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Amit Baum


You're awesome!



If your laptop has a native ethernet port you might also reduce power use by using wired ethernet instead of wifi when possible.



A similar experiment on another great blog: http://robrhinehart.com/?p=1331

Jim L


I see you mention wiring up panels, batteries, new bulbs, etc. I have worked with Ham radio in the U.S. for a number of years. They commonly use Anderson Power Pole connectors to make sure all of your gear can connect to any other person's gear.

I have found it very useful to put these connectors on all my portable solar panels, radios, battery chargers, inverters, etc. because it allows me to mix and match as necessary. The connectors are inexpensive and quickly eliminate wiring problems as you re-configure equipment.

(I do not work for any company associated with these devices, I just think they're cool in how they work.)

John Weber


The whole picture needs to be included not just the installed devices. I am not a supporter of fossil fuels or nuclear. I am concerned about continuing business as usual and its devastation of the earth and humanities future.

Solar and wind energy collecting devices and their auxiliary equipment have an industrial history. They are an extension of the fossil fuel supply system and the global industrial infrastructure. It is important to understand the industrial infrastructure and the environmental results for the components of the solar energy collecting devices so we don’t designate them with false labels such as green, renewable or sustainable.

This is a challenge to ‘business as usual’. If we teach people that these solar devices are the future of energy without teaching the whole system, we mislead, misinform and create false hopes and beliefs. They are not made with magic wands.


These videos are primarily concerning solar energy collecting devices. These videos and charts are provided by the various industries themselves. I have posted both charts and videos for the solar cells, modules, aluminum from ore, aluminum from recycling, aluminum extrusion, inverters, batteries and copper. Please note each piece of machinery you see in each of the videos has its own industrial interconnection and history.



Very nice blog post. About your problem with the external monitor: it is possible to rewire it and it should have 12V DC internally, but you'll lose your warranty. You could also sell your old Monitor and by one with an external 12V power supply. This might take you off-grid but it is not really better for the environment since there will be more CO² polluted during production then you save.

If you want to reduce the power usage of your laptop you can lower the screen brightness since the screen uses the most power if you're just writing. You could also buy an USB-keyboard, a USB OTG-Y-cable and a smartphone and write on that but you'll have of course a lot smaller screen and spend at least additional 150-250€ (depending on your phone).

kris de decker


@ Paul & Jim L

Great advice, thanks !

@ Jonatan

Interesting link indeed. But eliminating the need for a kitchen by switching to astronaut food is not going to work in the north of Spain, where enjoying tasty food is almost a religion.

@ John

I totally agree. This is one of the reasons why I try to achieve my goals with as few solar panels as possible. See the last paragraph of the article.

@ Mattis

I tried lowering the screen brightness but I did not achieve the reduction in energy use that I expected. I guess it depends on the laptop. Mine has a rather small screen but high performance.

Steve A.


I admire the tenacity you show with this project, however the size/efficiency of your solar panels seem exceptionally weak. If you were to upsize your PV panel (reducing wiring complication and cost), you could pull all of this off with a single panel and have ton of overhead.

On the top of my sailboat, I stitched in a flexible, 2.2 kg Solbian solar panel (125 watt). They even make a model with pre-attached zippers, making it easy to remove from cloth. For your window size, their square shaped 144watt panel seems to be perfect. The nice thing about thin film is that they weigh next to nothing, are flexible, and they aren't as sensitive to being perfectly angled towards the sun.

If you were to bump up your output, and use a sealed car battery, then you'd have no problem with your loads and on cloudy days. With my arrangement on the boat, I can run a small DC refrigerator, the ship's navigation equipment, lighting, autopilot and radios for days on end (although I power a huge 8D battery).

kris de decker



Indeed I could make the system more efficient by reducing wiring complication and cable length. I'm still reshuffling things in the office, which is why most of the cables are longer as they should be.

One larger battery instead of several smaller ones connected in parallel would also be more efficient, but this wasn't a deliberate choice. I started with a very small system aimed at powering an art work, and then gradually extended it into a home power system.

The higher efficiency of your system is probably also due to the fact that you're on a sailboat. My panels are shaded by the building itself in morning, while you can harvest solar energy all day.

The Solbian solar panel is interesting. Attaching it to the window of the office would ruin my view, but it could work in the bedroom. I can imagine it's also a solution for people who don't have the handy window sill that I have, or for those who have no window sills or balconies at all. Too bad it's four times more expensive than my solar panels.

betsy teutsch


Love all the specifics you give. I think this would be very useful info for Global South solar initiatives as they expand the appliances people can power off of small solar systems. There are lots of social businesses like http://www.azuri-technologies.com/ or Simpaworks in India. Also www.sunsaluter.com has a low-tech tracker for large, self-contained solar panels to rotate from east to west on a daily basis. Wonder if there's a way to incorporate that idea in your innovations? I have many of these tools in my book www.100under100.org - exploring low tech/high impact tools for the developing world.



Ha ha wow that is similar to my own efforts from 6 years ago. Like you I bought a laptop as they are way way more power savvy to desktops. I ran a dc bus 24V in my case as the adaptors I could get were rated 12-24Vdc (for trucks) and it halves the dc grid amperage. Light bulbs were swapped out to floor based fixtures but there LED's were prohibitively expensive at the time I used CFL's with a 24V electronic choke. Many of them burned out due to a dc bias on the ac. I later went to 24V LED's. I got the smallest bar refrigerator which takes only 90w when running and added external insulation layer - dense foam packing with a reflective film. I put a large bottle of water in it's freezer as a cold storage battery so that it did not run in the night at all to not tax my batteries. I bought 1x 24V 100w panel and added a second panel within a month of experimenting. I cook with a metho camping stove because I like using it. I used to feed excess power into the grid until I got into trouble by the utility. I hope to implement solar hot water this year so I can cut my gas utility off entirely. And a really tall rain water tank ( I have a courtyard) to see if I can live off rain (it's a massive roof). It's a wonderful way to live - in touch with the elements. Best of luck to you.



BTW I got a front load washing machine as they are way more power efficient. Mine only takes small bursts of 50w as it turns the drum over and over. Spin is 300w but only for 1 or 2 and also they use way way less water. Get one with an external hot and cold connection that way if you ever get solar hot water you can have warm wash (much more cleaning action for same or less detergent) without using inbuilt electric heater - tough on batteries/inverters.

Best wishes for your adventures.



Its impressive how you could get along with those quite small panels.

LED lighting usualy have built in resistors to limit the current. However if you buy the power Leds by yourself you can connect them in series and use a switched current constant source. The light emmited from LEDS depends on the current and voltage/current curve for LED is non Linear.

Use an SSD instead of a harddisk for your laptop. I also use powerpoles and for cabling loadspeaker cabling seems to be easiest. I mostly use 4 mm^2. You can buy the cheapest brand as only cross section matters.
A MPPT (Maximum point power tracking) could improve the yield of the solar panels too.



fyi, you are going to want to wire your battery bank differently. currently it will draw most from the first battery, if you unplug one wire and connect it the other way, it will draw more evenly across all the batteries.



Good ideas. My only suggestion would be to spend more time on the details. The construction methods and engineering of the mounts for the panels looks dangerous. How would you like to be walking below on a windy day and have that panel fall on your head? Attaching an aluminum frame to the building would look more professional and be safer.



Nice post!

One thing I would recommend is to check the voltage output with a scope to see how clean it is. For some devices cutting off the adapter and hooking it up to a noisy 10~14v power source might not make them last very long.
You also need to take the voltage drop of the cabling into account. Use a voltage drop calculator to see if you are within acceptable range.

One solution would be to up the power to something like 15v, and create various inlets in the wall. In each inlet you add a little circuit based on a 12v regulator and a 100uf capacitor to provide a clean power source.

You can now plug any 12v DC apparatus directly into these DC outlets



Your 12v lights could be easily powered by a small AC to 12vDC power adapter for on grid use/backup.

Hilton Dier


Interesting post. A few thoughts:

You yourself argued in an article you reference that a bicycle powered generator doesn't make back its embodied energy for something like a century. We are feeble creatures indeed. However, a modern solar panel will make back its embodied energy in a year or so. A bicycle generator might make you feel virtuous but sitting back and letting the sun do the work is actually more environmental.

Proceeding from that, a larger solar array will keep your batteries charged up even on semi-cloudy days. This will help them last longer. A lead acid battery below 50% state of charge is a dying battery. Likewise, depth of discharge (DOD) determines cycle life, with cycle life increasing exponentially with shallower DOD. A bigger solar array will not only pay back its own energy; it will also reduce the embodied energy expense over time of replacing the battery bank.

That said, a larger battery bank will actually end up with less embodied energy per delivered watt-hour because it will experience shallower DOD. See: exponential relationship above.

Short version: scrimping on hardware doesn't save you money or embodied energy.

Well done on the watt-hour scrimping, though.

Hilton Dier


Another thought, for future reference, on batteries:

It is better to have one large 12V battery than to parallel multiple small ones. Another option is to series wire two 6V batteries into a 12V bank.

The reason is that in parallel different currents can travel through different batteries. No two batteries are made exactly the same. One might be a bit weaker, or might be at a slightly different temperature. That one will charge up faster and get overcharged or discharge faster than the others and get over discharged. It will lose a little capacity, which will increase the overcharge/discharge. This will snowball until the originally slightly weak battery will be getting flogged on every cycle. It will die and drag down the pack.

I have seen this in the field a number of times.



Another thing you might consider is adding a small wind turbine. If you have access to the corner of the building and you are a few floors up there is often quite high wind currents there and even if your turbine only produces 20w that is 20w over 12 hours it will take care of some of the night time loads and top up your batts. The other thing is when the sun is not shining often the wind is blowing so you can get a little bit of power trickling in on those overcast days and nights. It can just round out the equation for you. Maybe something like this.... https://www.kickstarter.com/projects/rmrdtech/a-small-wind-turbine-for-a-big-difference/posts/1482182



And there is nothing wrong with mounting a way bigger panels below the windows against the brickwork. Make a triangular frame that hangs below the window but extends up and bends in to fix to the brickwork or the window frame. Just make sure you use appropriate fasteners as wind can apply quite strong forces ad over time loosen inadequate fasteners like nails. It would be best to affix the mounts to the brickwork if you get frequent strong winds. The benefits are that your window is totally unobstructed and you get plenty more power and daylight.

kris de decker


@ Jack

We had several storms since November 2015 and none of the panels has moved an inch. They may not look very professional but they won't fly away.

@ E

Thanks, but of course I'm not going to buy one of those because I prefer to adjust lighting to available energy supply. And it would be an unnecessary cost as the plan is to get the whole apartment off the grid.

@ Hilton

The problem with the bicycle generator is the battery, so the same problem exists for the off-grid solar installation. My previous article on bike generators did not take into account the fact that generating your own electricity may lower your energy use considerably. Consequently, bicycle generators, and even small windturbines, could be a sustainable choice if they encourage you to use much less energy.

Be careful with claims like "a modern solar panel will make back its embodied energy in a year or so". This is highly dependent on where the solar panels are manufactured and where they are installed: http://www.lowtechmagazine.com/2015/04/how-sustainable-is-pv-solar-power.html

Concerning the batteries: I stop using energy from the moment battery voltage drops below 12V. So I already oversized the battery pack in order to make them last longer. And indeed, when the batteries have to be replaced, I will switch to a large battery, and probably increase storage capacity a bit. At the moment, too much solar energy is wasted on sunny days.

@ Mirosan

I was thinking about a wind turbine but they are quite expensive and wind conditions are rather poor where I live. We have a strong sea breeze in afternoon but that's when the solar panels give their maximum output, too.

Mounting bigger panels below the windows against the brickwork would make the solar installation too visible and probably illegal -- it would involve changing the facade of the building which is often not allowed. And I want to keep the installation simple and mobile.

Matt Ritter


The sage of low technology powering a residence of electric devices? I think it's a shame. Here's my advice:

1. Ditch the computer monitor. I work 40+ hours a week on a laptop. Increase the screen resolution. If your eyes still need assistance, get reading glasses if you don't have them.

2. Ditch the electric lights. Follow the natural cycle as much as possible; wake when the sun comes up and go to bed when the sun goes down. During winter, use homemade lamps or candles from natural/reclaimed waxes or oils.

3. Ditch the clothes washing machine. Wash your clothes by hand. That will teach you to conserve and wash them only when needed. Underwear can be washed in the sink in 2 minutes - not much longer than it takes to wash your hands and face. Living in Spain, you probably don't need to wear socks most of the year. Wear sandals. Keep your wardrobe simple and easy to wash by hand. Make washing clothes part of your daily shower routine. If you can wash dishes by hand, you can wash clothes by hand.

4. As you've said, replace the electric stove with a solar cooker.

5. Replace the electric refrigerator with one of the lower-tech alternatives. Use a large, well-designed, heavily-insulated container in a dark corner of the residence. Eat a plant-based diet if you don't already, Shop for fresh groceries twice a week.

6. Ditch the laser printer. Join the paperless world. I use my printer only a few times a year. I could live without mine.



If you are looking for more solar capacity, you may be interested in the Voltaic Systems 17W Solar Panel as it offers various mounting options (including magnets) but more importantly it has a high energy density for the size : http://solar-panels.nz/products/17-watt-solar-panel

Michael Murphy


I think you will enjoy perusing this website run by a physicist who reports on personal efforts at sustainability: http://physics.ucsd.edu/do-the-math/

Jürg Steffen


Don't forget - the spanish gov is thinking of putting a charge on solar systems! Nice idea, next will be a charge on breathing.

Jason Olshefsky


I'll also plug the Anderson PowerPole connectors for 12V. Most radio folks seem to use this configuration: http://www.westmountainradio.com/kb_view_topic.php?id=ST166 ... where, when the wires are toward you and the plug-in device is away, and the "flat" part of the connector (not the interlocking part) is up, the red (+) is on the right. Making all your connectors the same way works great because the mating connectors are rotated 180° and always fit.

Second note—a bit of a nitpick, I know—the way you have the batteries wired is non-optimal. If you parallel batteries, hooking the positive connection to one battery and the negative connection to the battery at the opposite end of the chain ensures every battery has the same resistance to output. That is, the first battery has 0 hops to (+) and 2 hops to (-) = 2 total, the middle one is 1 hop to (+) and 1 to (-) = 2 total, and the last has 2 hops to (-) and 0 to (+) = 2 total. The way you have it wired now, the first battery has 0 total hops and the last has 2+2 = 4. It might not seem significant, but even small fractions of a volt make a difference when 0.5V represents 50% state of charge. After an inordinate amount of searching (why, Google, why are you so broken?) I found this article that explained the math to me: http://www.smartgauge.co.uk/batt_con.html

kris de decker


@ Matt

Why should we use candles instead of LEDs but not a typewriter instead of a computer? The "paperless world" that you are fond of isn't as low-tech as it seems. There's lots of coal burning going on behind the scenes. http://www.lowtechmagazine.com/2015/10/can-the-internet-run-on-renewable-energy.html

I don't want to go backwards. I investigate the possibility of adjusting modern technology to a sustainable level of resource consumption.

Great tip about the screen resolution, I will try that out.


Magnets may be handy, but my panels are six times cheaper.

@ Jason

Thanks for the details. Rewiring the batteries is on my to-do list.

kris de decker


@ Jürg

I don't think the Spanish "solar tax" will last forever. It's very much a project of the political party that is now in power. The former government did exactly the opposite. They will figure out a middle way, eventually.

The issue is discussed at hackernews (+ more comments on this article): https://news.ycombinator.com/item?id=11720584

Dominic Jefferies


Thank you so much for this excellent article, Kris. I read it in detail a few weeks ago and came back now to check exactly which solar charger you used. However, I can't seem to find any details of it. Could you specify the manufacturer and model of the charge controller? Apologies if I've overlooked anything...

kris de decker



You're welcome. The solar charge controller is a Velleman SOL10UC




If you wanted to get by without batteries, would it be possible to install diverters of some kind that would divert a certain portion of the electric current suitable for each appliance that you want to run? Any residual power could be used to run a heater or some other device that could operate at variable power inputs. I'm just thinking of how to cut down on the embodied energy of the batteries. Anyway, the question is essentially, could you do this without batteries?



I think the winter-time 400Wh is possible with the small panel constraint.
You can gain ~30% by switching to a MPPT charge controller, as it will use the higher voltage of the cold panels in winter.
As you said, switching to lithium batteries will save in roundtrip efficiency and better panels can provide a 50% production increase. Your panels have about the worst aperture efficiency I have ever seen :)
Perhaps some second hand e-bike batteries can provide you with 1 kWh of storage without breaking the bank?
A hot-fill washing machine is well in reach in that case (but not really low-tech).



Hey there great post.

I have good south orientation on my balcony, and i was thinking of conducting my own experiment, although I live in Bilbao and we don´t have as many sunny days as in Barcelona.

Can I ask what is the appliance seen in one of the photos which is all wired up and has a cigarette lighter receptacle? Is it the charge regulator which gives you also 12v outputs?

keep up the nice work

kris de decker


@ Connor (#32)

In principle, you could do without batteries if you would just use a laptop: it has its own battery. But if you need artifical light, you need batteries because the lights are used after sunset. And even with just a laptop, a very small battery would be essential because I don't think the solar charge controller works without it.

@ Leo (#34)

That appliance is indeed the solar charge controller. The usual solar charge controller has wires to which you can connect a cigarette lighter receptacle. This one is all wired up, which is handy. See comment #31 for the link.



Your solar panels would be more efficient if always at the proper angle relative to the sun, obviously. Mounting with electric motor and some electronic would do the trick, of course, but at a cost and it would eat some of the precious energy havested by the panel. But what if you had instead a manually ajusted mounting with a cable running into the house, a simple lever combined with a sun dial? From time to time, you set the lever to the shadow angle and its done. Not perfect but better. The sun dial is a simple idea that would allow you to find easily the proper angle, without even thinking about it. Put it at arm length from your desk and the gesture would become an habit, a background process, a human automatism. There is no need of "intelligent" device if you've got a brain...

Really enjoy your site. A good read, always.




Hi Kris
I recently bought and renovated a very small appartment in Barcelona; with the intention of living there in the schoolholidays and renting it the rest of the year.
We have a fantastic terrace with sun almost all through the day.
I was thinking about powering the lights with sunlight (...); which will be the main power consumption apart from the kitchen.

1. do you have more resources about installation? I have "two right hands" but I know very little about electricity, even less about solar panels.
2. is there specific regulation in BCN (opposed to our homecity Antwerp)
3. Can you show more of your installation? Size of the different components, etc?



I really enjoy your experiment and the tenacity you put into it. I only wonder how your 1st floor neighbour can proceed in the same direction?

Will the energy revolution be another step into individualism?




Hi, great piece.

Could you possibly let us know the brand / model of your solar charge controller (pictured) that includes both the USB ports and the 12v dc out in cigarette lighter form?

I've scoured the internet for hours but can't find anything like it!




Exactly what is the purpose here? If the aim is to lower the total use of energy, one should take into account the embedded energy in all these gadgets. Could you give some sources about the embedded energy of every component in this off-grid system? I'll be satisfied with coarse approximations if you don't have access to exact data. Thanks.
PS. You're doing a great work in this site.

kris de decker


@ Selim (#40)

Correct. That's why the article concludes that "The electricity I generate is likely more CO2-intensive per kWh than Spanish grid electricity. The only reason why my system is more sustainable than using grid electricity is because it forces me to lower electricity use considerably."

@ Chris (#39)

See comment #31

@ cafucio (#38)

Two-thirds of the tenants in the building could use a similar system -- they could even copy my design because all window sills have the same dimensions. There is a large square in front of the building, so the people on the first floor have almost as much sun as I do. The remaining tenants can't use solar energy because they are at the north side of the building.

@ Burt (#37)

All information I have is in the article. There will be a follow-up article early next year, with more details.

@ PdeMTL (#36)

Nice idea.



There are external monitors which are powered by USB. Some of them are even portable to use them on the road with your notebook.



Have you considered using a gravity light in the bathroom to not draw any power at all? Available on Amazon.



Hi Kris,

Kudos for trying this. I recently bought an energy meter and have been testing all my appliances, like this experiment, it forces you to acknowledge your energy demands and in turn it helps you reduce them significantly. Since reading this I have considered many times installing a system myself, perhaps starting out in our external off-grid garage (soon to be workshop) as a small scale proof of concept project, and if it goes well, later expanding into our home.

Will you consider writing an update on your PV setup now you have been using it sometime? What have you changed if anything since the article was written?

My main concern is being unable to settle on the daily Wh requirements of the system, because any underutilisation will result in EROEI decreasing as eventually the individual components will fail, on the flip side, over-capacity certainly allows for a little slack when adding extra load on the system, i.e. having guests, which requires things like simultaneous lighting in multiple rooms, charging more devices etc. I plan to build a cob oven in the garden, and install a wood stove with back boiler for hot water (living in the north of England means solar heating is not much use except during summer months). These would easily cover our cooking and water heating needs.

A PV system like yours would enable us to maintain a semblance of modern life with minimal demands on grid energy. However, my wife would have to agree give up her 800 watt hair dryer and curling tongs to support such a shift! :)



Will solar panels charge on a sunny porch (has roof) & is screened in



Hi, i'm from spain too. I see that the surface of panels you use is quite small, i supose that is cause it covers the windows. but there is a lot of wall surface wasted. Would be posible to get some of this wall surface for the panels (as, for example, when people hang plants) to achieve more surface?
the panels would hang from the window as pictures on a wall.



Have you thought of getting a laptop with a removable battery? If you had one battery in the laptop and the other one set to charge when you were at peak energy production it could allow you to keep your current work hours and also rarely run out of charge.



Very interesting article, nicely documented.
I have been thinking about installing similar setup on my balcony with Arduino and servos/steppers to aim a 115W panel at the sun every 10-15 minutes, use 12V/100Ah LiFePo cells, and MPPT charge controller (about 600 euro kit for a campervan. Budget restraints will likely reduce what will actually be used :) ). To power 12V equipment use 12V DC/DC converters, and wire the cells into a cheap UPS battery connectors to get high voltage for other equipment. That has the added benefit of having built in charger for the batteries, so in an emergency I could in theory power everything from the grid. For monitor and other equipment it is best to get ones with external power supply, then it's easiest to connect them to batteries.
Looking forward to next update.



Thank you, Kris, for this objective approach, and for not promoting renewable energies at every price.

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