Designing a high-efficient off-grid home

[dcjohn]

As a beginner and soon an owner of a house, I've spend last week watching Will's videos and generally reading about DIY solar power and electronics. Even my field (IT) is within the technology I didn't know a lot of things about electronics (and still don't). The main part I was missing is the huge amount of power loses/dissipation that we currently have. Those are mainly coming from

• Cables (Cheap/Undersized/Long/High-Resistance)
• Circuits (Cheap parts/Bad design)
• Power Supplies / Converters

I was really surprised when I realized that my PC's PSU has about 80% efficiency, meaning it wastes 20% (I guess that was the meaning of 80 bronze/gold etc. certifications coming from). And then I was thinking that's because of AC->DC converters (buck/step-down). And when start looking for off-grid realized an extra convert from DC->AC (inverter). My understanding for a general design of solar solution

Solar Panels (~20%) -> MPPT Charger (~90%) -> Battery Charge/Discharge (~95%) -> Inverter (~85%?) -> Power Supply (~80%) -> Load/Device

Note: Manufactures will usually claim max efficiency not average

If we exclude Solar Cells and cables that makes 0,9 * 0,95 * 0,85 * 0,8 = ~58% efficiency!

As an off-grid solar green energy dreamer I want to design a more efficient solution for my future home. Even if not possible to use it initially to be able to work in the future. All devices nowadays seem to (actually) work with DC instead of their AC inputs. Which means that they have an internal or an external (probably cheap/low efficient) converter. Ideally, we can use our DC source power directly (no inverter) to those devices without the need to convert from DC to AC and again AC to DC (at least for those with an external adaptor). The problem in this scenario is that each device uses different voltages. But we can still have a DC 48v bus and high efficiency buck converters to support all the main required voltages (such as 3.3, 5, 12, 19.5, 24 etc.). The advantage of this it would be having a single buck converter (for each common voltage) for all the devices and you just need to replace that with something new (e.g., GaN technology) to increase efficiency. USB-C would be another good solution however the device should support the communication protocol.

(What is not clear for me is the load that a DC buck converter can handle - e.g., ATX PSU 500W - and how you can make those converters to be able to handle high currents e.g., 3 PC power supplies x 500W)

Sorry for the long post and for my possible wrong calculations (forgive a beginner). Hopefully you will find it interesting too and contribute your thoughts!

Related: SMART DC/DC WALL PLUG DESIGN FOR THE DC HOUSE PROJECT

 

[BentleyJ]

Interesting that often times when researched, we find many products are not necessarily designed very well or are as efficient as they could be. I've explored the same line of thought as well. The answer to your pondering is in your own statement. "The problem in this scenario is that each device uses different voltages". While there are 12V or 24V or 48V Refrigerators, Water Pumps, Heat Pumps, Lights Etc. They tend to be specialty items that cost more are in shorter supply and may be difficult to repair.

This discussion has come up previously. Most of us on the forum have decided that all things considered its easier to simply add a few extra solar panels to compensate for inefficiencies and use just normal, brand name equipment that can be repaired or replaced with local stock.
Unless there is some specific limitation with regard to space and you must squeeze out every drop of efficiency, its not recommended to design a house with odd-ball equipment and a bunch of semi-DIY DCDC power supplies.
Unless of course you have the time and wish to do it as hobby.

 

[JohnTupper]

As a beginner and soon an owner of a house, I've spend last week watching Will's videos and generally reading about DIY solar power and electronics. Even my field (IT) is within the technology I didn't know a lot of things about electronics (and still don't). The main part I was missing is the huge amount of power loses/dissipation that we currently have. Those are mainly coming from

• Cables (Cheap/Undersized/Long/High-Resistance)
• Circuits (Cheap parts/Bad design)
• Power Supplies / Converters

I was really surprised when I realized that my PC's PSU has about 80% efficiency, meaning it wastes 20% (I guess that was the meaning of 80 bronze/gold etc. certifications coming from). And then I was thinking that's because of AC->DC converters (buck/step-down). And when start looking for off-grid realized an extra convert from DC->AC (inverter). My understanding for a general design of solar solution


Note: Manufactures will usually claim max efficiency not average

If we exclude Solar Cells and cables that makes 0,9 * 0,95 * 0,85 * 0,8 = ~58% efficiency!

As an off-grid solar green energy dreamer I want to design a more efficient solution for my future home. Even if not possible to use it initially to be able to work in the future. All devices nowadays seem to (actually) work with DC instead of their AC inputs. Which means that they have an internal or an external (probably cheap/low efficient) converter. Ideally, we can use our DC source power directly (no inverter) to those devices without the need to convert from DC to AC and again AC to DC (at least for those with an external adaptor). The problem in this scenario is that each device uses different voltages. But we can still have a DC 48v bus and high efficiency buck converters to support all the main required voltages (such as 3.3, 5, 12, 19.5, 24 etc.). The advantage of this it would be having a single buck converter (for each common voltage) for all the devices and you just need to replace that with something new (e.g., GaN technology) to increase efficiency. USB-C would be another good solution however the device should support the communication protocol.

(What is not clear for me is the load that a DC buck converter can handle - e.g., ATX PSU 500W - and how you can make those converters to be able to handle high currents e.g., 3 PC power supplies x 500W)

Sorry for the long post and for my possible wrong calculations (forgive a beginner). Hopefully you will find it interesting too and contribute your thoughts!

Related: SMART DC/DC WALL PLUG DESIGN FOR THE DC HOUSE PROJECT

Please excuse me if this isn't the most technical response (I work with heavy timber) but the most important variable when designing our off grid home in Tennessee was finding the right insulation (we went with 5.5" sips for wall system) and high efficiency windows and doors and of course insulating our gaps well. The home is oriented facing south on a south face at about 2300 feet above sea level and the placement and construction allowed us to spend less on batteries, panels and inverters as well as being efficient beyond the lifetime of the lithium and panels. I leaned on draftsmen at the homebuilder but the principles for passive house loosely dictated our approach. https://en.m.wikipedia.org/wiki/Passive_house

 

[dcjohn]

Interesting that often times when researched, we find many products are not necessarily designed very well or are as efficient as they could be. I've explored the same line of thought as well. The answer to your pondering is in your own statement. "The problem in this scenario is that each device uses different voltages". While there are 12V or 24V or 48V Refrigerators, Water Pumps, Heat Pumps, Lights Etc. They tend to be specialty items that cost more are in shorter supply and may be difficult to repair.

This discussion has come up previously. Most of us on the forum have decided that all things considered its easier to simply add a few extra solar panels to compensate for inefficiencies and use just normal, brand name equipment that can be repaired or replaced with local stock.
Unless there is some specific limitation with regard to space and you must squeeze out every drop of efficiency, its not recommended to design a house with odd-ball equipment and a bunch of semi-DIY DCDC power supplies.
Unless of course you have the time and wish to do it as hobby.

Well said, however companies should adapt to our needs with normal prices too. If they don't do it, we should not give them money for products that are not even doing what we want to do. For example giving the same product without the AC converter and make it more expensive it doesn't make sense at all. I agree that for the average person will need a company and a warranty to do this but I really thing going DIY and start electronics so I can do what I want freely. I'm an open source developer who hates capitalism and I will try to do the same with electronics and share my work that might be helpful for others

 

[efficientPV]

Nice you did the numbers, I don't think even lithium is that high in and out. This sure looks like a hobby to me, spending lots of money and not doing much useful with it. My friends race as a hobby. They read everything they can and go to great lengths for minimal improvement. Solar as a comparison would be showing up at the track with the engine skipping and blowing smoke. And when they made it around the track in two hours, they would be congratulating themselves. Easy is the dominant criteria.

 

[dcjohn]

Please excuse me if this isn't the most technical response (I work with heavy timber) but the most important variable when designing our off grid home in Tennessee was finding the right insulation (we went with 5.5" sips for wall system) and high efficiency windows and doors and of course insulating our gaps well. The home is oriented facing south on a south face at about 2300 feet above sea level and the placement and construction allowed us to spend less on batteries, panels and inverters as well as being efficient beyond the lifetime of the lithium and panels. I leaned on draftsmen at the homebuilder but the principles for passive house loosely dictated our approach. https://en.m.wikipedia.org/wiki/Passive_house

That's really interesting, I had to find time and check more details as I had no idea about passive houses. I will definitely look more in to it. Thanks!

 

[peakbagger]

While you are looking, take a look at the Pretty Good House Concept. I took a course on Passive versus other high efficiency house standards by someone heavilly involved in the Passive home business and came to the conclusion that certified Passive homes are way too rich for my budget with very poor payback. Pretty Good House is far more achievable and affordable design standard. The alternative is zero net energy homes, they are similar to Passive homes but allow active solar systems to offset somewhat higher building energy costs. Passive homes have to be truly net zero with no solar and with the solar resources in my area (44.4 degrees latitude) and heating and cooling curve, its a very expensive building especially one without a lot of square footage. IMHO, far better to build an efficient less costly home and install some PV panels to offset higher heating and cooling costs.

Passive homes do have the "bragging" factor as they can have a fancy plaque mounted on the door, but every component is expensive plus there are a lot of hidden costs associated with modeling, certifying and managing the construction. Yes it does potentially build a better house, but figure in 30% add to the cost. IMHO, unless someone is building a "forever" home unconcerned with resale, the upfront premium of a Passive versus a zero net energy home will not be paid back on resale.

 

[Leeds]

Rectifying power supplies can be very efficient but generally only in a tight design range. Rectifying power supplies that are meant for a wide range of loads are always a compromise of a design which bleeds energy at most power levels.

For this reason, things like the wall worts we all use to power USB cables are actually very efficient at converting from AC to 5vdc. They put out 2.1a or nothing at all, there's no wide range of power levels they need to accommodate. You can know that they are efficient by feeling for any warmth they generate- All of the inefficiencies show up as heat.

Running various voltages of DC all over your house is not going to save you any energy.

If you really want to save energy use independent inverters for each zone and turn them on and off as needed. That way they can each be sized for the required load and they themselves can be very efficient at making the dc->ac conversion. And of course when they are powered down and not in use they use 0 energy...

 

[50ShadesOfDirt]

Don't know if OP is buying an existing house or building one, and don't know if the goal is full off-grid or hybrid.

I'd pump more effort into design before anything else, and one of the concepts wrt loads is "reduce", as in, get rid of all energy-wasteful loads, and replace them with more efficient models. This may not be apparent until you run the numbers. Overpaneling is a key concept as well, and is easy enough to do. Could be many other "easy" design wins, vs having to do something to power existing loads, driven by members of the home who don't understand anything but being able to flip a switch (most of the public).

There are so many threads on this forum covering the design process, so do take advantage of them. Design theoretically for now, until you get closer to the real home. Perhaps even build a solar generator (on a handtruck, or similar project), and really get into the componentry. Might be a good launching pad for your exploration of dc efficiency efforts.

Hope this helps ...

 

[HalfBaked]

Please excuse me if this isn't the most technical response (I work with heavy timber) but the most important variable when designing our off grid home in Tennessee was finding the right insulation (we went with 5.5" sips for wall system) and high efficiency windows and doors and of course insulating our gaps well. The home is oriented facing south on a south face at about 2300 feet above sea level and the placement and construction allowed us to spend less on batteries, panels and inverters as well as being efficient beyond the lifetime of the lithium and panels. I leaned on draftsmen at the homebuilder but the principles for passive house loosely dictated our approach. https://en.m.wikipedia.org/wiki/Passive_house

This is my story as well, TN too, but west siiide. I built the house for solar, or rather for low inputs compared to a standard build. I was aware of passive house principles, but didn't have the money for that, so I did the best I could with available materials. I built into a south slope. Dug down about 4' in the back, the front is about a foot above ground level, all 8' windows (free). No sun can come in during summer because of 3' overhang. Posts, beams and roof built from whole logs from my property. Back wall is 18" thick limestone gravel compacted inside of polypropylene woven bags shaped like a dam to withstand the 16 truckloads of dirt I backfilled with. It is essentially underground, wild grasses on roof and invisible except for solar panels. Tons of thermal mass and semi underground means it resists changes in temperature very well. It still gets a little warmer than I would like in the summer due to me not wanting to overload the roof, soil is only about 4" average depth on roof. 1400ish sqft. The math says it can hold a lot more, but I sleep better this way for now. Plan to add about 6" of woodchips in the fall.
Pic from last year, turkey on roof.

 

[JohnTupper]

This is my story as well, TN too, but west siiide. I built the house for solar, or rather for low inputs compared to a standard build. I was aware of passive house principles, but didn't have the money for that, so I did the best I could with available materials. I built into a south slope. Dug down about 4' in the back, the front is about a foot above ground level, all 8' windows (free). No sun can come in during summer because of 3' overhang. Posts, beams and roof built from whole logs from my property. Back wall is 18" thick limestone gravel compacted inside of polypropylene woven bags shaped like a dam to withstand the 16 truckloads of dirt I backfilled with. It is essentially underground, wild grasses on roof and invisible except for solar panels. Tons of thermal mass and semi underground means it resists changes in temperature very well. It still gets a little warmer than I would like in the summer due to me not wanting to overload the roof, soil is only about 4" average depth on roof. 1400ish sqft. The math says it can hold a lot more, but I sleep better this way for now. Plan to add about 6" of woodchips in the fall.
Pic from last year, turkey on roof.

That's awesome, I always liked soil roofing conceptually but my timber framing mind just can't get over how moisture damage could be difficult to track down and address quickly

 

[HalfBaked]

That's awesome, I always liked soil roofing conceptually but my timber framing mind just can't get over how moisture damage could be difficult to track down and address quickly

The footings are a 3x3' French drain with sloped bottom and 4" perforated pipe then filled with tamped gravel. The roof membrane is 30mil industrial strength reinforced and a single piece covers the roof and entire hill beyond. Before I put on the soil, I covered the entire membrane with about 6" of gravel to allow drainage and keep burrowing critters at bay. No leaks so far. The only maintenance is pulling baby trees out once a year.
I knew nothing about building with timber and even less about using whole logs and not much on the web about it. It's been quite the learning experience, but nothing as challenging as solar!

 

[JohnTupper]

I had the opposite experience, solar was plug & play (except qc problems) and the grading and plumbing were where I ran into challenges

 

[HalfBaked]

I had the opposite experience, solar was plug & play (except qc problems) and the grading and plumbing were where I ran into challenges

Fantastic! My learning curve continues but I can feel it flattening. I can say everything has been working great for the last 2 days, maybe I'm in the clear but I know better than to celebrate just yet.
Grading for me was the worst as my biggest piece of equipment is a shovel. Plumbing was easy, I'm all one level, so ¼" drop per foot drainage. Black water goes to a homemade vermicomposting septic tank via flush toilets. Gray water plumbing goes off the other side of the ridge into a forested swamp. Incoming is all pex, super simple.

 

[dcjohn]

Rectifying power supplies can be very efficient but generally only in a tight design range. Rectifying power supplies that are meant for a wide range of loads are always a compromise of a design which bleeds energy at most power levels.

For this reason, things like the wall worts we all use to power USB cables are actually very efficient at converting from AC to 5vdc. They put out 2.1a or nothing at all, there's no wide range of power levels they need to accommodate. You can know that they are efficient by feeling for any warmth they generate- All of the inefficiencies show up as heat.

Running various voltages of DC all over your house is not going to save you any energy.

If you really want to save energy use independent inverters for each zone and turn them on and off as needed. That way they can each be sized for the required load and they themselves can be very efficient at making the dc->ac conversion. And of course when they are powered down and not in use they use 0 energy...

Yes, but generally the more conversations (I think that DC->DC can be more efficient though) that you will perform the more power you lose. I was looking into HVDC and Class 4, so might be better to run 100-380v (batteries) DC in to your house with safety and converters at the end to avoid cable loses/costs. One other thing (from what I've seen so far) is that, the more current you have the more efficiency you can get from the converters. Of course knowing the input/ output voltage can make them even more efficient. I also look for PoE, USB and the KNX bus (24-30V) which might be useful also for more general purpose including IoT (power+data).

 

[efficientPV]

"One other thing (from what I've seen so far) is that, the more current you have the more efficiency you can get from the converters."

Don't know where that observation would come from. I've never seen that in my experience. Voltage is king.

 

[Delmar]

My preference is to have a 12V system for direct running of small loads (lights, fans, USB ports) and a separate 48V system for only the AC inverter. Each have dedicated panels and batteries for redundancy. I have already converted most of my office to DC.

In the deep south our vast majority of power demand is air conditioning. Very interested in the EG4 mini-split that will directly receive DC from solar without the need for a battery. Perfect solution: the stronger the sun, the more cooling required, the more efficient the unit.

For water heating I installed an electric HPWH that uses a fraction of the power of a conventional resistance unit, and has a bonus of free air conditioning. A very welcome addition to my hot garage regardless of energy savings.

 

[Honuz]

Hi all, i'm fully off grid for 2 years now, here are my advices:
- Use 48V system if you need power (5kW), that's my case, i use tools for woodworking and most tools are 2.5kW. Even a small kettle is 1.5kW, that would already be 110A+ of a 12V battery....! 24V systems is kind useless in the middle solution imo.
- Use multiple inverters and good quality ones, i got a victron phoenix 800VA for 24h24 loads, 4G box, weather station, fridge (low 10W draw without load) and a bigger one a victron 5kVA on a different AC circuit (high loads circuit) which is working in eco mode, it pulse the circuit every 5s searching for a draw, if it find one it start and provide power, this mode eat 5w (expect a maximum 5s delay for an appliance to start). You could use just one multiplus ii in standard mode 20W draw with no load but my system is more secure in case of a failing inverter.
Tv, hifi are also on the 24h24 circuit, i advise you to use class D amplifier for their power efficiency and pretty good audio quality, i got Fosi amplifiers.
- Use low power devices, we use slow cook devices, they draw from 80 to 320W and cook for 4-8 hours, 98% of the time it use direct solar power (i got a 4kWc array) even in winter.
- Use mini Pc like NUC or whatever USFF, i got an INTEL NUC running 24h24, it draw 7W average.
- If like me you are a PC nerd, then have a tower PC (mine is Ryzen 5700X, 6700XT) and can draw 400W, i use it when the weather is good enough, the weather decide when i can use it and frankly this is not a problem at all.