3rd build, biggest I’ve done yet so any help appreciated

[Ro0ke]

Hi guys, new here, have a bit of experience building small systems but would like to run some numbers through you guys if you’d be so kind to read through them and point out where I’ve gone wrong or could be designed better

So basically this system is to run a detached garage for a car

The loads I have noted down in my garage are

1200 watt exhaust fan to exhaust fumes while the engine is warming up, it is on no longer than 10 minutes every weekend but I allowed 15 mins in calculations. This is an induction load so has a start up surge. 300wh used a day

Rated 1800watt compressor that’s on no more than 5 mins every month but I’ve allowed 15 mins every weekend in my calculations, I assume this is an induction load but I didn’t measure any surge with the clamp meter? Unsure on this. 450wh used a day

There are 5 led lights that pull under 100 watt each so I’ve allowed 500 watt for 2 hours a day. 1kwh used a day

Then there’s a motorised roll down garage door rated at 500watt that is used every weekend but I allowed 15 minutes a day. 125wh used a day.

This comes to 1875Wh a day even though it’s probably abit less I’ve bumped this up to needing a 3kwh battery as a little bit of autonomy which is 250Ah battery needed with a 12v system maybe 2 if I go down the lead acid route due to the 50% usage capacity.

I’m thinking of pairing this with 4x 300watt solar panels totalling a 1200 watt array
And a 3kw inverter with 6kw rated peak as none of those big loads will be on while the other is, only the lights.

Does this all look correct to you guys?
And is there anything you would change or anything I’ve done incorrectly?

I really appreciate any help and thoughts I get on this,

Thanks

Matt

 

[John Frum]

Suggest you get a clamp meter with in-rush function to measure the locked rotor amps for the fan and the compressor.

 

[Ro0ke]

Suggest you get a clamp meter with in-rush function to measure the locked rotor amps for the fan and the compressor.

Thanks for your reply, do I have to be this precise could I not add a factor in with an equation to cover it? The 1200watt exhaust fan has a rated wattage of 550watt but when I clamped it it’s pulling a steady 4.6A at 240v.

 

[John Frum]

Thanks for your reply, do I have to be this precise could I not add a factor in with an equation to cover it? The 1200watt exhaust fan has a rated wattage of 550watt but when I clamped it it’s pulling a steady 4.6A at 240v.

The in-rush/locked rotor amps could be 5x the measured duty cycle.
Might be more.

 

[John Frum]

I suggest you meause then add some fudge because these devices tend to draw even more on startup as they age.

 

[Ro0ke]

I suggest you meause then add some fudge because these devices tend to draw even more on startup as they age.

I’ve ordered a budget inrush meter to measure and will add 15-20% as well. Thanks mate

Does everything else look correct?

I’ve seen some 385watt panels that I’m thinking I could use, I can use them on a 24v system if my SCC can handle the VOC, right?

 

[John Frum]

loads
1200 watt exhaust fan
43 watt hours per day

1800 watt compressor
64 watt hours per day

5x led lights that pull under 100 watt each so I’ve allowed 500 watt for 2 hours a day. 1kwh used a day
1000 watt hours per day
*seems very high for led lights
*my led lights use 7.5 watts each.

garage door opener
125 watt hours per day

I get 1232 watt hours per day

How much energy you can harvest from your solar panels depends on where you live so I can't begin to guess.

I doubt a 3000 watt inverter is going to handle your compressor, not even sure it will handle your inductive loads.

You need to size the battery bank, bms, inverter and balance of system to cover the maximum instantaneous load.

 

[Ro0ke]

loads
1200 watt exhaust fan
43 watt hours per day

1800 watt compressor
64 watt hours per day

5x led lights that pull under 100 watt each so I’ve allowed 500 watt for 2 hours a day. 1kwh used a day
1000 watt hours per day
*seems very high for led lights
*my led lights use 7.5 watts each.

garage door opener
125 watt hours per day

I get 1232 watt hours per day

How much energy you can harvest from your solar panels depends on where you live so I can't begin to guess.

I doubt a 3000 watt inverter is going to handle your compressor, not even sure it will handle your inductive loads.

You need to size the battery bank, bms, inverter and balance of system to cover the maximum instantaneous load.

thank you, as im waiting for this clamp meter ive been having a look about online and im not convinced im going to get an inverter that can handle these motors. The really big inverters seem to be for 48v systems. Potential costs are rocketing.

re: LED lights, yeah i thought that aswell, clamp meter worked out just under that, they are similar to the square tile versions you see in shops in the suspended ceilings but surface mounted, so quite large. i was surprised to see they had a small surge too.

im in the north west, UK. i think its generally accepted we get 2.5 hours a day? unsure, i worked it out off that. i reckon summer will be great but winter will be in power saving mode, but generally the car wont be driven in the depths of winter anyway, so the demand will be less so would equal itself out.

your watt hours came to less than mine i think due to me not working out on weekend use like i said i did lol oops. i appreciate you going through that. Thats good news, only if can figure out this surge/inverter problem.

 

[Ro0ke]

Hi again I’m back, I got a surge current clampmeter and it’s confirmed the motor surges up to 14A on startup before dropping down to 4A on 230/240v

So that gives me a 3.5kw surge
Which off a 24v battery bank is 140A? Am I correct in this?

Any help appreciated

 

[John Frum]

Hi again I’m back, I got a surge current clampmeter and it’s confirmed the motor surges up to 14A on startup before dropping down to 4A on 230/240v

So that gives me a 3.5kw surge
Which off a 24v battery bank is 140A? Am I correct in this?

Any help appreciated

3500 ac watts / .85 conversion factor / 24 volts low cutoff = 171.568627451 dc amps.
But this is surge current so its not a huge impact.
Typically we size the balance of the system for the continuous rating of the inverter.
What are the continuous and surge ratings for your inverter?

 

[Ro0ke]

3500 ac watts / .85 conversion factor / 24 volts low cutoff = 171.568627451 dc amps.
But this is surge current so its not a huge impact.
Typically we size the balance of the system for the continuous rating of the inverter.
What are the continuous and surge ratings for your inverter?

What’s the conversion factor if you don’t mind me asking?

I haven’t picked an inverter yet, thinking a 3kw with a 5kw surge should be plenty. I’ve seen an epever upower which should fit the bill but I’m always open to any better suggestions.

 

[Ro0ke]

With regards to the surge current, do the batteries have to be sized to suit the temporary discharge from batteries because of the surge? For example a 170ah battery at minimum to cope with the 1c discharge of a 170A surge

 

[SparkyJJO]

Usually the cells have a surge rating too, i.e. 1C continuous, 2C surge for 10 seconds or something like that.

 

[Ro0ke]

Usually the cells have a surge rating too, i.e. 1C continuous, 2C surge for 10 seconds or something like that.

Ah I never knew that, would I find that in the specs of the battery chemistry?

 

[John Frum]

What’s the conversion factor if you don’t mind me asking?

Its a guess at the efficency of the inverter conerting dc to ac.
It takes ~1175 watts dc to make ~1000 watts ac.
Higher quality inverters are generally more efficient.

I haven’t picked an inverter yet, thinking a 3kw with a 5kw surge should be plenty. I’ve seen an epever upower which should fit the bill but I’m always open to any better suggestions.

Most high frequency inverters have a 2x surge capacity ~1/30th of a second.
That is not long enough to cover the surge to get a motor spinning.
You will need the surge rating to be a few seconds.

 

[John Frum]

With regards to the surge current, do the batteries have to be sized to suit the temporary discharge from batteries because of the surge? For example a 170ah battery at minimum to cope with the 1c discharge of a 170A surge

Generally its the BMS that limits the continuous and surge current we can pull from a battery.
LFP cells should not be drawn harder than .5c continuous which is 85 amps for a 170ah battery.
The same cells can easily handle a surge of 1.0c which is 170 amps.
The bms may or may not accommodate the surge.

 

[Bluedog225]

That fan must be huge.

 

[labeeman]

I see no power for the inverter in your calculations inverters draw power all the time and will depend on the inverter.

 

[Ro0ke]

Its a guess at the efficency of the inverter conerting dc to ac.
It takes ~1175 watts dc to make ~1000 watts ac.
Higher quality inverters are generally more efficient.

Most high frequency inverters have a 2x surge capacity ~1/30th of a second.
That is not long enough to cover the surge to get a motor spinning.
You will need the surge rating to be a few seconds.

Oh course the efficiency makes sense.

so would my inverter have to have a continuous rating high enough to cover the surge of the motor? My inverters going be huge.

 

[Ro0ke]

Generally its the BMS that limits the continuous and surge current we can pull from a battery.
LFP cells should not be drawn harder than .5c continuous which is 85 amps for a 170ah battery.
The same cells can easily handle a surge of 1.0c which is 170 amps.
The bms may or may not accommodate the surge.

If I DIY the battery which is a possibility if I go lithium I could potentially have a look in the BMS settings. But I think for ease and more of a fit and forget, I might go acid.

Do we have any recommendations for an inverter?