption data, I estimated the system size needed to cover the bulk of my daytime usage and determined how much storage would be needed for days where panels would be insufficient. For example, on a really hot day, my hourly consumption might exceed12 kW. But that would only be for 3 - 4 hours, nearer midday when solar heating is at its peak. The rest of the day, my hourly usage would be well below that. Based on these estimates, I sized 10 kW solar and a 30 kWh battery, as this would have enough generation to cover most of the day, along with supplemental battery to cover loads most mornings, evenings, and hours where solar wasn't enough. I added another 15 kWh to reduce C loads for power hungry appliances (car charger, electric dryer, double ovens) and for grid out conditions. Ultimately I settled on 10 kW solar and 45 kWh of batteries.

So what would it take to go 100% off grid, never having the ability to use grid or generator .. This kind of usage would be a lot of batteries.. 120kwh? more? the part that make this kind of hard to get right, if for us the sun determines the power we have and the AC we need.. Cold really puts a twist on the supply and demand ..

I briefly evaluated going battery only, but I figured the utility might be unhappy with me doing 0 kWh in daytime while pulling 80 kWh nightly, so the panels were a hedge against that and daytime power outages. It's only been three weeks since I finally connected the batteries, but so far I'm very happy with the way everything has turned out.

Y

 

[AlaskanNoob]

I'm looking forward to tackling this question as well. We have 7KW of solar up and this summer we'll get the rest installed for a total of 25KW of solar panels. We also have 38KWh of lithium. The question is...does it make sense to double the lithium storage?

Still collecting data, but it seems to me that in order to really know, I'll need to set up an irradiance device because once the batteries are full, I no longer have a measure of what PV is not being utilized. I need to find an irradiance device that can be networked and that will log the data, then I'll know if it makes sense or not. Until then, I'll just be guessing.

 

[JäseH]

So what would it take to go 100% off grid, never having the ability to use grid or generator .. This kind of usage would be a lot of batteries.. 120kwh? more? the part that make this kind of hard to get right, if for us the sun determines the power we have and the AC we need.. Cold really puts a twist on the supply and demand ..

The panels could be larger challenge unless you have ample space. I can easily add another 8 or 9 boxes of batteries, as they're fairly compact, stackable, and DIY. But getting all the panels and doubling up inverters and wiring to accommodate additional solar generation would be hard.

Still collecting data, but it seems to me that in order to really know, I'll need to set up an irradiance device because once the batteries are full, I no longer have a measure of what PV is not being utilized. I need to find an irradiance device that can be networked and that will log the data, then I'll know if it makes sense or not. Until then, I'll just be guessing.

PVWatts has regional solar insolation stats. If your panel angle, facets, and shading are not changing, it's probably safe to assume you'll get whatever each panel is already generating from additional panels. Or you tweak the PVWats calculator to match your current generation, then scale up for additional panels. If you do net metering, you can configure your system to feed the surplus to the grid and that will give you a measure of total solar generation potential.

 

[CaySol]

Seems to me the best goals/policy would be #1 - never have a full battery and #2 - never have an empty battery .. This would allow you to keep everything you make, and always have enough to never run out.

BUT, there is practical challenges to this .. Cost, Necessity, Available Space, City Codes .. I am really not interested in them today, but more in the perfect world if I had 1,000,000Kwh battery that was 50% charged and 50Kw of Panels, I would be set .. Now, how can I achieve this type of independence without going way way way overboard, and with real accuracy..

I assume if i can produce 10% more then I used on any given day, then I can bank the extra 10% and use it on a rainy day .. The question becomes how much do you need to in extra each day, and batteries to ensure both Goal #1 and Goal #2 can be met? If there a good way to calculate this, are their too many variables?

I would think - 10% extra power, and 7-10days of operating power would allow this to happen ... But that is a very simplistic view of the problem .. Would love to see what others think..

(Please do not point out its not cost effective .. that don't matter, there are 1000's of things we all do because of other reasons and not because it is cost effective)

Yeah. I think my perfect system could get my battery bank to 100% in a single day and have enough capacity to cover the maximum days with little to no solar production. So I wouldn’t worry about having wasted production because of a full battery. So 7 days of use worth of storage and 7 days worth of production in panels( in a single day), for example.

 

[MattMan119]

Yeah. I think my perfect system could get my battery bank to 100% in a single day and have enough capacity to cover the maximum days with little to no solar production. So I wouldn’t worry about having wasted production because of a full battery. So 7 days of use worth of storage and 7 days worth of production in panels( in a single day), for example.

Yeah that would do it.. Just a lot of extra panels .. I wonder if 110% 120% 140% would be level of production, and I wonder if there are location that may need more or less then 7 days..

What I am thinking.. if I have 15Kwh use a day .. then
100Kwh would be good for 7 days
but with using only 15kwh a day could i get away with 15+1.5kw of panels (16.5kw) or should it be 15+7.5=22.5kwh ..

Then again, not sure anyone is putting that much though into it.

Oh i bet one of the panel calculation websites will give us a ZERO your electric bill number or usage, then we can use that to just calculate how much battery is needed to never get full or empty ..

 

[Daddy Tanuki]

Seems to me the best goals/policy would be #1 - never have a full battery and #2 - never have an empty battery .. This would allow you to keep everything you make, and always have enough to never run out.

BUT, there is practical challenges to this .. Cost, Necessity, Available Space, City Codes .. I am really not interested in them today, but more in the perfect world if I had 1,000,000Kwh battery that was 50% charged and 50Kw of Panels, I would be set .. Now, how can I achieve this type of independence without going way way way overboard, and with real accuracy..

I assume if i can produce 10% more then I used on any given day, then I can bank the extra 10% and use it on a rainy day .. The question becomes how much do you need to in extra each day, and batteries to ensure both Goal #1 and Goal #2 can be met? If there a good way to calculate this, are their too many variables?

I would think - 10% extra power, and 7-10days of operating power would allow this to happen ... But that is a very simplistic view of the problem .. Would love to see what others think..

(Please do not point out its not cost effective .. that don't matter, there are 1000's of things we all do because of other reasons and not because it is cost effective)

depending upon circumstance, totally cost effective. I use 10-15kWh per day once the sun goes down so roughly the same when the sun is up. assuming that the panels get covered by snow thats 20-30kWh per day. i have 72kWh of storage and have never hit a zero production day... last week panels covered at 0600.... half clear and producing by 1100 clear by the next day. so it is totally doable, depends upon how much you can spend and how much it costs to bring power in. (the real issue in remote areas). BTW remember if it floats, flies, or F***s it is cheaper to rent.. yet we all get married do we not?

 

[MattMan119]

The panels could be larger challenge unless you have ample space. I can easily add another 8 or 9 boxes of batteries, as they're fairly compact, stackable, and DIY. But getting all the panels and doubling up inverters and wiring to accommodate additional solar generation would be hard.

Yes, agreed but we have to start with a problem statement, and it would seem most of us look at the first part of the equation and not the full list of problems prior to make a determination of what it would really take.. This is not wrong because most of us don't what to solve the entire problem statement, we only want to trim something off the electric usage in peak, or have a refrigerator in case of power outage. It is clear, not all will be practical even in most cases, but how would or could we even get to the the right numbers..

When you know the full problem, and something new comes out. You will look at it and say .. Wow, that can solve for all my issues and recognize the solution. Some of us have solutions today and don't know it. (I would bet)

PVWatts has regional solar insolation stats. If your panel angle, facets, and shading are not changing, it's probably safe to assume you'll get whatever each panel is already generating from additional panels. Or you tweak the PVWats calculator to match your current generation, then scale up for additional panels. If you do net metering, you can configure your system to feed the surplus to the grid and that will give you a measure of total solar generation potential.

I will take a look at a few of these.. Maybe, I and rethinking a solved problem..