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Grid Tied Battery Strategy?

ARHillBilly

New Member
Joined
Aug 22, 2024
Messages
15
Location
Arkansas
I am grid tied with net metering. The power company gives me 1kWh for every 1 kWh I produce.

I am rural and we lose power more often that I believe to be common. We lose power for 4+ hours at least 1-2x a month, for a day at least a few times a year and for days every couple years.

I have 6.3 kW of PV (limited by power company), a Midnite AIO 11.4 kW Inverter and three Midnite PowerFlo 16 kWh Batteries (48 kWh Total). I also have a gas generator I can drag out, hook up and use to charge battery if necessary.

If I blasted all electric heat I can use upwards of 30 kWh per day. I normally use 10-15 kWh per day. If I am really trying to conserve I can get that under 10 kWh per day.

My goal is to use solar to offset my monthly costs but also to avoid the frequent power outages.

What is the best strategy for my batteries?
Should I use them over night (down to about 75%) and use PV to recharge daily?
Just keep them topped off at 100%?
Something else?
 
Last edited:
Starting the day at 75% gives you 33 kwh at the start of the day so that should cover all except the several days outage. For that an extra 8 kwh is not going to fill the gap so use the generator once the batteries get to say 15% soc.
 
that's a great question. looking forward to answers from more knowledgeable people.

using the battery costs you money. there is probably a 90% round trip efficiency to use the battery so everything else equal your bill will increase.

but, not using the battery and keeping it full is bad for the battery so thats not good either.

but, if you keep the battery at lets say 50% soc without using it , then you only have 50% available when the power goes out.

its a tricky one.
 
Starting the day at 75% gives you 33 kwh at the start of the day so that should cover all except the several days outage. For that an extra 8 kwh is not going to fill the gap so use the generator once the batteries get to say 15% soc.

The math and weather start to work against me. Typically when we lose power there is some type of weather event. So if I have 48 kWh I really have about 42 kWh useable. With winter weather giving me days of clouds it doesnt take long at 10-15 kWh to run low on juice.

But there is also DC to AC conversion losses, the design cycles of every battery and what is best for the long term health of the battery. So many factors.
 

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that's a great question. looking forward to answers from more knowledgeable people.

using the battery costs you money. there is probably a 90% round trip efficiency to use the battery so everything else equal your bill will increase.

but, not using the battery and keeping it full is bad for the battery so thats not good either.

but, if you keep the battery at lets say 50% soc without using it , then you only have 50% available when the power goes out.

its a tricky one.

A lot of competing factors.

I wonder if keeping the battery at 100% as the standard but running it down to 10% once a week or twice a month would be the right option. I could time this out so Im draining it when we are going to have 2 days of good sun.
 
A lot of competing factors.

I wonder if keeping the battery at 100% as the standard but running it down to 10% once a week or twice a month would be the right option. I could time this out so Im draining it when we are going to have 2 days of good sun.
Everything ive read says keeping the battery at high voltage/high soc is not good for the long term health of the battery.
 
Everything ive read says keeping the battery at high voltage/high soc is not good for the long term health of the battery.

I know every battery has so many cycles by design. I wonder which is worse, a day sitting at 100% or a 100% to 60% to 100% cycle daily.
 
This is not a 'tricky one' or a complicated question with difficult math involved.

You have bought and paid for batteries, use them every day, that is the purpose of a battery! It does not cost money to use the battery as someone said. It costs money to get energy into the battery if you are buying from the grid or using a fossil fuel generator. If you are using PV, wind or hydro this is free

The battery is getting older every day: this is called 'calendar aging'. The cycle aging a battery experiences every time it discharges and recharges is concurrent with calendar aging. The only way you could speed this up is to discharge, recharge, discharge & recharge in a single day. Your depth of discharge to minimum, typically 20%, and recharge to 100% is healthy and expected for your Midnight Solar Lifepo4 Powerflo 16 battery. It can do this more than 8,000 times easily, but, 8,000 cycles is every day for 21 years! (after these cycles, it may retain less energy, it may have 99% capacity instead of it's full 16KwH potential for example, so don't str as on this). If you use the free energy from the sun to run your home you will pay off those batteries and inverter and PV panels. If you don't use your batteries you just wasted a bunch of money and effort.

Use the sun (hey, it's free!) to charge your batteries and power your house. This is the why we have solar. The grid and the generator are there for you in case you are not getting enough solar to charge your battery. These two power sources are what costs money daily.

Lastly, I struggle to understand how you can burn 30KwH of electricity in a day - maybe try closing your windows when the heater is on? Try covering the jacuzzi when you are not using it? Maybe don't run the in-lawn heater coils outside that keeps the grass snow free? I highly recommend taking a look around your house to see what is so energy greedy, and improving its efficiency or limiting its use.
 
@ARHillBilly, I too am grid tied, with an occasional 'normal' grid outage, and the more rare hurricane or tornado related multiday outage. After much thought, I have finally settled on charging the battery bank (lithium) up to about 85% daily, using the Enhanced Grid Support setting on my Schneider XW inverter. This setting uses the Charge Controllers to 'administer' to the batteries, and when float is achieved, as set in the charge controller, it will sell all to the grid. I can adjust the level of charge that I want in the batteries by adjusting the float charge voltage. My MPPT CC's float setting is 54.4v, which gives me about an 85% charge at the end of a sunny day. This 54.4v CC float setting gives me about 53.35v after resting a few hours at the end of the day. Should I see a big storm, or such coming, I can adjust this float level higher for a higher SOC.
 
This is not a 'tricky one' or a complicated question with difficult math involved.

You have bought and paid for batteries, use them every day, that is the purpose of a battery! It does not cost money to use the battery as someone said. It costs money to get energy into the battery if you are buying from the grid or using a fossil fuel generator. If you are using PV, wind or hydro this is free

The battery is getting older every day: this is called 'calendar aging'. The cycle aging a battery experiences every time it discharges and recharges is concurrent with calendar aging. The only way you could speed this up is to discharge, recharge, discharge & recharge in a single day. Your depth of discharge to minimum, typically 20%, and recharge to 100% is healthy and expected for your Midnight Solar Lifepo4 Powerflo 16 battery. It can do this more than 8,000 times easily, but, 8,000 cycles is every day for 21 years! (after these cycles, it may retain less energy, it may have 99% capacity instead of it's full 16KwH potential for example, so don't str as on this). If you use the free energy from the sun to run your home you will pay off those batteries and inverter and PV panels. If you don't use your batteries you just wasted a bunch of money and effort.

Use the sun (hey, it's free!) to charge your batteries and power your house. This is the why we have solar. The grid and the generator are there for you in case you are not getting enough solar to charge your battery. These two power sources are what costs money daily.

Lastly, I struggle to understand how you can burn 30KwH of electricity in a day - maybe try closing your windows when the heater is on? Try covering the jacuzzi when you are not using it? Maybe don't run the in-lawn heater coils outside that keeps the grass snow free? I highly recommend taking a look around your house to see what is so energy greedy, and improving its efficiency or limiting its use.
30kwh a day isn't much depending on the situation. We are now all electric in our house with mini splits, two electric cars, and a hot tub (with the cover closed!) in Colorado. Our biggest use day was this January 4, 183 kwh after subtracting 26kwh of solar during that day (so glad for net metering and using credit from last summer)! It was an outlier though because we came home from a trip with 4% left on a max pack Rivian, plugged in and then cranked up the hot tub! It all depends on how that energy is being used, and if you are aware of where it's going.
 
Wow.... Fair play to you. Two electric vehicles, alongside air-con / heat pumps and a hot tub to boot!
Forgive my amazement at the 30kWh consumption. We live in Hawai'i where the weather is perfect nearly all the time, so heating and cooling are generally provided by the breeze. We have one hybrid vehicle and a thirsty V6 Tacoma. I BBQ on the sundeck at least 50% of our meals using gas. So our daily energy budget to power the house (lights, fans, refrigerator, TV, stereo, office devices, water pumps, hair dryers etc is 3.5 ~ 4kWh. However to go full electric on cooking, water heating and transportation, I can easily see 20+ kWh daily. And, in a genuine winter climate like yours it is much easier to grasp.

I digress to your initial question though. Use the batteries? Yes, as much as possible. That is why the are there, and that is how you maximize your investment in this technology.
 
I am grid tied with net metering. The power company gives me 1kWh for every 1 kWh I produce.

...

My goal is to use solar to offset my monthly costs but also to avoid the frequent power outages.

What is the best strategy for my batteries?
Should I use them over night (down to about 75%) and use PV to recharge daily?
Just keep them topped off at 100%?
Something else?

My situation is similar to yours - rural, questionable power, grid tied, net metered, batteries plus solar plus gas generator if needed.
And our goals are the same - offset monthly costs and have enough to cover power outages.

The best strategy to offset costs depends on your utility rate structure. I use my batteries+solar when the utility rates are high and use the grid+solar when rates are low.

As for the batteries - ever hear a guy say when he refers to a hot car "Drive it like you stole it!"?
I use the batteries as much as needed and recharge as needed.

For me, specifically, this means that I start charging via solar at dawn, then power household loads while exporting surplus.
When rates increase from 2pm-10pm, I use my batteries+solar to power all household loads.
When rates drop overnight, I will charge from the grid after midnight if the batteries are below my desired reserve capacity (about 40%).

What approach have you settled on?
 
"Blasting electric heat" means what exactly? Mini-split in Arkansas shouldn't take too much even in the depths of winter, a normal heat pump should be sufficient, and 30KWH is pretty light usage.

If the goal is backup for the grid, I would have enough battery to last 2 days, and at least enough solar to charge 1/2way on a perfect day. I would never let my batteries fall below 1/2 way unless the grid is down. ie keep a battery buffer minimum of a full evening's power worst case unless the grid is out. If it's out for more than a day, hopefully you will get enough solar the next day to compensate.Adjust appropriately for your budget, to keep at least 1 'nights' worth of battery at all times. (~ 18 hours). Build it out to scale up as budget permit, for backup purposes, battery first then add more PV to keep it charged, or use the grid.
 

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