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Old 09-25-2018, 06:53 AM   #1
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To invert or not too? that is the problem

Hey everyone! Long time lurker first-time poster here.

We've been diligently trying to design our energy system over the last few months and seem to have hit a bit of a decision point, we're not sure which way is best and are looking for your opinions on which option to choose . Any help would be much appreciated.

We have managed (we think) to get a reasonable estimate of our power needs (see attached pdf), with a peak consumption of about 10400 watts, we're aiming for a 48v 500AH battery bank made up of 24 2v 500AH solar batteries, connected in series, with just over 1100 watts of solar panels. (let us know if any of these numbers seem out of place).

We plan on full timing in our bus without relying on access to hookups. We would like to build a good quality system without throwing the checkbook at it unecessarily.

Our problem lies in deciding on invertors, chargers or convertors?
-Is it worth buying 48v appliances for more significant drain items?
-is one big expensive inverter/charger all we really need?
-is it better to have a small inverter for small loads and a bigger one for the peak?
We have distilled our choices down to three options.
Option1
Buy 48v LED lights, fridge/freezer, 4g wireless modem, Airconditioner.
48v lights are easy to find, the next two are always on, and the Airconditioner is a high drain appliance.
Then run a medium size inverter/charger to handle the general AC appliances.

Pros:
less invertor loss ( 5% drop in total daily needs )

Cons:
expensive appliances (fridge)
Complexity (two big circuits 48v + 240)
Cost (thicker cabling 48v circut)

Option2
48v LED lights, everything else through one big expensive inverter/charger using house appliances for quality/price ratio.

Pros:
One circut
Cost (Cheap household appliances)
Cost (thinner wiring)

Cons:
Cost (expensive inverter/charger)
Bigger invertor running all the time (less efficient at lower loads)

Option3
Two invertors, one smaller one for the fridge/freezer, lights and modem. Then a second bigger inverter to handle the rest of the AC load when needed. (not sure where the charger fits into this, maybe the bigger inverter is also a charger?)

Pros:
Small inverter handles min load
The larger inverter can be turned off frequently
Cost (thinner wiring)

Cons:
Cost (two invertors)
not using any 48v

It seems like our three options all have their pros and cons and we are not sure which one would suit our needs best? Also any advice on invertor sizing or brands for the above options would be much appreciated
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File Type: pdf power-consumption.pdf (28.2 KB, 37 views)
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Old 09-25-2018, 09:54 AM   #2
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My preference for off-grid use is all-DC as much as possible.

12V is default, worth the heavier wiring to save money on standard fittings. Can convert up if needed.

Focus on reducing consumption, not using appliances designed for mains grid power. Heating devices use propane much as possible.

So "only" connection when plugging in to shore power is charging the battery bank.

For appliances not available in DC, each gets its own right-sized inverter, switches on / off with the load device.

Exception is aircon, not run off the bank at all, genny must rum when that is needed.
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Old 09-25-2018, 10:34 AM   #3
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Why not 12V/48V lights and two or more inverters. I already have two small inverters myself, the 1500 watt peak runs my kitchen blender for smoothies with the proper cable (heavy) attaching it to the battery. I bought a home-kitchen stove-hood (Coleman propane stove) with a 120V-AC fan, and I will turn on the inverter as I need it, but I would replace it with a 12V-DC fan if I could get the right size to fit.

With LED lights becoming more available, and since they consume so little energy, they are the way to go, in my bus at least. They are DC by design. I tore apart the STOP sign on the side of my bus, and pulled out the LED circuit boards that lit it from inside. With a decorative cloth to cover (since they spell out STOP), they will mount on the wall or ceiling.

Two or three years ago I heard a story on NPR news about a company in North Carolina (S.E. USA for you Kiwis) that invented a new light source that could be made into wall-paper like sheets, used very little power like an LED, was dimmable I think, and was cheap and easy to make. It offered the promise of diffused lighting without shadows.
It was going to be made and sold to private companies for the first few years of production to offset the cost of development, then made available to the public. Before I spend too much on any LEDs, I am going to look into covering my ceiling with this stuff. They say cheap enough to cover your walls, also, but my walls are windows and cabinets....
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Old 09-25-2018, 11:07 AM   #4
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in my system, I have 880AH of AGM batteries, up to 2KW of solar (realistically 1200 to 1600 watts), a 12V battery system 3kW inverter (9kW peak). My fridge is 12V, cooktop is 120V induction (no propane)... hot water is currently solar/ engine / 120V electric. microwave and TV are 120V. I too plan on being able to be off grid..

Advantage of the 12V marine fridges is efficiency (they only draw about 70W when running).

The Induction cooktop will draw up to 2 KW but it's extremely efficient at heating things... not much wasted energy.

I have gone with a number of components from the marine world.. they seem to be better built and more capable.

I agree with the point of using larger wire and 12V.. much easier to deal with.
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Old 09-25-2018, 11:22 AM   #5
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10kW seems like a HUGE peak consumption. I would echo what's been said above, look into reducing usage first and foremost. Off-grid energy 101 is basically that, reduce consumption as much as possible. That said, the higher quality inverters have very low parasitic draw, 25-50 watts for the larger 4kW units from Magnum and Xantrex. If you put all your lights and other assorted electronics on DC, the inverter can spend the majority of it's time in idle mode and draw even less.

Going propane with any heating applications, such as cooktop, water heater, etc will help tremendously towards this goal.

What's your plan for Air Conditioning? AC is a huge load, the modern inverter driven split systems can be easily powered by a 3+kW inverter with good surge capacity, but they'll pull amp hours out of your battery bank very quickly.

Your battery bank, 24ea 2v 500AH cells...is going to be very expensive, very heavy, or both. Not sure if you had a specific product picked out already, but here's one: https://www.wholesalesolar.com/99601...0ah-2v-battery

Cost is $535 each, and they weigh 76 pounds, so you're looking at $12,480 and 1,824 pounds for your bank, before making up any connections at all.

That much battery gives you 24kWH of total power, of which you can actually use 12kWH assuming a 50% discharge. If your AC uses 1kW running (8.3 amps at 120V) then it will use all of your available power in 12 hours. I saw a post about a 48v powered air conditioner that was supposed to be pretty efficient, don't know the exact numbers on that but worth looking into.
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Old 09-25-2018, 11:55 AM   #6
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Quote:
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Air Conditioning? AC is a huge load, the modern inverter driven split systems can be easily powered by a 3+kW inverter with good surge capacity, but they'll pull amp hours out of your battery bank very quickly.
Understatement.

Trying to run any aircon from batteries is just going to give a little time-shifting of genny run-time. It will not actually reduce that runtime much,

no matter the size of the bank.

Many gennies are more quiet than most aircon units, so IMO pretty pointless to make that huge investment in consumables, for what benefit?
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Old 09-25-2018, 11:58 AM   #7
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Agreed. And I still want to see the list that adds up to 10+kW peak load.
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Old 09-25-2018, 12:41 PM   #8
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Look at the PDF.

Hey OP, there is no such thing as watts per day, makes no sense

Watts (and amps) are a flow rate, measured instantaneously.

For "per time" have to use watt-hours or AH.
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Old 09-25-2018, 02:37 PM   #9
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Ah, totally missed that

OP, excellent that you're doing your homework on this, but as John said you need to fix your methodology. Your list looks like nameplate numbers for each item rather than actual measurements. spend the $20 and get yourself a Kill-a-Watt, it will tell you the actual amps, wat, and much more importantly kilowatt hours for each of your appliances. For example, instead of just putting 500 watts for the washing machine, plug it in and run a couple of Cycles and measure the actual value, and then divide that by the number of Cycles you intend to run per day.

The standard number you need to develop for each load is kilowatt-hours, not amps or amp hours or Watts or any other. Once you have kilowatt hours, you can then go about sizing your battery Bank and inverter based on expected usage.
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Old 09-25-2018, 02:52 PM   #10
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Way too much batteries, too little solar power. FLA batteries should ideally be charged at 5% to 13% of their 20-hour rate. Without running your numbers, I'd be surprised if you even reach 5%, and anything less than that will result in deficit charging and an early death of your expensive battery bank. You really need to rethink things, a lot! FYI, I have 2kW of PV to charge about 850aH of golfcart batteries for a 12V system, and that's at about the top end of the recommended charging rate to ensure I can get a good charge into them even on a winter day.

Spend some time (a lot of time) reading what the solar experts on the NAWS forum have to say, and also read Handybob's thoughts on the subject.

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Old 09-26-2018, 03:37 AM   #11
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Quote:
Originally Posted by john61ct View Post
My preference for off-grid use is all-DC as much as possible.

12V is the default, worth the heavier wiring to save money on standard fittings. Can convert up if needed.

Because we are running a 48v battery bank we would have to convert the voltage down to 12v (most likely) anyway and arent convertors only 90% efficient? As well as inverters.



Quote:
Originally Posted by john61ct View Post
Focus on reducing consumption, not using appliances designed for mains grid power. Heating devices use propane much as possible.

So "only" connection when plugging into shore power is charging the battery bank.

One thing we probably should have said is that we're going to try and rent the bus out once it's built, so we have to cater for people who are "less efficient" so these needs aren't ours but what someone may use given the opportunity.



Quote:
Originally Posted by john61ct View Post
For appliances not available in DC, each gets its right-sized inverter, switches on / off with the load device.

Exception is aircon, not run off the bank at all, genny must rum when that is needed.

Thanks that does make sense, what sized genny would you recommend?
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Old 09-26-2018, 04:01 AM   #12
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Ah, totally missed that

OP, excellent that you're doing your homework on this, but as John said you need to fix your methodology. Your list looks like nameplate numbers for each item rather than actual measurements. spend the $20 and get yourself a Kill-a-Watt, it will tell you the actual amps, wat, and much more importantly kilowatt hours for each of your appliances.

Thanks for the advice, we actually went out and got a meter today. Our problem is, we dont own lots of the items we want yet. How do we get a better idea of what they will drain?
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Old 09-26-2018, 04:25 AM   #13
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Way too much batteries, too little solar power. FLA batteries should ideally be charged at 5% to 13% of their 20-hour rate. Without running your numbers, I'd be surprised if you even reach 5%, and anything less than that will result in deficit charging and an early death of your expensive battery bank. You really need to rethink things, a lot! FYI, I have 2kW of PV to charge about 850aH of golfcart batteries for a 12V system, and that's at about the top end of the recommended charging rate to ensure I can get a good charge into them even on a winter day.



I ran the numbers were at 4.7% so i guess more solar is in order, i also saw on some solar guides/forums that you have to multiply your peak daily consumption by a factor, and also only account for the brightest sunlight in a day. that changes the values quite a bit.. see attached revised sheet.


would 3kW be enough of a solar setup?
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File Type: pdf power-consumption1.pdf (27.9 KB, 4 views)
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Old 09-26-2018, 06:04 AM   #14
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10kW seems like a HUGE peak consumption. I would echo what's been said above, look into reducing usage first and foremost. Off-grid energy 101 is basically that, reduce consumption as much as possible.

//see above

One thing we probably should have said is that we're going to try and rent the bus out once it's built, so we have to cater for people who are "less efficient" so these needs aren't ours but what someone may use given the opportunity.


Quote:
Originally Posted by cycle61 View Post
That said, the higher quality inverters have very low parasitic draw, 25-50 watts for the larger 4kW units from Magnum and Xantrex. If you put all your lights and other assorted electronics on DC, the inverter can spend the majority of it's time in idle mode and draw even less.

Thanks for this info, i am starting to feel like this is the way to go (plus way more solar)



Quote:
Originally Posted by cycle61 View Post
Going propane with any heating applications, such as cooktop, water heater, etc will help tremendously towards this goal.

We plan on using propane for hot water, heating and cooking



Quote:
Originally Posted by cycle61 View Post
What's your plan for Air Conditioning? AC is a huge load, the modern inverter driven split systems can be easily powered by a 3+kW inverter with good surge capacity, but they'll pull amp hours out of your battery bank very quickly.

Your battery bank, 24ea 2v 500AH cells...is going to be very expensive, very heavy, or both.

The plan was to only really have it there incase its sweltering heat, or we're in a bug infested region (half of the south island of New Zealand) and cannot open the windows. We were hoping just just rely on an oversized battery bank and solar array, but maybe a small genny would be nice to run when needed.



Expensive? yes, but not that bad, there are plenty of solar batteries around for $330 AUD, heavy? also yes, ill have to fab up a big tray and weld it someplace (under the bus) securely.
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Old 09-26-2018, 06:05 AM   #15
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Many gennies are more quiet than most aircon units, so IMO pretty pointless to make that huge investment in consumables, for what benefit?

what sized genny would you reccomend? just enough to run the AC? or bigger for "wiggle room"?
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Old 09-26-2018, 06:42 AM   #16
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Too many variables, including bank chemistry, how big and well insulated the space is, ambient temps etc.

Each setup requires a system design approach
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Old 09-26-2018, 06:44 AM   #17
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One thing we probably should have said is that we're going to try and rent the bus out once it's built, so we have to cater for people who are "less efficient" so these needs aren't ours but what someone may use given the opportunity.
Well no way I would trust others that much with quality battery banks.

A genny based system would be easier and safer.
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Old 09-26-2018, 06:48 AM   #18
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Thanks for the advice, we actually went out and got a meter today. Our problem is, we dont own lots of the items we want yet. How do we get a better idea of what they will drain?
Buy load devices with efficiency in mind. Before sizing the energy system.

Then make sure to measure AH per 24 hrs in DC as

Quote:
Originally Posted by littlekiwibus View Post
Thanks for the advice, we actually went out and got a meter today. Our problem is, we dont own lots of the items we want yet. How do we get a better idea of what they will drain?
Buy load devices with efficiency in mind. Before sizing the energy system.

Then for inverter powered appliances, make sure to measure AH per 24 hrs in DC as drawn from the bank, not just AC.
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Old 09-26-2018, 06:51 AM   #19
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Because we are running a 48v battery bank we would have to convert the voltage down to 12v (most likely) anyway and arent convertors only 90% efficient? As well as inverters.
Don't trust the best-case overly optimistic brochure numbers.

Measure the power consumed both sides and compare yourself.

I've seen many inverter setups wasting close to half.
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Old 09-26-2018, 06:51 AM   #20
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Why are you using 48V?
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