Help with backup solution: capacity calcs

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c3n0byt3

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Hi all, long post ahead...

I've been looking for a backup solution and think inverter+batteries are the best choice for me.
It's in home and will power a few appliances for ~4 hours.
It need not have a UPS feature and will be plugged into appliances when needed.

So on the inverter size needed I have this.
Appliancewattssurge facpower facfinal wattsVA load
TV12510.9125139
Mibox1010.91011
Fridge15020.9300333
ONT-Router2510.92528
Total460511
Considering inverter efficiency of say 80% then I have total size of ~640VA.
Yes I know some thumb sucks, but hopefully on the conservative side?
We have one of those Eskom smart meters and spot usage is always <300W with all above running (fridge too) + some lights (with geyser switched off). If the fridge is quiet it's around 100W.

On the battery side is where I'm having trouble. I have this so far.
Run time (hrs)VoltsDC A/hInit DC AOther facs*Final DC A
41243 (Total VA / Volts)1702.5~426
* Other facs: Temp, age, DoD, wiring
I'm assuming 12v as this seems most popular.
Ultimately, I don't know what this all means in terms of actual available batteries.
If a battery is say 105ah then do I need like 4?
Can I balance cost of lithium batteries vs adding more non-lithium batteries and discharge >50%??

On set up, I'll probably place it behind our couches (closest to fridge) and run extensions from there.
Then on actual purchasing, stock seems to be a problem right now.

I'll need a pure sine wave inverter for the fridge right?
Then it needs to be >640VA? Does speccing higher help?
For 4 hours do I really need total 430A? The surge factor on the fridge is purely for that startup.
Am I missing some pieces wrt inverter+batteries? Looking at Geewiz makes it easier to add on trolley and batteries.
What else do I need?
 
The fridge will spike up to 1000W or more at startup, consider a 24V setup, it's much better to work with low dc current.
 
Lupus said:
You do not need a fridge for load shedding, why do people keep putting this on their requirements. Your fridge if sealed correctly will hold 24 hours without power.
Click to expand...
I don't get it either. Max we had was 12 hour outage in last 20 years and nothing was spoilt in the fridge.
 
surface said:
I don't get it either. Max we had was 12 hour outage in last 20 years and nothing was spoilt in the fridge.
Click to expand...
I've had a 24 hour outage and everything was fine, sure my dad has had a 55 hour outage, but he borrowed my generator to keep his fridge going.
But for load shedding you often see it, does this keep a fridge going? I mean how bad is your fridge that 2 to 4 hours of no power is going to warm it up.
 
c3n0byt3 said:
Am I missing some pieces wrt inverter+batteries? Looking at Geewiz makes it easier to add on trolley and batteries.
What else do I need?
Click to expand...
subscribed to thread as I am also looking. IMO, this trolley type setup will not help you with fridge.
 
surface said:
subscribed to thread as I am also looking. IMO, this trolley type setup will not help you with fridge.
Click to expand...
Well there is one that would but it has a 3kva, if you get a 3kva+ inverter it will handle the fridge easily.
 
Lupus said:
You do not need a fridge for load shedding, why do people keep putting this on their requirements. Your fridge if sealed correctly will hold 24 hours without power.
Click to expand...
Yes it should be sealed. Never had an issue.
However, LS with 2.5 hours in-between and +26C ambient indoor could play a role in why we are seeing stuff in the crisper going bad real quick.
Also, we have two kids (three if you count thoughtless spouse) that keep opening the fridge.
But I'm leaning toward the short time between LS and higher summer temperatures....
 
TheChamp said:
The fridge will spike up to 1000W or more at startup, consider a 24V setup, it's much better to work with low dc current.
Click to expand...
Ok so it looks like 90% of my system is going towards catering for the fridge, or is it just the startup?
Or is it also when the compressor kicks in from time to time?
 
c3n0byt3 said:
Ok so it looks like 90% of my system is going towards catering for the fridge, or is it just the startup?
Or is it also when the compressor kicks in from time to time?
Click to expand...

Fridges have a duty cycle

Typical example: 30W idle with a 1000w/2000w 1 second startup and then 300W while its cooling. A typical duty cycle is normally rated 35% of the time for fridge and 65% for a freezer.

Check your fridge yearly rating and you can work it out from there.

Personally We just use those blue ice blocks to replace empty space, that way it can stay cooler for longer even when you open the door.
 
Last edited:
AlphaJohn said:
Fridges have a duty cycle

Personally We just use those blue ice blocks to replace empty space, that way it can stay cooler for longer even when you open the door.
Click to expand...
I am so envious that you have empty space in your fridge. :D:D We had empty space when we bought it.
 
Lupus said:
You do not need a fridge for load shedding, why do people keep putting this on their requirements. Your fridge if sealed correctly will hold 24 hours without power.
Click to expand...
My wife insisted, I was looking at something costing around R3k, still remember making a thread about it. Then I went F@4k it, let just get solar.
 
AlphaJohn said:
Fridges have a duty cycle

Typical example: 30W idle with a 1000w/2000w 1 second startup and then 300W while its cooling. A typical duty cycle is normally rated 35% of the time for fridge and 65% for a freezer.

Check your fridge yearly rating and you can work it out from there.

Personally We just use those blue ice blocks to replace empty space, that way it can stay cooler for longer even when you open the door.
Click to expand...
The ice blocks are a plan. Similar to power backup but stored in water rather than battery.

Without the fridge it's looking like lithium could be a possibility.

The only remaining problem is stock.
 
rh1 said:
My wife insisted, I was looking at something costing around R3k, still remember making a thread about it. Then I went F@4k it, let just get solar.
Click to expand...
That will power the fridge
 
  • Haha
Reactions: rh1
c3n0byt3 said:
The ice blocks are a plan. Similar to power backup but stored in water rather than battery.

Without the fridge it's looking like lithium could be a possibility.

The only remaining problem is stock.
Click to expand...
Stay off the acid.
 
So I've identified a potential.

Inverter Trolleys

Buy Inverter Trolleys online at the Best Prices! Door to Door and Overnight Delivery. Voted the best online store in SA. View Here
www.geewiz.co.za www.geewiz.co.za

Really don't need the solar stuff. More so since according to Geewiz the batteries (Hubble S100) that they use aren't compatible.

How are these Axpert inverters? Have read some posts that say not good.
But then for the Mercer ones also not good in conjunction with Hubble batteries?.
 
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