The Walden Effect: Farming, simple living, permaculture, and invention.

Roland Smith

About me


Those who are curious can check out my homepage.

I'm an design and manufacturing engineer specializing in composite materials. I live in the city of Eindhoven my native Netherlands. Next to my work I'm interested in al least anything technical, but also history etc. Wikipedia is a major sink of my time these days. :-) I'm also involved in Free Software, as a hobby. I try to answer questions on the freebsd-questions mailing-list. I also maintain several applications that have been ported to FreeBSD (next to my own), and I occasionally submit patches to the FreeBSD OS.

How I stumbled on this blog, I can't remember. But the discussions are interesting.

Anna Hess's books
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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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We love hearing your technical expertise, and non-American perspective. Thanks for making a page and posting!
Comment by anna Fri Feb 5 14:45:57 2010

Hi Roland (and everyone else),

I've seen you offer interesting and useful comments on many of anna and mark's tool and energy related posts. I have an off-grid property that I don't yet live on, but do have a young caretaker couple camping on. We finally have a small gas generator (35 years old from when my parents ran cattle in the Oregon mountains, but it still runs fantastic!) to run power tools off of for maintenance projects. So far we all charge our cell phones and laptops off of our cars. And we are excited that we can now plug in our personal electronics whenever we are running the generator as we saw or power the cordless drill. BUT we are wondering if there is a first step battery set-up (not the $3,000 inverter/charger and ten 12-volt deep cycle recommended by the off-grid store for when we are ready for running freezer/refrigerator, etc.) that we could occasionally charge efficiently to power our personal electronics . . . where we won't have to run the generator specifically for this purpose even every other day.

I thought about the 5in1 power pack, but have duly noted the spotty performance/experience of Mark's. thoughts/suggestions?

Thanks, Charity

Comment by Charity Tue Jan 10 11:37:02 2012

Hi Charity,

A question like yours has many different facets, so there will be no clear single answer. (This is not unusual in engineering problems, btw. :-) ) The answer depends to a large extent on the constraints (e.g. budget, required capacity).

I'll try to touch on the main points. My background is mechanical engineering, so electrical engineering is not my specialty even though I know the basics.

Basically this is all about energy and power. Power (the flow of energy) is measured in watt (which is technically joules/second). Energy is officially measured in joules, but watt-hours also works, al long as you are consistent. With DC, power (in watt) is voltage (in volts) times current (in amperes).

If you are just going to use it for powering electronics, I would suggest making a DC (direct current) ditribution system, since most phones and laptops run on or be charged with either 5V DC or 12V DC. This means you can leave the mains chargers at home, and you also won't need an inverter. That alone saves a lot in efficiency. :-)

This means you will need a battery, a charge controller and some other electronics (loke a voltage divider but more efficient) because batteries tend to be 12V and you might want 5V as well. Furthermore you'll need some wiring and DC connectors for you devices (like the ones that are on your mains charger). You'll also need a dry place to store the battery. Depending on the type of battery, you'll need more or less ventilation for the storage compartment, and it is probably best for it to have an above-freezing temperature.

Since this is a stationary application, a lead-acid battery is probable the cheapest solution. Don't use a car battery. Those are optimized for generating the huge currents that a starter motor requires, not for deep discharging. My suggestion would be to use a subtype of the lead-acid battery called a VRLA battery, especially the AGM ("Absorbed Glass Mat") subtype. They have a lot of advantages compared to "wet" batteries, mainly less maintenance and a low self-discharge rate, but see the linked article for a full list.

How big the battery needs to be depends on e.g. how long you want it to last and how much current the electronics need. A worst-case number for the latter is to look at all the mains chargers for your gadgets and multiply the output voltage and current. For each charger, multiply the DC output voltage (in volts) with the maximum rated output current (in amperes). That will give you the power in watts. Sum that all up and you'll have a worst-case amount of power that the electronics can absorb.

A more realistic appraisal would be to look up the battery capacity of your laptop, cellphone and other gadgets (usually in ampere-hours) and measure how low they last on a full charge. For each gadget, divide the battery capacity in ampere-hours by the typical time between recharges in hours and you have the average current in amps. Multiply that by the battery voltage and you have the average power requirements.

Sum up all the power requirements, multiply with the amount of hours you want the big battery to last between recharges and you have the needed capacity in watt-hours. Divide by 12 for a 12V battery and you have the appoximately needed capacity in amp-hours. Now multiply this with a factor of at least 1.25 for an AGM battery and 2 for a wet lead acid battery (because you can't discharge them completely).

It also depends on how you want to charge your battery. If you want to use solar panels, youd have to look up the amount of irradiation you can expect at your location, and calculate in the number and efficiency of the solar panels you can afford. Furthermore you'll need to take into account the number of days with low sunshine you want the battery to last.

If you want to charge the battery with the generator, the calculations are a bit different. Typically a generator will be most efficient at full load. But then you'd have to size the battery so that the charge current doesn't get too high (batteries doen't like that at all).

IIRC, Anna's brother Joey has a solar-powered DC system in his house, there are some articles about it in the archive.

Comment by Roland_Smith Tue Jan 10 16:18:05 2012

Roland --- I was just about to email you to make sure you noticed you had a personal question. :-)

The articles Roland mentioned are:

An old solar-powered house

Freedombox for a solar powered house

Comment by anna Tue Jan 10 18:49:58 2012
Comment by anna Tue Jan 10 18:54:07 2012

Looks like the error that occurred while I was responding actually meant my comment didn't go through. Thanks so much for your detailed response. Looks like I have some math to do. I will absolutely need to use the generator for now. Eventually we will put in micro-hydro, but solar really isn't a year round solution on the rainy Oregon coast.

I will post again if I have more questions!

Thanks (to you both)!


Comment by Charity Thu Jan 12 19:29:44 2012

We've had discussions in about hydro- and solar power here before. Check out the comments on the articles "friday we lose power" and "microhydro", among others.

Comment by Roland_Smith Fri Jan 13 13:28:06 2012

Regarding your comment on solar grid tie inverters.

Hello my name is Danny, I found your comment and email from the blog located at

And was wondering if you could help me.

I noticed that part of the blog and comments recognized the legal nature of the issue. I want you to know that before I ask my question that I am in no way concerned of legalities, all I'm concerned with is paying as little into my PUD Electricity bill as possible. If I have to break the law then so be it.

Could you help point me into the direction of from whom and where I could purchase the easiest and cheapest solar panel and grid tie converter kit? I would be grateful for any advice and send you a finders fee to return the favor of your generosity.

Any help is much appreciated, and needed.


Just another cheapskate and proud of it.

Comment by medical billing Sun May 13 20:09:12 2012

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