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Battery draw down at night?

scottmag

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Jan 16, 2023
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Florida
for you off-grid folks who run on batteries only at night, what is your draw down % at night?

We run one small minisplit AC and usually draw down to about 75% of battery each night. Sometimes the AC has to work extra hard and we’re wake up at 50% battery in the morning. Curious what other folks experience.
 
There are a lot of variables to consider for this.
Each system, location, battery capacity, solar energy available, and amount of loads are different.
You can't really compare two with each other unless they are exactly the same and installed next to each other.
That said, I usually stay above 80%. Unless I don't.
 
I'm hybrid but I'll jump in anyway.

I have 3.8kW solar, 20kWh battery, and loads that vary between 24 and 75kWh year round.

Right now while heating and cooling loads are low we would coast down from 100% at 6:30pm to 40% at 11:30am the next day when the solar starts out running the load again.

But, we only get about 10kWh excess per day, so I have the grid charger kick on at 50% and back off at 55%. We usually do 1-3 50-55% bounces between 7am and 11:30, and then kicking off from there we hit 100% again by 6:30pm about half the time.
 
It Depends:
My system runs my business and my home next door, typically the overnight draw-down on my 99.6kWh ESS will be 15% running both properties.
If you have three days of ESS available, then any one day is about 33% and depending on what runs at night you may use 10-15% overnight.
 
As others have said, it is all relative, really. How much battery one has, their solar system capacity and the equipment they run through the different hours. Most typically, one looks at their "worst / heaviest" usage periods to determine the "minimum daily energy usage" they have to cover in the worst conditions (meaning no input/generation). Then most folks triple that number for 3 days autonomy.

My offgrid usage is on average 4kWh per day (year round) but I built this house for hyper efficiency & use very efficient gear in it. 3 Days autonomy means I would need 12kWh stored. BUT I am deep north and can see a week with no sun of any value.... Therefore my battery bank reflects the amount of storage I need for those stretches so I have 1680AH or 43kWh of LFP.

Simply put, everyone's situation & conditions is different & unique so questions like your are hard to answer.
 
As others have said, it is all relative, really. How much battery one has, their solar system capacity and the equipment they run through the different hours. Most typically, one looks at their "worst / heaviest" usage periods to determine the "minimum daily energy usage" they have to cover in the worst conditions (meaning no input/generation). Then most folks triple that number for 3 days autonomy.

My offgrid usage is on average 4kWh per day (year round) but I built this house for hyper efficiency & use very efficient gear in it. 3 Days autonomy means I would need 12kWh stored. BUT I am deep north and can see a week with no sun of any value.... Therefore my battery bank reflects the amount of storage I need for those stretches so I have 1680AH or 43kWh of LFP.

Simply put, everyone's situation & conditions is different & unique so questions like your are hard to answer.
Thanks yeah I’m just curious about other folks’ experience of how much battery they use each night In terms of % since actual kWh is entirely unique
 
It all comes down to how many days of autonomy you want.
Currently I can go 3 days with little or no production. (Without adjusting my lifestyle) final plan is for 7 days.
 
This is an interesting question for me. I’m hybrid with a new system that just came online earlier this spring - so no experience yet of fall and winter. I currently have my battery discharge limit set at 50% and depending on what we do in the evening and how much cloud cover there is, in the morning we’ll have between 50% and 75% left in the tank by the time the panels take over. This is with not much or any AC usage yet which will start changing things pretty soon. We have about 13KW solar production and about 31KWh of storage. Based on pre-system calculations and also our experience so far, I think that we could squeeze 2-3 days out of the batteries if we only ran what’s needed in the house - but that would be very non-typical usage.

My primary reason for having put in the system was not to minimize grid usage (though that’s a welcome secondary) but to be more resilient to the grid going down. We are very rural and more vulnerable to outages than nearby population centers. Long winter outages can be uncomfortable.

Given what I’ve experienced so far during weather and what I think that means for winter solar production, I’m thinking about potentially not using the batteries for anything but grid backup during winter months (so we’d always have 100% SOC in case of a grid outage) and also considering adding more storage.
 
for you off-grid folks who run on batteries only at night, what is your draw down % at night?

We run one small minisplit AC and usually draw down to about 75% of battery each night. Sometimes the AC has to work extra hard and we’re wake up at 50% battery in the morning. Curious what other folks experience.
In the summer, we run our mini split (9k BTU) on low and use about 50% of our battery (including everything else) over night. We have a 20kwh LFP bank.

Our sleeping area is insulated walls so the rest of the house doesn't get cooled on the over night. This really helps and we are nice a cool at night with minimum power.

A 50% draw down is a good balance on bank size, not considering cloudy day reservers.
 
Not running Aircon yet so it is going to change once that happens. When I have a previous full charge, from a decent PV production day, my 24vDC liFePO4 nom. setup shows 26.5vDC on the AIO's from the battery bank upon morning wakeup at around 5am. Usually only a minor load is on at that time so there is little voltage slump. I figure my overnight loading (including AIO's idle) to be in the 3-4kWh area.
 
It has been 3 weeks since my batteries hit 100% ... yesterday we made it to 98%. At 06:00 this morning I am at 79%, but we had one A/C running all night.
We're feeling the effects of Spring weather, too. We are struggling with the balance between making full use of the batteries and just knowing that one bad storm system will knock out the power (and solar production) for days. Generally, we use about 20% of our batteries before we start charging again the next morning. This includes power usage for dinner cooking and morning hot water for showers. During the stormy season, I have my inverter set to switch to grid power when the batteries get down to 60%. I figure using grid power is cheaper than running my generator if power goes out and the sun takes a vacation for a while.

I am curious how others handle this balancing act.
 
We're feeling the effects of Spring weather, too. We are struggling with the balance between making full use of the batteries and just knowing that one bad storm system will knock out the power (and solar production) for days. Generally, we use about 20% of our batteries before we start charging again the next morning. This includes power usage for dinner cooking and morning hot water for showers. During the stormy season, I have my inverter set to switch to grid power when the batteries get down to 60%. I figure using grid power is cheaper than running my generator if power goes out and the sun takes a vacation for a while.

I am curious how others handle this balancing act.
We do something similar during Winter here, since most power outages are snow-related ie trees pulling down power lines. My experience has been to keep an eye on the weather forecast - if a winter storm is coming, then we charge up the ESS to 100% using the utility while it is available, and save the fuels (gasoline and propane) for the generator as the "back up to the ESS" in case the storm takes down lines for longer than a few days.
If the weather looks tame, we let the system run each day and collect what solar it can. At night (using off peak rate) we will add whatever the solar missed for the day - what I mean by that is: our typical normal day is 37kWh and if the solar collected was say 20kWh on a day in December, we charge the ESS an additional 17kWh that night using the lowest cost ToU rate to make up what the solar missed that day. If the weather is clear, we don't worry about charging up to 100% SOC, just ride the averages.
I don't recommend you charge up LFP cells to 100% SOC and leave them at that high SOC for long periods of time - not good for these cells. Let them exercise up and down the SOC range, but also keep and eye on the weather coming, take action when you see a need.
 
As many others have said it depends... The season, the cloud forecast, what plans are, the weather forecast, how much I want to fight a perfectly sunny day are all variables.
 

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So, here’s my setup:

5.5kw solar
24kwh batteries
6K XP

During the day I’m in a surplus of PV input, loads are usually 1-1.5kw, batteries are usually charged by 10-11AM if sun is decent.

Loads at night start around 1.1kw, drop down to around 500w by 9pm and then get as low as 190w.

I usually wake up at 60-75% depending on how long I stayed up (was on nightshift so loads were often a bit higher)

I’ve been trying to discharge a bit lower here and there so it’s not the same or close every night.
 
Generally we don’t pay any attention to loads, live life normal. 3 mini splits and all the normal aspects of a house in the city. Wake up to between 40-70% SoC depending on a ton of things like, how dinner was made, when laundry was done, dishwasher etc.

We’re fortunate to be grid connected, dipping lower isn’t an issue. 30kWh battery bank.
 
2.2kw Solar Panels
13kw Lifep04 Cells
2.2kw 24Volt Samlex EVO Inverter/Charger

Run a critical loads circuit panel with solar (furnace, fridge, freezer, computers & router gateway, dehumidifier, TV and a few lights). Rest of the house is run off the grid (AC, stove, laundry, etc.). During sunny and partly cloudy days (spring, summer & fall) system normally charges to 100% during day and down 40 to 60% and night. During winter and with a couple rainy days (spring, summer & fall) it is common for the battery packs to drain to about 5% where the Samlex EVO Inverter/Charger transfer switch switches over to the grid to supply the critical loads and charge the battery packs.

As others have suggested if bad weather is predicted, the battery packs are charged to 100% with the Samlex so can get maybe a couple of days of solar power if the grid goes down with bad weather (No Sun). Longer than that it is time to pull out the generator.
 
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Prob the lowest the battery has been (12.5kWh) since I moved the fridge, wfh setup and TV over to my battery inverter (48/1200 running pass through into a Delta 2 max). I have to unplug the 48/1200 if I'm going to run a bigger load like the air fryer or four slices of toast, so the d2m can deliver up to 2400W. Two slices and I'm good to go with the 48/1200. Around 300W of steady load. Meant to be a little better solar wise tomorrow so we'll see. I don't have a 48V charger so if theres insufficient solar would just plug the d2m into the grid.Screenshot_20240513_234333_VRM.jpg
 
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