Electrical calculations

[Mat7783]

So I crunched numbers tonight for fridge, microwave,coffee maker, lites, exhaust fan,water pumps, etc based on average use during 24 hr period and came up 167.2AH load. This including a 80% efficiency to be safe . My stove , water heater , and space heater will be on propane. Will a 200AH battery bank suffice or should I go with 300AH to add as a buffer. What are your thoughts. Thanks again.Also using 3KW inverter for fridge, microwave, coffee maker, and a few minor outlets.

[Ronnie]

Generally speaking you do not want to use more then half your batteries capacity. So double the 167ah figure. Then that only gets you one day. Not sure how you plan to charge your batteries, but if by solar then you would be assuming all sunny days.

[matthews2001]

look at thread:

Voltage drop using inverter (Multi-page thread 1 2)
matthews2001 (couldnt figure out how to post link to thread)


maximum inverter size really looks like 1000 watts, bigger one will decrease battery life

[Mat7783]

I plan to use inverter generator with 8,3amp 12v outlet or RV site power thru generator fir recharging of batteries.

[Mat7783]

What about the startup wattage and amperage of a microwave and coffee maker and hair dryer. Wouldn’t any combination of these appliances run over 1kWatts?

[matthews2001]

I think thats the point. Microwaves and hair dryers are really too many watts to use from normal rv/bus 12v setups for battery life and draw. This assumes 4 6 volt t-105 trojan deep cycle batts, about $800, a typical setup.

It will "work" but at great cost to batteries

[TheHubbardBus]

Originally Posted by Mat7783

So I crunched numbers tonight for fridge, microwave,coffee maker, lites, exhaust fan,water pumps, etc based on average use during 24 hr period and came up 167.2AH load. This including a 80% efficiency to be safe . My stove , water heater , and space heater will be on propane. Will a 200AH battery bank suffice or should I go with 300AH to add as a buffer. What are your thoughts. Thanks again.Also using 3KW inverter for fridge, microwave, coffee maker, and a few minor outlets.

First off, battery chemistry matters. For deep cycle LA chemistries (including FLA & AGM) you have ~50% usable capacity. And cycle life will improve the less of that you use. So best case (or worst case, for your batteries), 200AH is 100AH. With Lithium, you've got ~80%.


Also, you have to consider how much you're pulling at any one time. The AH rating is usually based on a C/20 discharge rate. So for 200AH, that's 10A continuous. Any higher discharge & you're going to see less in actual capacity. Also, high discharge rates - particularly w/ LA - will lead to shorter battery life.


I'm assuming this is a solar system. If so, you'll have to consider sizing the panels so that you'll be able to restore the AH used during the course of the day. You'll also need to determine how many days of no-sun autonomy you're comfortable with. For instance, you've got one day planned for above (before considering the other factors mentioned). If it's a cloudy day, you may not get all that back. If it's a rainy day you may not get any back. So in one day, you could be done. If you're ok w/ that (generator power, etc), then no problemo. Otherwise, your AH capacity needs to double for each day you need storage for.


Finally, don't forget to add in the amount of juice your inverter is using, both in the process of inverting, as well as in standby. These numbers, particularly the former, can be considerable. Your inverter will basically be sized by your anticipated peak loads... the max amps you plan on pulling at any one time. Best bet is to try to tailor your inverter size so it meets this amount, with little excess, as the larger the inverter, the more power it draws.


EDIT. Just saw the bit about planning to use a generator. Sorry. Feel free to discount all the solar stuff then ;) Everything else applies however.

[Mat7783]

Thanks for all the info.

[john61ct]

A 3000W or 5000W inverter is not the issue, so long as the bank and daily energy inputs support them properly.

Obviously costs and weighs a lot more than one stressed by 1000W.

And if the gennie is supporting running the big loads, then even the small bank will be fine.

Make up a detailed energy budget, figure out your Ah per day needs, where the inputs for 115% of that are coming from, and design the system to support those numbers.

[JDSquared]

Originally Posted by TheHubbardBus

First off, battery chemistry matters. For deep cycle LA chemistries (including FLA & AGM) you have ~50% usable capacity. And cycle life will improve the less of that you use. So best case (or worst case, for your batteries), 200AH is 100AH. With Lithium, you've got ~80%.


Also, you have to consider how much you're pulling at any one time. The AH rating is usually based on a C/20 discharge rate. So for 200AH, that's 10A continuous. Any higher discharge & you're going to see less in actual capacity. Also, high discharge rates - particularly w/ LA - will lead to shorter battery life.


I'm assuming this is a solar system. If so, you'll have to consider sizing the panels so that you'll be able to restore the AH used during the course of the day. You'll also need to determine how many days of no-sun autonomy you're comfortable with. For instance, you've got one day planned for above (before considering the other factors mentioned). If it's a cloudy day, you may not get all that back. If it's a rainy day you may not get any back. So in one day, you could be done. If you're ok w/ that (generator power, etc), then no problemo. Otherwise, your AH capacity needs to double for each day you need storage for.


Finally, don't forget to add in the amount of juice your inverter is using, both in the process of inverting, as well as in standby. These numbers, particularly the former, can be considerable. Your inverter will basically be sized by your anticipated peak loads... the max amps you plan on pulling at any one time. Best bet is to try to tailor your inverter size so it meets this amount, with little excess, as the larger the inverter, the more power it draws.


EDIT. Just saw the bit about planning to use a generator. Sorry. Feel free to discount all the solar stuff then ;) Everything else applies however.

Not OP but it helps me. I want to get as dependent on solar as I can. And even if I read it 15 times today, if I could be the 16th time it finally makes sense in my head LOL. So thanks for typing it out.

[TheHubbardBus]

Originally Posted by JDSquared

Not OP but it helps me. I want to get as dependent on solar as I can. And even if I read it 15 times today, if I could be the 16th time it finally makes sense in my head LOL. So thanks for typing it out.

I am extremely pleased you found it helpful

[bubb, the real one]

Is that number in ac amps or dc amps,

[mitchk]

Originally Posted by matthews2001

look at thread:

Voltage drop using inverter (Multi-page thread 1 2)
matthews2001 (couldnt figure out how to post link to thread)


maximum inverter size really looks like 1000 watts, bigger one will decrease battery life

So I'm trying to figure out this whole inverter thing and I feel like I keep seeing different opinions. I realize that the process of inverting to 120V is (I believe) around 80 percent efficient. But you are saying that a bigger inverter is less efficient so I should go with as small of an inverter as possible? Originally I was planning on going with a slightly bigger inverter then what I needed just to make sure it would cover everything I needed.

[john61ct]

Good efficiency comes from servicing a load that is close to the inverter's actual maximum continuous capacity.

So a 2000W inverter powering a 150W load will be very inefficient.

The other factor is only turning the inverter on when it is needed.

https://sprinter-source.com/forum/sh...d.php?p=842667

[mitchk]

Originally Posted by john61ct

Good efficiency comes from servicing a load that is close to the inverter's actual maximum continuous capacity.

So a 2000W inverter powering a 150W load will be very inefficient.

The other factor is only turning the inverter on when it is needed.

https://sprinter-source.com/forum/sh...d.php?p=842667

This helps a lot. Thanks! I will probably try to go with a 1000W inverter. If I need to go a little bit bigger it probably isn't a big deal as long as I turn off the inverter whenever I am not using it.

[Doktari]

I’ll be going with a combination medium sized 2500 watt inverter charger and a small 600 watt inverter. The big inverter is 92% efficient but will be in Search mode most of the time.

[bubb, the real one]

167 amps calculated draw, is this AC amps or DC amps

[Doktari]

that 167 amps would have to be DC. AC would be 20,040 watts at 120 VAC. 167 amps at 12 volts about 2338 watts at 14 volts.

[Mat7783]

That’s 167ah load on DC 12V. Battery. Will use inverter to power 10cu’ fridge and 700w microwave 5 minutes daily, hair dryer on low setting, coffee maker in morning etc. fridge is only constant draw of ac voltage. Everything else is 12VDC such as lighting, fans, water pumps etc. size of inverter is the puzzle and battery bank size??

[JDOnTheGo]

Originally Posted by Mat7783

...size of inverter is the puzzle and battery bank size??

I think you are asking your question again. If that is not the case, ignore the following, I'm mostly just repeating what has already been said.

1. Assume your daily power consumption estimate is low and/or batteries are less than 'brand new' - round up to 200Ah/day. (plus it makes the math easier )

2. Assuming lead-acid batteries, you need a 400Ah bank to provide 200Ah/day (more complicated than that but good enough for now). How many days do you want to be able to operate this setup from battery bank alone? Three?? You need a 1200Ah bank (to provide 600Ah over three days).

3. Determine your maximum AC power requirement (everything running at the same time) being aware of start up current and pick an inverter to provide that.

Sadly, there are very few easy answers - there are many "it depends." All of the above can be picked apart and reasons found to make it inaccurate.

... and, include a 'better' battery monitor in your system so you can take reasonable care of that battery bank.