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UPS or Solar system?


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Way I explain to people:

- What do you want?

- What do you need?

- Why?

There is a HUGE difference in price.

 

An inverter is like your car's engine. It does not matter what size it is, in the end, the engine size is of no consequence if the tank is empty.

 

With power solutions, the fuel tank is the batteries. And that is where we miss the pot easily.

 

Inverters: You can get a 5000w inverter or a smaller 1200w inverter.

The question you need to ask yourself: What is my load ... and you need to differentiate between: Wants and Needs.

And then you need to decide: 48v or 24v?

 

24 versus 48v batteries - Solar and UPS:

You tend to find 1 battery starts losing efficiency so you need to replace all of them in one go, for you cannot replace just one. 

- 24v = 2 x batteries

- 48v = 4 x batteries

It makes a huge impact 1-5 years later.

 

Inverters ... size does not matter, run-times do. Estimates based on 2 x 105ah batteries:

- 300w load will run +-3.57 hours to 50% DOD

- 600W will run +-1.79 hours to 50% DOD

- 5000w - 21 minutes tot 50% DOD

Unless you use solar daytime, then a larger inverter is ideal to power all the devices you can between 10h00ish and 15h00ish.

BUT, the inverter must match your panels input!

 

Conclusion:

The lower your load, the less batteries you need, the cheaper the inverter becomes.

On solar, if used daytime, the inverter and panels must match to save optimally, to get a ROI that makes you smile.

 

What are you options to power loads: Questions to answer:

a) Is it to power stuff during daytime?

b ) Or is it to power stuff during load-shedding?

 

During load-shedding: UPS - solves the problem as cheap as possible.

1: Plan for 2.5 hours @ max 50% DOD.

2) And if you can, plan for, or have the option to add more batteries, if load-shedding comes twice a day.

This is to make sure the batteries last as long as possible.

 
Power stuff during daytime: Solar system - to save money

1: Use the power generated daytime - cheaper - less batteries.

2: Store for night use - gets really expensive for batteries are much more expensive than Eskom.

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And then you need to decide: 48v or 24v?

 

24 versus 48v batteries - Solar and UPS:

You tend to find 1 battery starts losing efficiency so you need to replace all of them in one go, for you cannot replace just one. 

- 24v = 2 x batteries

- 48v = 4 x batteries

It makes a huge impact 1-5 years later.

 

I agree with some of the things you've mentioned and would like to add the following.

 

The battery voltage of the UPS/inverter will depend on a lot of things, not only on replacement price of the batteries.

 

24V in terms of batteries does not equate to 2 batteries and 48V does not equate to batteries. A battery bank can consist of 2V cells or 4V, 6V, 8V etc. batteries and you can even have multiple strings of batteries in parallel.

 

Back to the inverter battery voltage...

An important thing to keep in mind when deciding on the system voltage / battery bank voltage is the size of the inverter. The bigger the inverter the more current would be drawn from a lower voltage battery bank than from a higher voltage battery bank for the same inverter output. Here is a comparison without including any losses etc. If you draw 3000W from a 24V inverter, you will draw approximately 3000/24 => 125 ampere from the battery bank. If you draw 3000W from a 48V inverter, you will draw approximately 3000/48 => 62.5 ampere from the battery bank. In real life you rather want to draw less current from a battery bank for various reasons which include the following:

  • The batteries are discharged at a lower rate and will last longer per cycle.  For the same reason running the inverter for a specific amount of time the batteries on the lower voltage inverter will be discharged (DoD) more than the batteries on the higher voltage inverter.
  • The battery cables on a higher voltage inverter can be thinner and accessories like fuses and fuse holders etc. will also be cheaper because they don't need to handle such high currents.
  • The connection and electronics inside the inverter can be designed to handle lower currents in the higher voltage inverters which  can also result in a lower purchase price for the inverter.

 

Most of the time small inverters <2000W are designed to work on 12V, medium sized inverters 2KW-3KW are designed for 24V and bigger inverters for 48V or higher.

 

Bottom line is that inverter sizes (KVA or KW) usually determine the battery bank voltage. :)  

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Agreed. I have that some issue trying to convince people to spend their money on a smaller sine wave inverter instead of that big honking 3kva MSW inverter (that's never going to get anywhere near that because they are tied to a 100Ah battery :-)

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Agreed. I have that some issue trying to convince people to spend their money on a smaller sine wave inverter instead of that big honking 3kva MSW inverter (that's never going to get anywhere near that because they are tied to a 100Ah battery :-)

 

I've came across a post in an overseas forum the other day where someone mentioned a 12V10KW inverter.  :D

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Excellent!!! Open a door and the wisdoms flood in. I like!

 

superdiy, yes, the more watts the inverter can feed, the better it is for the batts to have higher DC volts. You also save on wiring.

 

But the core of the post is: Do you need a 5000w inverter or do you want to power your lights, TV, DSTV, alarm system? Therein lies the crux.

 

So to explain that to the layman, as plonkster also tries to do, is best done when you bring in the costs of batt replacements. Then one start to think: Uhm, do I need it, or want it?

 

The person who needs a i.e. 5000w inverter, they have no choice for you do not get, that I am aware, good pure sine wave inverters in that range at 24v. Only 48v or higher, and the chances are good that you need bigger ah batts than 105ah.  :D

 

I sell CyberPower inverters that max at 2450w, using 24v. Very good inverter / solution, but they recommend 2 x 200ah as min batts. 2 x 200ah are cheaper than 8 x 105ah, and less wires.

 

@ Plonkster, where can I find good pure sine wave inverters, that are under 2000w?

I sell a lot of 1320w modified sine wave inverters (as a sideline), but I am now getting bored with them. Time to upgrade.  :D

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Victron do make a Multiplus 24/5000 :-) I agree that at that wattage you want a 48V bank :-)

 

I personally went with a 24V bank precisely because I realised I don't NEED to run the whole house. In fact, I have a very small 1600VA inverter. Decision based simply on two factors, 1) I don't need to power the hole house when Eskom is out, 2) most of my consumption is caused by low-wattage things that are on a lot of the time (baseline, fridge, freezer, etc). The high wattage stuff are on only a few minutes (vacuum cleaner, hair dryer) or hours (stove) per day.

 

Sine wave inverters under 2000W? Well, there's your Victron phoenix range, starting at 180W, then it goes up to 800W (in that particular packaging) after which packaging changes to the blue case we know and love. Thing is they are not cheap. I looked at the Cotek too. Apparently that's the same thing that's sold as a Samlex in the states: Not top of the range but apparently not bad. Then there's these Scott Power inverters that some people in my home town sell. They are from UAE, looked at the spec sheet, haven't yet seen one in real life. The literature is very honest though, not embellished like Chinese products (you know, with those impossible peak power numbers).

 

But you know every time someone asks me that question, I realise I'm a Victron groupie... :-)

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Great information. The most important thing before you even start is to know exactly what you are using. You need to be able to size your battery bank properly. If you do not know what your peak loads are and how much power you are using, you just cannot determine what you need to get. 

 

Since I have started my solar project, I have had several people asking me about these R8 000 avoid loadshedding kits that consist of 2 x 100ah batteries and about a 2kw inverter. Looking at that, its just big enough to cause enormous frustration. That is about just big enough to power a few lights for a few hours.

 

I have also had backup power installers telling me that 4 x 100ah is more than enough battery power to power an average house for 8 hours. Its quite interesting when you do the sums. 4800w x .85% efficiency x 50% DoD = 2kw. Average houses likely use 700 - 900wh at least, taken that most haven't changed lights to LED or gone for power efficient fridges etc.

 

Best investment you can make before you decide on sizing anything is a proper usage meter and run it for at least a month so you can get an idea of your usage patterns. I have used the Efergy meter when planning mine and It gave me a decent picture of exactly what I need. Probably the best investment I have ever made.

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Man, I went nuts with my measurements. First I got the OWL meter (same as the efergy really, CT you wrap around the big red wire, wireless LCD panel). Then I got an efergy plug-in to measure some appliances more directly. Finally I ordered ten cheap current transformers from RS and an arduino mini with 12 analog inputs, and built something to measure every circuit separately. That last one wasn't too much of a success, measurements aren't very accurate, and I suspect it has to do with my voltage divider resistor values... long story. Then I replaced a few appliances and lamps... and only then did I get the inverter.

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Ran a Efergy meter for over a year, for that is how long it took me to a) find them bastard loads, 2  replace inefficient equipment (the right way) and c) get the family in line (which took the most effort). 

 

Today I smile when solar installers come to the door and say: Give us your electricity account, that we can quote you to reduce it.

 

 

And I cannot agree more. Most people selling 'load shedding solutions' out there today has NO clue that a maintenance free batt must NEVER go below 50%.

And at 50% you have theoretically 200 cycles ... in real life, about 20-40 (+-7 months) ... because the users are draining them to near inverter switch off point.

 

And to sell user maintenance batteries, eish, they do not like that either, checking the water thing, as one guy put it.

 

Even had a IT guy who sent a UPS back to a supplier, which was a few months old, due to it not 'working' anymore. Supplier replaced the entire UPS, no questions asked. When they tested the inverter on a new set of batteries, it purred perfectly. The batts however, where drained beyond their abilities ... now how is that a warranty claim ... supplier learned very fast! Ps. The same IT guy told me the story.

 

Then he started buying from me and tried the same trick, I just laughed for I did make it very clear to him what goes for what before he bought. He is not selling UPS'es anymore.  :P Ps. he was in it for the bucks, not to help. 

 

No-one is telling users to reduce their load to match the batteries, or, if they do not want to reduced, to increase the batteries. Watch, next year, when the batts are dead, they are going to mistrust everyone, with a lot of 2nd hand inverters flooding the market.

 

I sold over 60 UPS'es the last 3 months as a sideline. If the client does not agree to the 300w / 600w runtimes at 50% on 2 x 105ah batteries, I double the batts ... resulting in every one acknowledging the limitations once they saw the price difference.  :lol:

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Man, I went nuts with my measurements. First I got the OWL meter (same as the efergy really, CT you wrap around the big red wire, wireless LCD panel). Then I got an efergy plug-in to measure some appliances more directly. Finally I ordered ten cheap current transformers from RS and an arduino mini with 12 analog inputs, and built something to measure every circuit separately. That last one wasn't too much of a success, measurements aren't very accurate, and I suspect it has to do with my voltage divider resistor values... long story. Then I replaced a few appliances and lamps... and only then did I get the inverter.

It is to be noted, I was told, that these meters are guideline only. To measure it spot on, can cost a few grand for as you suggested, there are a few variables that one needs to take into account.

 

So OWL and Efergy, gives you a large picture. But, interestingly enough, my Efergy is as close as dammit to the actual municipal reading per month.

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The meters with a simple current clamp only measures apparent power (VA), not real power (W). The apparent power and real power will be exactly the same for pure resistive loads like a geyser element or kettle, but as soon as the load becomes inductive or capacitive the power factor needs to be taken into account.To measure real power, you have to measure the current, the voltage and the power factor. For example, I've measure a few items in and around the house and two interesting examples are: A specific 3W LED downligher consumed 3.1W (real power), but 6VA apparent power and the 40 inch LCD TV consumed 3W (real power) on standby, but 18.2VA (apparent power) on standby. Best would be to invest in a true power meter - check if the meter can measure power factor, because then it should be able to measure real power (W) as well.

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I put a Uniontech 5W LED downlight on my scope a while ago. The efergy meter told me it had a power factor of around 20%, so I wanted to see what it looks like. This isn't a perfect picture, I had the voltage sense on the wrong side of my shunt resistor and the shunt resistor itself was on the high side, so that little dip in both waveforms likely has to do with that. Also, my current and voltage wave form appears out of phase here but it isn't, I just had the wires the wrong way round :-) To note... useful work is only done on the rising side of the voltage waveform, and not even the full half of it. So in order to arrive at the same 5W, the peak current has to be way higher than what it would be if you were using DC, which would be just below 500mA. As a result, I suspect this thing is rather a lot closer to 20va than 5.

 

uniontech-mr16-power-factor.jpg

 

Now my efergy will say it uses 18 watts, because it assumes a power factor of 0.9.

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