To battery or not to battery!

I am just finishing off the spec for a 6.5Kw array. The system will be grid tied with a hot water diverted. I am getting different companies telling me the pros and cons of going with a battery system. The power company has a decent feed in tariff that will allow me to sell back excess power. A lot of people telling me the time it takes to get a financial return will be close to the end of my life if I got for a battery option.

I like the idea of topping up the battery in the winter evenings with lowre-priced power from the grid but I would save on the initial install price without a battery.

Any thoughts/opinions on this?

Without a lot more data on your specific situation it is impossible for us to make a judgement on the financial return on adding batteries to the system.

Generally speaking, putting in batteries is often difficult to justify financially for grid tied systems, particularly if you have good feed-in tariff.

Think of it this way: When you sell power to the utility and then buy it back later, it is like renting storage space on the grid. The rental price is the difference between what you buy power for and what you sell power for. So, if your 'rental' cost is $300/year. Then divide the upfront cost of the battery install by $300 and that is your payback time. (Be sure to factor government rebates/tax breaks in the up-front cost). Also note that right now, building a DIY battery system can be a *lot* lower cost than buying it commercially. Consequently, a DIY system is much more likely to make financial sense than a commercial, professionally installed system

Note: None of the above considers the time value of money. If you invested the cost of the battery instead of buying the battery.... what return would you get?

However, there are other reasons to put in batteries.
* If you don't have grid access, batteries are about the only way to have continuous power.
* If you have frequent power outages, then it might make sense to have the batteries.
* Even if you don't have frequent power outages, there is an emotional value in knowing you are safe if there is one.
* There is also the emotional value of being able to stick it to the utility.

BTW: I am a firm believer that to be viable over the long term, a technology needs to make sense without government rebates or subsidies. Those of us in the DIY community are coming close, but I don't think the home-battery industry is there yet for most of the population.

BTW: I hope and expect that over time, the cost of commercial home-battery systems will come way down. The interesting thing is that as they come down, the cost delta between Commercial and DIY will shrink significantly. The DIYr will still be able to by components and build their own, but the cost of the DIY system will not go down as fast as commercial systems. I suspect it will be like PCs. You used to be able to build a PC for a lot less than it cost to buy one.... You can still do it but the delta is so small it is not worth doing unless you have a very custom need.

The answer to your question lies in what your expected use case is. If you have frequent grid failures and want to maintain power for some/all of that time then batteries are the key. The size of that battery can be as little or as much as you want. Do you want just a few hours of backup or days?

Do you have existing back up generator that you use and have systems in place to quickly implement? (transfer switch?) If so perhaps battery is not that important as you have your generator to run the house/freezer/furnace.

Enviorment can be key. Are you in the fridgid north and want some way to keep the heat on if there are frequent ice storms? Or are you at the sunny equator and dont want to loose everything in the freezer every time the grid goes down...

wheresthesun said:

I am just finishing off the spec for a 6.5Kw array. The system will be grid tied with a hot water diverted. I am getting different companies telling me the pros and cons of going with a battery system. The power company has a decent feed in tariff that will allow me to sell back excess power. A lot of people telling me the time it takes to get a financial return will be close to the end of my life if I got for a battery option.

I like the idea of topping up the battery in the winter evenings with lowre-priced power from the grid but I would save on the initial install price without a battery.

Any thoughts/opinions on this?

Click to expand...

I would never buy from anyone other than bigbattery.com, you are just wasting money on marketing, esp with todays prices for cells. I cringe when I see people pay to have companies install them. It was $8k for my my main parts - two 6kw inverters, 28kwh battery, and 12.5kw array. If you are smart and handy the next best option is a diy 16s or 32s 48v battery - $4k will get you 28kwh. That's more than half a tesla model 3s' 50kwh battery system.

Granted this is a DIY forum so these are the types of answers you will get

I'm with the "have a battery crowd" but you're going to pay a lot for a decent size bank from a commercial vendor.

An option for you might be to go with a batteryless commercially system now and add a battery backup to it on your own later. You'd need to be careful to have the installers use a inverter that can have a battery added to it either directly or via whats called 'AC coupling. An example of the latter would be a SMA Sunny Boy and you add a Sunny Island to it to it later. It'll cost a bit more this way though.

Another option would be to use something like a Outback Radian (which I can't recommend enough) as your primary inverter, but have the installers only put a minuscule cheap battery (golf car batteries for example) on it which you then replace with a DIY lifepo4 solution on your own.

Unless you get crazy the hardware cost of a 'string' inverter (no battery) vs a "hybrid" inverter (grid-tie w/ battery) isnt going to be all that different especially as at least half of your install cost is going to be labor.

Just make sure whatever inverter they use doesnt use something odd like a 96V DC battery voltage - stick to a nominal 48V battery as you can get parts and pieces to DIY a big battery bank yourself a heck of a lot easier and cheaper.

What are your utility rates? High/low time and buy/sell?

Some people here have a utility which will buy their production for 50% of retail, sell back to them at 100%. But rates are so low that the spread costs no more than the cheapest DIY battery.
Others have a bigger spread, and they might come out ahead if the battery does last its predicted life.

My choice has been excess PV production, get enough credit to pay for the smaller amount of power I will draw during peak times.
I do have a battery, but it is for operation during grid outages, not for rate arbitrage.

I recommend avoiding batteries for the time being. We think they will continue to get cheaper.

I might be considered a little eccentric but the value of educating myself during a battery self build project was part of the considerations. We never know the full benefits or where they may lead to but this attitude has rewarded me well in life.

Vince14 said:

I might be considered a little eccentric but the value of educating myself during a battery self build project was part of the considerations. We never know the full benefits or where they may lead to but this attitude has rewarded me well in life.

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Yup.... A lot of the $$$$ I have spent I just consider the cost of a hobby I enjoy.....and no further justification is needed.

schmism and FilterGuy​

voice my views as well. It depends are WHAT and WHY you are doing solar.

If your primary goal is to lower the cost of electricity that you use, then absolutely go batteryless, and bear the consequences without whining about when the consequences show up.

Yet if your primary goal is be totally independent of the grid (in the long run) or to only use the grid for backup purposes, then you buy the batteries. The cost of such is not primary concern (within reason).

When the grid goes down for any reason and you do not have batteries, your solar production turns OFF. But that is okay if your primary goal is the cheapest solar plan.

If you want electricity when the grid fails or is turned off by PG&E for fire protection, then your goal is to have a UPS system backed up by the grid when it is available.

Summary, what are your goals?

For me coupling my goals with enjoying the Tech aspect of solar and its Hobby value, it is the features I want, not the cost of those features, that matter most to me.

dougbert said:

Yet if your primary goal is be totally independent of the grid (in the long run) or to only use the grid for backup purposes, then you buy the batteries. The cost of such is not primary concern (within reason).

When the grid goes down for any reason and you do not have batteries, your solar production turns OFF. But that is okay if your primary goal is the cheapest solar plan.

Click to expand...

Basically agree, but with some adjustment.

If using batteries fits your goal in the long run, plan for it. Maybe put in an undersized battery now. Probably wait for the long run before putting in a large battery.

It is possible to have solar production of AC while grid is down, without batteries. Some hybrid inverters are batteries optional. Newer Sunny Boy will deliver up to 2000W AC direct from PV. This could keep refrigeration going and charge device batteries while the sun shines.

Dabbling now for educational purposes is good, but going big perhaps should wait for further market growth and cost reductions.

Hedges said:

Basically agree, but with some adjustment.

If using batteries fits your goal in the long run, plan for it. Maybe put in an undersized battery now. Probably wait for the long run before putting in a large battery.

It is possible to have solar production of AC while grid is down, without batteries. Some hybrid inverters are batteries optional. Newer Sunny Boy will deliver up to 2000W AC direct from PV. This could keep refrigeration going and charge device batteries while the sun shines.

Dabbling now for educational purposes is good, but going big perhaps should wait for further market growth and cost reductions.

Click to expand...

Yeah great points. I didn't touch much on the actual solutions for going battery, but I agree if batteries are in the future for a project, then get the right hybrid inverter and other equipment that supports batteries. They can operate without them or (as you mentioned) with a small capacity bank that can power the critical loads for a short time, where the majority of grid-outs occur for less than an hour. Then add more battery capacity as money allows over time.

I would like to learn more on the Sol-Ark inverter, as it has such abilities. I have not researched into the Outback line nor the Sunny boy, but have heard of them. Hard to practice with the various models since you have to actually buy them to program them and it is costly to buy them all - LOL

Even with my house batteries, I have small 600wh UPS units for my wife's and my desktop computers, since when I work on my system, I have to invoke the inverter bypass switch now and then. With the UPS units, the computers stay happy. The other laptops have their own built in UPS batteries.

Wow. Thanks for all the replies. Really gives me a lot to think about!!
We do get some power outages and I work from my house so it would certainly be a consideration for that