DC off grid solar system

Hey all!

I am trying to educate myself about electricity and solar in particular since I am aiming at installing my own off grid solar system eventualy.
I have read in a few places about the ineffectiveness of inverting DC into AC and since it would be completely off grid I am thinking about using my power as DC.

Is that possible? How would that work? How would I know which machines need to be converted and do I need to convert the Volt as well?
If anybody here doe it this way I would love to get some details about it.
Thanks

pjeter schornstein wrote:...the ineffectiveness of inverting DC into AC...

This statement does not make much sense to me. Do you actually mean "inefficiencies"? You will loose maybe 10-15% of your power converting from DC to AC. That is perfectly normal and just an unavoidable fact of life.

Or instead, are you referring to the issue that "modified sine wave" inverters struggle to run some appliances properly, and you need to use "true sine wave" instead?

pjeter schornstein wrote:How would I know which machines need to be converted and do I need to convert the Volt as well?

So, instead of converting DC to AC, you want to convert all your AC appliances to DC? Am I understanding that correctly? That sounds completely impractical (much more work than just using an inverter) and also very dangerous if you don't know excatly what you're doing.

In my town there are four homes that use Photovoltaic power. One of these homes is grid-tied net-metering, and three homes are off-grid. One home is all 12vdc [it works just fine].

The 12vdc home has been on solar power since the 1980s, and it is the simplest and cheapest system I have ever seen.

Hey Matt, yea that's what I meant and 15% sounds like much. From what I understand a lot if not most instruments are using DC input anyway and are using a convertor.. is that not correct ?
Thanks Galen, in which sense is it cheeper and simpler?

Galen Young wrote:One home is all 12vdc [it works just fine].

That's pretty cool. Would like to learn more.

It really depends on what you want to run.

It's certainly possible to have everything run on DC, but often the cost of DC powered devices is more than simply buying more solar panels and an inverter.  DC powered refrigerators are a good example.   A relatively small sunfrost refigerator costs around $3,400, you can buy an equivalent energy star rate fridge, inverter, a couple extra panels, etc. for less than that.

Anything that requires a lot of power would also not be a good candidate for DC.  For the same power level, low voltage DC requires more amps than high voltage AC.  For example 1200 watts =120V AC at 10 Amps, or 12V DC at 100 amps.  Higher current needs large wires.  Wire rated to handle 10 amps runs about $0.10 per foot, wire rate to handle 100 amps runs about $2 a foot.

Once upon a time off gridders would run all dc and either not have an inverter or use it infrequently due to inefficiencies and such. That is going back to the days of solar being in the 6$ per watt range. Also that was before inspections on off grid houses. It is possible to get a dc only house code compliant but it's not easy and usually not worth the hassle in this day and age. Some problems I have run into:
DC current arcs so by code in a house all cabling should be armored or in metal conduit. All standard switches are made for ac so you will have to source dc rated recepticle switches which I have not found. You can derate an ac switch but it's 90 percent (15 amp switch is now 1.5 amps) all plugs are ac rated only so again you need to source a dc plug... if it's a cabin or a trailer you can get away with it but if it's a full sized house that will be mortgaged or permitted you will have problems. Up to you if they are worth overcoming. I still run some dc loads and have rejigged my greenhouse for 12 volts only but we never go that route for clients.
Cheers,  David Baillie

From "Once upon a time" land ;-)

We've been off grid for about 27 years and run most of our home using 12v or 24v appliances and as we built our house ourselves, we installed heavy wiring to accommodate the amperage, with all the distribution circuits protected by fuses. I used heavy duty UK plugs and sockets throughout the house.

I don't know where you are and what the laws are but here in France there are (So far) no "norms" or controls applied to DC off grid homes. You just have to learn as much as you need to do the job securely yourself and hope nobody takes any notice of you. We have no mortgage.

We have about 1400 watts of assorted and some quite old solar panels. We run several solar battery systems and use/direct the energy depending on how we are using the (large) house.

Using DC, we run :

A mixture of LED and compact fluo lighting - about 36 light sources
24v chargers for 'phones and tablets
3, 4, (often more), laptop computers using 12v adaptors
24v 120 litre Gram low energy fridge
12V/24V compression fridge/freezer
12v Internet livebox
12v pump for solar water heating system

Using a Victron 24v 800 watt inverter, we can also regularly use :  

450 watt vacuum cleaner
450 watt twin tub washing machine (Hot and cold water fill)
Shearing equipment
2 music systems (Not big boomboom - but they do the job)
Chargers for all our hand tools, computers etc.

All our space heating, cooking, indoor drying and water heating is done using wood or solar with a bit of gas for cooking in the summer.

We run a PDC in late autumn here when it's chilly and there's not a lot of sun; we have around 30 people for about 15 days. The house runs like a "normal" house and people have to be reminded constantly that we're off grid and they need to charge their equipment during the day.

We watch what we're doing with heavy loads (No washing machine, no vacuum cleaning) around the winter solstice but that suits me just fine. :-)

If you have an efficient inverter sized appropriately for your most common loading, the conversion loss is more like 8-10%. For a kilowatt hour, that would be 100 watt hours or about a 20th (edit; 20 minutes) of an hour of full sun on a 300w panel... if it is not on 24/7.

The standby load is usually larger than the difference in efficiency. That needs to be addressed within the system.

Dc power to all long duration, moderate current loads that are far removed from the batteries is no problem. For hard to get, expensive or otherwise impractical loads, ac power is convienient.

Dc lighting, computing/entertainment/communications, low load water pumping (sometimes high volume and pressure... distance and voltage) and like the post above illustrates, most everything if not all loads can be dc, it just takes a little more effort at integration.

This monitor/tv/display screen is available 32" at 16.5 watts lit and .5 watt on standby.

https://www.semiconductorstore.com/blog/2014/devices-that-can-be-powered-with-poe/657

If you look around, there are great and common appliances which translate well to off grid even with an inverter.

And this is a great way to power stuff with dc power and have some control an monitoring to boot, if you are into that. Witing couldnt be easier. 24v and 48v makes distance easier. Network switching can save power automatically when not in use.

https://www.google.com/amp/s/www.versatek.com/blog/why-power-over-ethernet-will-power-the-smart-building-of-the-future/amp/

https://www.google.com/amp/s/planetechusa.com/blog/poe-lighting-how-poe-is-revolutionizing-led-lights-in-smart-homes-and-offices/amp/

Almost forgot. Backwoods solar electric and Altestore.com have great tutorials.

These are my favorite sources from my start as an installer. There was not much out there and im sure there are some more modern sources than New england, but the basics of electrical and solar are comlplete and valid.

Solar energy international has arguably the best curriculum and has been one of the greatest educators worldwide for about 15 years now.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.newenglandsolar.com/pdf/Books.pdf&ved=2ahUKEwj4zqqZyuTaAhXwSt8KHSa6AncQFjADegQIABAB&usg=AOvVaw0Mo2anKmgwtGGNxaGHqwrM

https://www.solarenergy.org/shop/publication/photovoltaics-design-installation-manual-previous-edition/