12v vs 120v

Telco · February 2008

I've done a lot of reading on this site, and see that it's recommended to go with 12V stuff wherever possible to run stuff directly off the batteries rather than running them through an inverter to get 120V due to inverter losses.

Let's assume that a person wants to go from on grid to off grid, without worrying about whether it's cost effective to do so. Let's also assume that said person has already maximized his conservation efforts.

How much more power, in rough percentages, would this person expect to need if he chose to run everything at 120V instead of 12V, over a similar setup that combines 12V where possible (say, lighting, refrigeration, ect) and 120V when not possible? I understand that it costs power to rectify 12V to 120V, but how much wastage are we really talking here? Is it the difference of running a couple of extra solar panels, or something major? I don't expect an exact answer as there's no way that an accurate answer can be provided to a currently theoretical setup, but perhaps if someone here who is running a dual voltage setup might be able to post what he has, and what he estimates he'd need to go all 120V, it would be very helpful to get an idea for my own planning. I'd like to just go all 120V when I build, but if the cost and complexity difference is going to be huge then I might go ahead and go with a dual voltage setup. I'd really like to nail all this down before the building begins, as rewiring after the fact can really suck.

Thanks in advance for any info on this.

mike95490 · February 2008

Re: 12v vs 120v

The inverter process, from 12/24/48 vdc to 120VAC , as a general rule of thumb, is about 15% loss. So if you needed 500watts of lighting, it would require 575w from your battery. The extra power is radiated away by the heat sinks in the inverter.

Inverters are rated in efficiency, and average 15% loss. They all have a "sweet spot" where they can reach 95%, but that's generally only for a given load, say about 80% of their full load. At night, after you shut off your radio, the inverter is still running, in idle, till your fridge kicks in, or the furnace lights at 6AM. They are getting better in both efficiency, and idle modes.

I may be wrong, and they are now in the 10% loss, instead of 15%.

BB. · February 2008

Re: 12v vs 120v

A typical inverter is around 80-85% efficient (except at very low power levels--less than 5-10% of capacity--where fixed operational losses start to dominate). Those fixed losses for a "typical" 1-2 kW inverter seems to be around 8 watts or so. Usually, you will see something in the specs about power consumption with no load (and a lower power "search" function).

I, personally, for a "house sized" off-grid system would be perfectly OK with going 120 VAC. 12 vdc are good for small systems (camping trailers, cabins, etc.). But for larger systems, it is a pain in the rear to run 12 volts everywhere (plus needing very large gauge copper wiring to run the 12v power vs 120v). Also, for larger systems, you are going to probably need 24-48 volt for the inverters (to keep their wiring reasonably sized). And it now becomes a pain to get 12 volts from your system (down converter--losses, second 12 volt only system, etc.).

My own concern--you ask if "cost effective" is not an issue--is that the overall efficiency of a Grid Tied system is around 95% and the efficiency of a off-grid system (with flooded cell batteries) is around 60% (inverter and battery charging losses).

So, combining an off-grid's system ability of only storing power for a few days (versus grid tied net metering of one month to one year of "storage" capabilities), with its lower efficiency... An off grid system would need to be, roughly almost twice as large as a grid tied system, plus you would need to generate alternate power some other way and manage fuel issues for those periods of low sun/high(er) power needs.

Having off-grid power available with solar PV for backup is neat for natural disasters, but is usually worth the $5 per month minimum charge (at least for my electric utility) to have grid tied solar vs the issues of running my own off-grid system for my suburban home.

I know the question was what if cost was not an issue--but, in the end, cost and size are almost always part of the equation. And cost for Grid Tied power (for my area near SF California) is around $0.25 per kWhr, and every time I run the basic numbers for an off-grid system, I come up with a minimum of $1.00 per kWhr. And add that a grid tied system is eligible for rebates (in some states, and a federal tax credit), and (I believe) a pure off-grid system is not.

In the end, a hybrid system (off grid capable grid tied system) where utility power was reasonably available would be my choice for a "cost is no object" system. I have the advantage of utility power for "storing" 1 year of power (summer over capacity carried into winter with little sun) and the UPS nature of off-grid system, with minimum battery losses, and wear & tear (since the batteries are only used when there is a grid failure). And the emergency generator could be natural gas or propane powered (if available) would hardly ever have to be run.

You can size the off-grid capacity to either power your minimum emergency loads (few lights, TV, central heat), or you could size it to power your normal loads (A/C, full lighting, washing, cooking, etc.).

-Bill

crewzer · February 2008

Re: 12v vs 120v

Here are links to useful technical information on efficiency specs and conditions as well as tare losses for several true sine wave inverters:

Exeltech: http://www.exeltech.com/xpspecs.htm
Morningstar: http://www.morningstarcorp.com/products/SureSine/info/SI_DataSheet.pdf
OutBack: http://www.outbackpower.com/pdfs_spec/FX_Domestic.pdf
OutBack (CEC Test Results): http://www.consumerenergycenter.org/erprebate/inverter_tests/summaries/Outback GVFX 3648.pdf
Xantrex (CEC Test Results): http://www.consumerenergycenter.org/erprebate/inverter_tests/summaries/Xantrex%20XW6048.pdf

HTH,
Jim / crewzer

nigtomdaw · February 2008

Re: 12v vs 120v

Hi Your theoretical post doesn't indicate when you start to build if your property has grid facility or not. If you have grid then it seems that the efficiency of grid tied over off grid is a no brainer as per BB.

If your new build is off grid from scratch again BB's previous post would lend to using 120 ac as the easiest way to go. ac wiring and sockets and hardware will be far cheaper that DC counterparts. Equivilant 12v Dc equipment over 120 ac can show a severe price premium ie fridges freezers and limited choice. if you already own 12 Dc equipment then thats a personal factor to consider. Some goods just are not available in DC or if they are are hard to find and source.

USA may be different to Europe but I would struggle to find DC stuff like Cooker hood/extractors, Home Compressor, Small cement mixer , power washer, food processor, Laptop, or Computer, Vacumn Cleaner, Microwave, which all run off my 3kw inverter also most small equipment like Modems , Wireless Internet, Phones, Cordless Power Tools , all use AC chargers or wall warts sourcing individual Dc supplies would be difficult and costly, Most power tools like bench saws pillar drills and angle grinders are far better sourced in AC. I have just about every Dewalt 18v XRP tool you could own and there not cheap, but when it comes to stuff like angle grinders and SDS drills and a long hard days work a head for them then AC is the way forward. Today I used my cordless Dewalt 4.5inch angle grinder to cut some fencing 30 minutes use soaked up 3 batteries. I should have pluged the mains unit in on an extension lead. So at some point in your off grid home you will need a genny or an inverter to provide that AC power, so why not design it in from the start and probably save money in the long run.

PS I love my Dewalt XRP gear but when you push them hard ie drilling or cutting you do go through the batteries. But I wont be with out them for convienence and flexability. I own two hammer drills, SDS drill, circular saw,demolition saw, angle grinder,jig saw and 3 torches, all 18 v XRP but also have Dewalt Angle Grinder and SDs in240v AC plus a lot of other AC power tools which double up the XRP stuff.

But I digress, I thought long and hard over 12 volt only... 12volt circuit and AC Mains circuits and AC only. I choose AC only and 18 months in I am sure the right choice was made. We are a family of three with a Large American Fridge Freezer 2 Large LCD TVs internet, auto washing machine and all the other on grid luxuries you would expect. A new washing machine AC would cost $300 a new AC microwave $75 a small Garage Compressor $150 a new Vacumn cleaner $70 all with plently of style and choice, how much would they cost in Dc and what choice would you have. I use AC 240 v LED and flourescent lighting and dont have to worry about excessive equalizing voltages damaging them, my only reccomendation is buy an inverter charger with PFC charger as per the new Xantrex XW or Magnum series and cut a lot of genny compatability out of the equation but thats another topic. !

Telco · February 2008

Re: 12v vs 120v

Good deal, thanks folks. This answered my question. From what you are saying, it'll be:

1. All 120V, with a few extra panels and batteries
2. Half 120V half 12V, meaning a few extra panels and batteries, plus two electrical networks in the house with expensive, thick wiring on half of it, plus the added time and expense to find the 12V appliances to begin with.
3. All 12V, which would require all hefty wiring, almost impossible to find appliances that mostly work crappily, no compatibility with anyone else that might come to visit with 120V gadgets, ect.

I think I'll just go for the simple path of all 120V for my project. It'll be a lot simpler all the way around, plus will have the added advantage of being able to sell the house to anyone, not just another solar nut. If necessary, all that would need to happen is the line would need to be run and a meter installed.

Crewzer, thanks for the links. I'll check them out next week during the slow times at work.

On whether grid power is available, most likely it will be since I'll be building within 25 miles of the Tulsa area. But, the whole point (to me) of going with solar will be to avoid dealing with the power company at all. I'm going to be building on acreage, and last I heard it can cost 10-20 grand or more to run wiring from the pole to the house if the power company does it, if you need it run, say, a quarter mile.

The way I see it, I'm going to pay either way, I can either pay them to run the wire and continue to pay them a monthly charge, or I can invest the money in making my own, and not have that 100 dollar a month charge every month forever. Considering the gear can last 20 years or longer and the batteries will last at least 5 years if not longer, that'll mean 6 grand more MINIMUM (if power prices stay the way they are now) in the bank paying me interest when it's time to replace the batteries. And, I'll not have to worry about some jackwad two miles down the road not trimming his tree in the fall, causing an ice storm to pull the line down and take my power out like what happened this winter. The neighborhood I live in is all underground wiring, but we are in the outer suburbs mingling with the farmers and ranchers who all have, or rather had, trees growing along the utility right of way. When the trees collapsed with the ice, my power went with them.

With any luck, I'll be able to achieve my goal of a zero payout house, where the house doesn't require me to make any monthly payments. 8)

12v vs 120v

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