Going from 12 volt to 24 volt system

[kittysmitty]

My next question. I currently have a 12 volt 800 watt Victron Energy PSW Inverter. What are your thoughts on a step down DC to DC converter such as one of these, instead of buying a new 24 volt inverter. Thanks

http://www.samlexamerica.com/products/ProductDetail.aspx?pid=72

http://www.victronenergy.com/upload/documents/Datasheet-Orion-DC-DC-converters-EN.pdf

Thanks

[madlabs]

At those power levels I don't see any real benefit going to 24V. What are your goals in the switch? If the system is running and meeting your needs, leave it. I wouldn't switch voltages until you want a new inverter. And then, I wouldn't go to 24V. Most new stuff is going 48V and the 24V stuff will left behind, kinda like the ZIP drive. Remember those?

I personally started with a 12V system, then when I outgrew it and wanted to upgrade went 48V. Mind you, I stareted with a 2.5kW inverter and then went to a dual stack 8.8kV system, very different scenario.

Jonathan

[southpaw]

That inverter has a maximum power draw in excess of 150 amps which far exceeds the capabilities of that converter.

[kitestrings]

We similarly upgraded form 12V to 48V several years ago.  I agree with Jonathan, I think I'd stay with 12V until you have, or need, a different inverter.  If you do end up needing 12V after the switch, I'd suggest you look at Solar Converters equalizers.  We've had great luck with them.  You can easily do a 12V "tap" on a 48V (or 24V) bank, than equalize with one of these.  We still have a lot of 12V lights and small electronic gadgets.

http://www.solarconverters.com/index.php/products/82-battery-equalizer

~ks

[Mary B]

I switched to 24 volts and will never go back. Smaller wiring needs so less expensive copper needed. Better charge efficiency from my charge controller also. The Samlex SDC-60 would be marginal on that inverter, assuming 90% inverter efficiency then you would need 65 amps peak at 13.6 volts. If your load on the inverter is under 600 watts you can probably get away with it but running anything near its maximum ratings shortens the life of that device

[kittysmitty]

First, let me thank everyone for their comments. I find this site, compared to others, offers more advise and less criticism. My setup is off grid and a one weekend a month visit. This time of the year, hunting season, is every weekend for 6-7 weeks. I have a setup that most would say, will not work, but it has been working for 5 years. 10 (ten) 12 volt AGM batteries  in parallel. 200 watts total solar. This past weekend, 3 days in total of 4000 watt hours used.
Next, I'm a scrounger.  I pay full price for very little. If you have the time and connections, you can find almost anything. My batteries, Inverter, charge controller were  free. I had to pay for the panels, close to $4 a watt at the time.
My system has been working fine, but, times are changing. I now use more power after we go to sleep. Up to 5 CPAP machines.
So, maybe I will stick with 12 volts and just add more solar. Cheap 30 volt panels and a MTTP controller, or 12 volt panels and a 12 volt controller? And a good backup generator.

[ChrisOlson]

I'll chime in and offer that if you don't see your power needs increasing any time soon, just stick with the 12V system.  There's LOTS of off-grid installations here in the Wisconsin North Woods and just about every part-time hunting cabin or weekend lodge is using 12V systems.  There's quite a few full-time, year 'round off-gridders up here as well, and just about every one is using a 24V system.  Only a couple I know of that are 48V, and we're one of those.

It would be absolutely senseless to go from 12V to 48V unless your total system capacity needs to exceed 1.5 kW on 12V or 4.0 kW on 24V.

[kittysmitty]

Chris, when you state
 "total system capacity needs to exceed 1.5 kW on 12V or 4.0 kW on 24V"
Do you mean the total daily total use, or the total draw on the system at any one time?
Thanks

[ChrisOlson]

Basically inverter size.  If you can keep the amps to the inverter below 200 at nominal voltage and 88% inverter efficiency you will not exceed the capability of 4/0 cable to the inverter.  1.5 - 2.0 kW is fine on a 12V system and be able to use the inverter at full capacity.  4 kW is fine on 24V.  Above that you should go to 48V.

The batteries don't really care because a given bank of batteries, whether they are wired 12, 24 or 48V all have to deliver the same amps at the same load.  It's just that with 12V you have more parallel connections with 12V batteries, and parallel connections are a little harder to keep balanced.  But it's pretty easy (although not cheap) to build a big 12V system with two 1156ah 6V batteries, or 6 big 1150ah 2V cells too.

Looking at total daily usage, say you have a big 12V system with a 2 kW inverter and your average load is 600 watts.  In a 24  hour period you will use 14 kWh with the only limitations being that your total load on the system cannot exceed 2 kW continuous.  And you would be operating your inverter at 30% rated load, which is good.  Inverters don't reach their peak conversion efficiency until they get to at least 15% rated load.  Below 15% rated load on an off-grid inverter you are simply wasting your stored energy in your battery in losses in the inverter.

 For off-grid homes, 14 kWh/day is a big system and it is obvious that the 12V setup will handle it just fine.  The reason we use 48V systems is because our average loads run around 1,500 watts during peak evening and morning times, 500-600 watts the rest of the time, and peak loads exceed 6 kW.  Our 48V inverter will deliver close to 8 kW for 10 minutes, 7.2 kW for 30 minutes, and 6 kW continuous @ 91% conversion efficiency.  Conversion efficiency at peak evening and morning loads is 96-97%, and around 94% at 500-600 watt load.  With a 12V system this would require almost 800 amps to the inverter(s) @ < 85% full load inverter efficiency, which is not practical.

Basically you want to size your inverter so it runs at 10-15% rated load or better most of the time for peak conversion efficiency from DC to AC power.  Putting in a 6 kW 48V system, then running it at 250 watt average load, is just plain nuts because your inverter efficiency is down below 75% once you get below 10% rated continuous load (with a quality inverter).

[ChrisOlson]

As an addition to above, and unable to edit a post here, I am making a new post.  This is a chart that shows inverting efficiency of XW-series inverters (only as an example of how to size an inverter for your system to get peak efficiency):



As you can see, these inverters require ~25% rated continuous load to achieve the peak of its efficiency curve.  Much below 500W rated load and their efficiency very rapidly drops off to less than full-load efficiency.  This is why it is important for an off-grid installation to NOT buy an inverter that is too big and then run it at <15% rated load, average.  You are wasting a good portion of your power you produce from renewable sources.  It is much better to choose a smaller unit, operate it at > 15-25% rated continuous load and use generator support for intermittent peak loads that exceed the inverter's capacity.

[kittysmitty]

The only reasons I was thinking about going to 24 volts was, 1. the less parallel batteries for 24 vs 12, and 2. the lower cost of panels. My inverter is sized just fine, so I think I will just add more panels, and in the long run, the higher cost of 12 volt panels should be offset with not having to replace my inverter, charge controller, etc. Thanks again.

[DamonHD]

It's the number of batteries (and their capacities) that determines how much energy is stored in them, not whether they are wired serial or parallel (providing you do it right), so that shouldn't be a reason to change.

Rgds

Damon

[ChrisOlson]

It's the number of batteries (and their capacities) that determines how much energy is stored in them, not whether they are wired serial or parallel (providing you do it right), so that shouldn't be a reason to change.

While I agree with the battery thing, kittysmitty did provide one huge incentive to go to 24V vs 12V.  Those "grid-tie" 30Vmp solar panels work GREAT on 24V off-grid systems without MPPT controller.  If you run out of capacity with your controller and want to put in more solar you can hook those 30Vmp panels direct and use them for "boost" during bulk charging, then they taper off at AbsorbV and let the solar controller finish absorbing.

Being able to use those vs the so-called 12V (~17 Vmp) panels is a pretty significant cost savings because those 30Vmp panels are so cheap compared to the ~17Vmp ones.  And they pretty much automatically regulate their voltage at Absorb V without having to use a controller on them.  This is from experience when we had our 24V system with stacked 4 kVA inverters.  I used those 30Vmp "grid-tie" panels hooked direct when I didn't have enough controller capacity for them, and they worked pretty sweet.

[David HK]

Many years ago someone wrote the following on this web site. The subject was voltage -


"The more volts you've got, the easier it is to play with the electricity."


Dave in HK

[DamonHD]

(Hey, welcome back!)

[OperaHouse]

I went from 12, to 24, to36V and next year it will be 48V panels.  This is tough when you start out with a lot of mismatched panels from different sources.  As I bought more I made the decision to buy additional in sets that would make 48V.  The electronics is a lot easier to build and wire requirements are a big savings.