What size inverter + battery kit do I need?

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moothemoo

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I read online how to do the calculations but I want to check that I'm not missing something or have calculated incorrectly. I want to run 3 standing fans that I have and a CPAP. For the CPAP I know I will need a pure sine wave, so I'm looking at this system.

Fans: 360 V x 0.25 A = 90W (3 x 90W = 270W)
CPAP: 240V x 2 A = 480W
Total = 750 Watts per hour requirement

Assuming a 75% efficiency on the battery, 75Ah divided by hourly rate of 2 hours (load shedding window) gives discharge current of 37.5 Amps which multiplied by 12 Volts gives me 450 W per hour per battery, so 900 W per hour on a 2 x 100Ah system

Am I correct that the system I'm looking at will do the job or am I missing something?
 
moothemoo said:
I read online how to do the calculations but I want to check that I'm not missing something or have calculated incorrectly. I want to run 3 standing fans that I have and a CPAP. For the CPAP I know I will need a pure sine wave, so I'm looking at this system.

Fans: 360 V x 0.25 A = 90W (3 x 90W = 270W)
CPAP: 240V x 2 A = 480W
Total = 750 Watts per hour requirement

Assuming a 75% efficiency on the battery, 75Ah divided by hourly rate of 2 hours (load shedding window) gives discharge current of 37.5 Amps which multiplied by 12 Volts gives me 450 W per hour per battery, so 900 W per hour on a 2 x 100Ah system

Am I correct that the system I'm looking at will do the job or am I missing something?
Click to expand...
accepted depth of discharge on lead acid batteries is only 50%, so you need to adjust for that, as well as for the inverter effeciency.

As for you calculations, it might be easist to work in watt hours (Wh). You require 1500Wh (2h x 750w) at 50% DOD that is 3000wh. Assuming 80% router effeciency that becomes 3750Wh. A 12v 100Ah battery is 1200Wh, so you would need 4 of them.
 
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Sorry for hijack.

I am looking for the cheapest pure sinewave UPS, and please not Mecer.
APC is the best apparently.
 
If you frequently discharge a lead-acid battery to 50%, you will be lucky to get 12 months life out of it. You could do 20%

Consider a lithium-ion battery. Much more expensive but you get what you pay for in spades
 
CamiKaze said:
Sorry for hijack.

I am looking for the cheapest pure sinewave UPS, and please not Mecer.
APC is the best apparently.
Click to expand...

See what Geewiz are selling

A Mecer is a rebranded Axpert. I do recommend APC, they deliver what their spec says
 
@moothemoo

(Battery voltage) x (battery ah) / (load in watts) / 2 x (power factor of 0.9) = hours

Your load is only 750w so suggest you get a 1.5kva inverter (alway double) which will probably be a 12v or 24v battery system. Makes no difference though to the calculation.

2 x 12v 100ah batteries in parallel = 12v / 200ah
2 x 12v 100ah batteries in series = 24v / 100ah
Multiply the volts by the ah =12 x 200 or 24 x 100 = 2400
Now divide by your expected load = 2400 / 750 = 3.2
Now divide by 2. This is a 50% depth of discharge on your batteries = 3.2 / 2 = 1.6
A good inverter will be pure sine wave and will have an efficiency (power factor) of around 90 - 95% = 1.6 x 0.9 = 1.44 hours
A 2 x 100ah battery system will give you 1 hour and 25 minutes of 750w until the batteries reach 50% depth of discharge.

I would suggest 4 batteries to get a minimum of 2 hours 50 minutes of 750w to easily squeeze in a load shedding session and have the batteries fully charged before the next session.

As a matter of interest i have a 3kva 24volt system. It has 4 x 100ah batteries. My maximum load, with everything on is less than 1kva. My normal daytime load is 0.3 - 0.4kva. Busy time in the house is 0.8kva. Maximum load is 2 tv's with media players, 1 pc with multifunction laser printer, 1 pc, 1 laptop, 1 wifi router, 2 wifi ap's, 2 x 5 port switches, cooker hood, 2x 20w spots, 5x20w pot lights, 4x5w pot lights, 2 x box fans, 1 x evaporator fan, 1 ceiling fan with 2x8w cfl's, 4x15w led bulbs, 1 bathroom extractor, 2x18w fluorescent light, 3 x cell phones on charge, 2 x tablets on charge. 620L fridge/freezer add 300w "idling" and 600w going flat out.

Forget kettle (2.4kw), convection oven (3kw fan or 2kw grill), microwave (2kw), geyser (3.5kw), iron (2kw), 8kg washing machine (2kw), tumble dryer (3kw), dishwasher (2kw)
 
Last edited:
Dairyfarmer said:
@moothemoo

(Battery voltage) x (battery ah) / (load in watts) / 2 x (power factor of 0.9) = hours

Your load is only 750w so suggest you get a 1.5kva inverter (alway double) which will probably be a 12v or 24v battery system. Makes no difference though to the calculation.

2 x 12v 100ah batteries in parallel = 12v / 200ah
2 x 12v 100ah batteries in series = 24v / 100ah
Multiply the volts by the ah =12 x 200 or 24 x 100 = 2400
Now divide by your expected load = 2400 / 750 = 3.2
Now divide by 2. This is a 50% depth of discharge on your batteries = 3.2 / 2 = 1.6
A good inverter will be pure sine wave and will have an efficiency (power factor) of around 90 - 95% = 1.6 x 0.9 = 1.44 hours
A 2 x 100ah battery system will give you 1 hour and 25 minutes of 750w until the batteries reach 50% depth of discharge.

I would suggest 4 batteries to get a minimum of 2 hours 50 minutes of 750w to easily squeeze in a load shedding session and have the batteries fully charged before the next session.

As a matter of interest i have a 3kva 24volt system. It has 4 x 100ah batteries. My maximum load, with everything on is less than 1kva. My normal daytime load is 0.3 - 0.4kva. Busy time in the house is 0.8kva. Maximum load is 2 tv's with media players, 1 pc with multifunction laser printer, 1 pc, 1 laptop, 1 wifi router, 2 wifi ap's, 2 x 5 port switches, cooker hood, 2x 20w spots, 5x20w pot lights, 4x5w pot lights, 2 x box fans, 1 x evaporator fan, 1 ceiling fan with 2x8w cfl's, 4x15w led bulbs, 1 bathroom extractor, 2x18w fluorescent light, 3 x cell phones on charge, 2 x tablets on charge. 620L fridge/freezer add 300w "idling" and 600w going flat out.

Forget kettle (2.4kw), convection oven (3kw fan or 2kw grill), microwave (2kw), geyser (3.5kw), iron (2kw), 8kg washing machine (2kw), tumble dryer (3kw), dishwasher (2kw)
Click to expand...

Thanks so much for your reply, it is very helpful.

I just want to power 3 x standing fans which I’ve heard on average are 50 W / 0.25 A and my husbands CPAP machine which says 120-240 V / 2 A so assume that would use 480W? And only for 2 hours once a day. It’s actually only every second day at the moment on Stage 2 (only during the night), but probably worth preparing for future stages.

Regarding charging, how long do the batteries take to charge, if for e.g. in your example it gives power for 2 hr 50 min?
 
For your cpap machine you can get a 12V adapter from your machines manufacturer. I use one too. Got a lithium ion battery from my manufacturer that's about the size of two cellphones and delivers 6hours extra power. You can also get a 12v car cigarette lighter adapter with a transformer in. Call your local machines dealer or any medical company that sells your machine
 
moothemoo said:
Regarding charging, how long do the batteries take to charge, if for e.g. in your example it gives power for 2 hr 50 min?
Click to expand...
About half the time the batteries worked to get them charged.

If you go the inverter route, then get your house light (after converting to led's) and your tv onto the inverter too. You won't even realise eskom is ever off.
 
Dairyfarmer said:
About half the time the batteries worked to get them charged.
Click to expand...
You seem to really know your schit with this stuff, so hopefully you can also enlighten me:

I have a 1000W Modified sine wave Inverter with 2 000W Peak capacity.

I have 4 x 102Ah batteries connected in parallel for a 12V system.

I use a 30A smart charger, permanently on the batteries, charging in a cross over setup ( + connected to Battery 1 and - connected to battery 4).

I only run a "old" 55" Plasma and a small media player for max 2 hours during shedding.

Two "questions":

I get a huge amount of "theories" on the internet on exactly how much power my Plasma consumes but it seems to be in the 300W range. What would my DoD be and how long can I run my Tv etc, before getting to only 70% DoD?

Also after using my setup for 2 hours, it definitely is not fully charged after only 1 hour. Not according to my charger.

Your feedback/advice would really be appreciated.
 
Dairyfarmer said:
@moothemoo

(Battery voltage) x (battery ah) / (load in watts) / 2 x (power factor of 0.9) = hours

Your load is only 750w so suggest you get a 1.5kva inverter (alway double) which will probably be a 12v or 24v battery system. Makes no difference though to the calculation.

2 x 12v 100ah batteries in parallel = 12v / 200ah
2 x 12v 100ah batteries in series = 24v / 100ah
Multiply the volts by the ah =12 x 200 or 24 x 100 = 2400
Now divide by your expected load = 2400 / 750 = 3.2
Now divide by 2. This is a 50% depth of discharge on your batteries = 3.2 / 2 = 1.6
A good inverter will be pure sine wave and will have an efficiency (power factor) of around 90 - 95% = 1.6 x 0.9 = 1.44 hours
A 2 x 100ah battery system will give you 1 hour and 25 minutes of 750w until the batteries reach 50% depth of discharge.

I would suggest 4 batteries to get a minimum of 2 hours 50 minutes of 750w to easily squeeze in a load shedding session and have the batteries fully charged before the next session.

As a matter of interest i have a 3kva 24volt system. It has 4 x 100ah batteries. My maximum load, with everything on is less than 1kva. My normal daytime load is 0.3 - 0.4kva. Busy time in the house is 0.8kva. Maximum load is 2 tv's with media players, 1 pc with multifunction laser printer, 1 pc, 1 laptop, 1 wifi router, 2 wifi ap's, 2 x 5 port switches, cooker hood, 2x 20w spots, 5x20w pot lights, 4x5w pot lights, 2 x box fans, 1 x evaporator fan, 1 ceiling fan with 2x8w cfl's, 4x15w led bulbs, 1 bathroom extractor, 2x18w fluorescent light, 3 x cell phones on charge, 2 x tablets on charge. 620L fridge/freezer add 300w "idling" and 600w going flat out.

Forget kettle (2.4kw), convection oven (3kw fan or 2kw grill), microwave (2kw), geyser (3.5kw), iron (2kw), 8kg washing machine (2kw), tumble dryer (3kw), dishwasher (2kw)
Click to expand...
What is the difference between a 12v and 24v setup?

IE which one one should I go for?
 
Thor said:
What is the difference between a 12v and 24v setup?

IE which one one should I go for?
Click to expand...
In terms of power and storage it doesn't make a difference. It is recommended to increase voltage to keep the amps down on the battery lines though.

One needs to think about your needs though, if you need a small solution a 12v makes sense, and if you go for 24v you need to add batteries in pairs.
 
Thor said:
What is the difference between a 12v and 24v setup?

IE which one one should I go for?
Click to expand...
Like @johnjm says 12v you can add a single battery at a time, with 24v it is always in pairs. Although you don't want to be adding new batteries to an old system. The system will only ever be as good as the worst battery.
 
MightyQuin said:
Your feedback/advice would really be appreciated.
Click to expand...
Do yourself a favor and buy or borrow a Kill-A-Watt type device. You need the maximum watts as well as the average. The maximum will help you spec a system that will handle everything getting turned on at once. the average will help you spec the amount of battery storage you need.

I spent 2 weeks testing most devices in the home to see what they actually used. Take a fridge. I have double door that "idles" at 300w but can run for a while at 600w. So the inverter must be able to handle the extra 300w for more than a few seconds (inverters can handle a certain overload for very short periods to account for things like motor startup). But my batteries don't need to account for 600w all the time, just the 300w.
 
Dairyfarmer said:
Like @johnjm says 12v you can add a single battery at a time, with 24v it is always in pairs. Although you don't want to be adding new batteries to an old system. The system will only ever be as good as the worst battery.
Click to expand...
That bit I understand but what's the benefit

Pros and cons of the two?

IE why would I choose the schlep of a 24v system over a 12v?
 
Dairyfarmer said:
Do yourself a favor and buy or borrow a Kill-A-Watt type device. You need the maximum watts as well as the average. The maximum will help you spec a system that will handle everything getting turned on at once. the average will help you spec the amount of battery storage you need.

I spent 2 weeks testing most devices in the home to see what they actually used. Take a fridge. I have double door that "idles" at 300w but can run for a while at 600w. So the inverter must be able to handle the extra 300w for more than a few seconds (inverters can handle a certain overload for very short periods to account for things like motor startup). But my batteries don't need to account for 600w all the time, just the 300w.
Click to expand...
Yea the fridge is a silent killer.

Mine idles at 100watt but goes to 750watt when the compressor kicks in.Screenshot_20200202-214057_eWeLink.jpeg
 
When I spec'ed my system there was so much contradictory info out there. I had already decided what i wanted to be on the inverter. Firstly tv's and pc's had to be on. They were already on UPS's that lasted upto an hour. Then I wanted all my lights to be on. And lastly provision for the fridge if needed. The system needed to be more than 1.5kva so I went straight for the 3kva. The batteries were another story, but being a 24v system I needed more than 2 and it had to be in pairs. So 4 seemed to meet the requirements. I bought "cheaper" batteries for R1800 each. They cost more than the inverter. I didn't want to lay out tens of thousands on batteries and they didn't meet requirements or were a complete overkill. I did a Rands per Cycle calculation and a R3500 vs a R1800 worked out at much the same cost per cycle.
 
Dairyfarmer said:
When I spec'ed my system there was so much contradictory info out there. I had already decided what i wanted to be on the inverter. Firstly tv's and pc's had to be on. They were already on UPS's that lasted upto an hour. Then I wanted all my lights to be on. And lastly provision for the fridge if needed. The system needed to be more than 1.5kva so I went straight for the 3kva. The batteries were another story, but being a 24v system I needed more than 2 and it had to be in pairs. So 4 seemed to meet the requirements. I bought "cheaper" batteries for R1800 each. They cost more than the inverter. I didn't want to lay out tens of thousands on batteries and they didn't meet requirements or were a complete overkill. I did a Rands per Cycle calculation and a R3500 vs a R1800 worked out at much the same cost per cycle.
Click to expand...
Where does your inverter link to the house electrics? Have you wired it to the DB?
 
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