Newbie help with battery sizing - which calculation is right?

Hi,

I will be building an offgrid system for my cabin and am trying to start design by sizing my batteries. I am set on lithium and my largest draw will be an AC fridge.

My question is the right way to calculate the energy consumption of the fridge.

According to the Energystar rating the fridge uses 345kh a year. I calculate that at ~39w an hour or ~1kw a day. If I was to have two, 100ah batteries, is it accurate that I would get 60 hours of runtime from the batteries? 200ah X 12v / 39w = 60 hours.

I'm a little confused because I found this data for the fridge amps/watts on the fridge manufacturer's website.

If I assume the startup is just a few minutes each day, but the runtime is ~8 hours each day, I come up with a very different AH need. Which is right and any guidance if 200ah of lithium batteries is enough for the fridge plus another 50ah of misc appliances/lights a day?

Thank you!

Update, I called the manufacturer and they stated typical running current ranges from (pasted below), so perhaps the elevated numbers I posted initially are max values for when the freezer defrosts itself? Do these lower number ranges seem more in line with a 345kw a year and what a 200ah battery could support?

mainersolar said:

Hi,

I will be building an offgrid system for my cabin and am trying to start design by sizing my batteries. I am set on lithium and my largest draw will be an AC fridge.

My question is the right way to calculate the energy consumption of the fridge.

According to the Energystar rating the fridge uses 345kh a year. I calculate that at ~39w an hour or ~1kw a day. If I was to have two, 100ah batteries, is it accurate that I would get 60 hours of runtime from the batteries? 200ah X 12v / 39w = 60 hours.

Click to expand...

That will be around 60 hours runtime indeed, well done! Though do take care of the units: electrical energy is expressed in Wh and electrical power is expressed in W.

mainersolar said:

I'm a little confused because I found this data for the fridge amps/watts on the fridge manufacturer's website.

Startup amps/watts	Running amps/watts
6.0 / 690	2.5 / 287.5

If I assume the startup is just a few minutes each day, but the runtime is ~8 hours each day, I come up with a very different AH need. Which is right and any guidance if 200ah of lithium batteries is enough for the fridge plus another 50ah of misc appliances/lights a day?

Thank you!

Click to expand...

Interesting question and well spotted. As you noted earlier, a consumption of 345 kWh/year translates to an average power of 39 W. However, the fridge is equipped with a far more powerful motor that draws 287.5 W. In normal use the fridge motor will not run continuously. In fact, to reach the 39 W average power, it will only run 39/287.5 = 14 % of the time. The startup current is normally only drawn during the first second that the motor has been started. This happens several times a day.

The power of the fridge motor does not influence the overall energy consumption much. More powerful motors will simply run less often and all other things being equal (size, insulation, temperatures etc.) the overall energy consumption will be very similar.

As you see, the fridge motor is far more powerful than the average power consumption. This is necessary to cool down warm contents quickly. If you put a large amount of warm contents in the fridge, the motor will run continuously drawing the 287.5 W for several hours until the fridge reaches target temperature.

Something that you might want to add in your calculations is the inverter efficiency. They tend to waste at least 10 % in heat. Be aware that inverters draw quite a bit of power just by being turned on.

A few things to work out first.
Typically, a system should not exceed 250A draw from a battery bank.
The Uncorrected Math. Corrections are required for efficiency and that varies on Inverter Type & Grade.
12V@250A=3000W, 24V@250A=6000W, 48V@250A=12,000W
120V@3000W=25A, 24V@6000W=50A, 120V@12,000W=100A {divide Amps by 2 for 240VAC)

Examples; (from my system, 120VAC with 24VDC Battery Banks)
- when my Fridge starts up it pulls 7A but drops to 1.5A when running (EnergyStar-5 fridge)
- My Deep Well pump starts at 550W and ramps to 1100W by the time it reaches 52PSI cutoff. (Grundfos SQ-5 with SoftStart)

I am 100% offgrid, use on average 3.5-4.0kWh a day can run a Heavy MIG or 3HP Compressor - pretty much whatever I want but I do not abuse the system with Big Ugly Dirty draws like MIG/Compressor, I have a Genset for them...

Have a look at the First Link in my Signature which give you diagrams, schema's & photo's of how it is all put together. In 2021 You now have access to some really good AIO's (All-In-Ones) which contain the Inverter/Charger & Solar Charge Controllers and even Midnite Solar has their new DIY Series out.

A Few Thoughts:
I installed 120VAC only, as I saw no use for 240VAC my well is 300' deep and a 120V softstart deep-well pump works perfect.... BUT that was 6 years ago and product availability & options was considerably less. We could not even get Victron back then. IF I was building from scratch today, I would build with Stackable AIO's most likely Growatt and have it provide 240VAC SplitPhase (I'm Canadian so) which would still provide all the 120VAC I need but also support an EV and other E-Toys.

A few bits you need to know.
12V/100AH=1.2kWh or 1280Wh.
24V/100AH =2.65kWh or 2560Wh.
Battery Packs in Series Increase Voltage (NOT the best solution for Lithium Based Packs)
Battery Packs in Parallel increase Storage (Amp Hours) and is the preferable solution as it shares load/charge and adds fault tolerance / failover.
It is always best to Build to your Target ! If going 24V or 48V Battery, buy/build 24V or 48V.

Inversion Info:
Pure Sine is the only way to go for the health of your electronics and sensitive devices (even LED Lights).
Depending on Grade/Tier, the Lower Grade inverters can be anywhere for 84-90% efficient. Tier-1 High Grade premium Inverters can be up to 96% Efficient. As well, Lower Tier Inverters tend to use a LOT more power (watts) in "Stand-By or Sleep" modes whereas Hi Grade Tier-1 grade equipment may only use as little as 8 Watts idle. THIS ADDS UP over 24 Hours !
Low Frequency Inverters are heavier but more efficient and capable of 3X Load Surge. IE a 3000W Inverter can handle 9000W surge.
Hi Frequency Inverters can only handle a Max of 2X Surge and can have issues with certain devices & electronics.
REF: https://www.magnum-dimensions.com/k...rsion-methods-explained-high-frequency-vs-low

Currently the most popular cells being used for DIY Battery Packs are the 280AH & 304AH cells coming out of China. One Known Good Reputable Vendor can be found here.. Contact Amy Wan by Chat/Messenger for a "proper quote with shipping"
LINK to Luyuan Tech.
SPECIAL NOTE: Almost all vendors tell you they sell Matched & Batched cells, they DO NOT, they are Voltage & IR Tested at the storage voltage as received. Luyuan Tech actually has the cells tested & batched BY EVE themselves and you get a report with QR Serial Number & test results of your batch. Luyuan Bulk cells are also Manufacturer Direct, no broker junk.
! Matched cells are ALWAYS above Spec, so a 280AH is more often 290AH if not more.
! Bulk Cells rarely reach full capacity and maybe a few AH short or can be New Old Stock, Grade-B, Blems, or even Used.

Have a look at the links in my Signature there are a few useful links to docs/resources that may be of help to you.
Hope it Helps, Good Luck.