In 2011 I wrote a series of posts about living off-grid. I am going to re-post them here with some updates and then add to the series. While this post wasn’t written first it seems like the right starting point. New posts will talk about doing grid-tie systems in Guatemala.
After a few years of living off-grid I can add practical knowledge to theory. This article offers a short story on the good things I learned and some tips on what not to do.
I elected to design and build my own off-grid system. My goal was to meet most of my electricity needs with close to no ongoing costs and an initial investment of less than what a grid connection would cost. For the most part, I have met those goals. As the price of off-grid equipment has dropped, it is easier today to do this than when I did it.
The major components of my system are:
- Nine photovoltaic panels of 210 watts each
- A controller for these panels
- A battery bank consisting of eight 6V, 400AH Rolls batteries hooked up for 24V use
- Two 20A and 24V to 12V DC-DC converters
- 120VAC, 3500W inverter
The following additions were made:
- 400W (800W peak) wind generator — basically to see if wind was feasible
- Tri-Metric battery monitor
- Additional 600 watts of photovoltaic panels — I already had these and just decided to add them to the system
- Home-made 24V, 40A diesel backup generator
- Commercial trash 5KW gas generator
Last but not least are an assortment of fuses and circuit breakers. While I am not detailing them, they are extremely important if you want to avoid disasters.
This can be described as a 24V system where other voltages, as needed, are produced by devices running off 24V. The first question most people as is “Why 24 volts?” It turns out that there were more reasons this was a good choice than I had initially anticipated.
The first part of the answer is simply because this is too big a system for 12 volts. Too big means that with the currents involved at 12 volts, it would be hard to prevent serious losses. For example, for an inverter running on 12 volts to output 3500 watts, it would have to draw around 300 amperes. The peak charge current for all the PV panels and wind generator at 12 volts is also pretty close to that. Doubling the voltage cuts the current in half. As you have half the current and the same voltage drop at 24V is only half as significant percentage-wise, you actually get a 4-times improvement.
A higher voltage would, of course, have additional advantages as far as reducing losses but there are many more choices for equipment such as PV controllers at 12 and 24 volts than at 48 volts. In addition, there are lots of low power consumption refrigerators and freezers made to operate on 12 or 24 volts.
The down-side is that while there are lots of lighting options available for 12 volts and 120 volts, there are few at 24 volts and those that are available tend to be much more expensive than similar units at 12 or 120 volts. My solution was to go with 12 volt lights but run them off of DC-DC converters. That is, a unit that runs on 24 volts and outputs 12 volts.
These units have two advantages I had not thought about when I first decided to use them:
- They supply a constant 13.8 volts output independent of the battery voltage or load. Thus, lights do not flicker when, for example, I start my 120V radial arm saw even though there will be a drop in the battery voltage.
- They are self-protecting. that includes against over-temperature and output current overload. So, if you short out the output of the 20A unit, the current will fold back to a low level until the short is removed. Very different from when you drop your screwdriver across a big lead-acid battery.
The DC-DC converters are inexpensive, very efficient and very reliable. One of the 20A units runs all the lights and also the 12 volt RV-type pump I have to supply water pressure in the house. The second 20A unit then runs the 12 volt outlets in my house. The two reasons for this are first I do the step-down closer to the loads then if the unit in the garage was being used to run the loads in the house and that an overload on one of the outlets does not cause the lights to go out.
As I was already running 24 volts into the house to run the refrigerator, no extra wiring was involved.
While some will suggest getting more 24 volt devices to replace 120VAC devices will save losses in the inverter, this is, for the most part, a non-issue. Running the coffee maker for 10 minutes a day or the radial arm saw for five minutes once a week is just not worth the concern. And as both are high-power devices, decreased losses in the wires because of the higher voltage probably make up for inverter losses.
I do run my laptop off a car adapter plugged into one of the 12 volt outlets and have a DC power supply that I put in a desktop system which can run on any voltage between 12 and 32 volts. Thus, I can connect it directly to the batteries eliminating conversion losses.
Tips and Tricks
- While there are 12V lights designed specifically for home lighting, there are lots of LED units now offered as replacements for incandescent units in automobiles. They are inexpensive and offer very long lives. Units designed to replace car domelight bulbs are well-suited for under-the-cabinet use.
- If you have a lead-acid battery bank, size it such that under normal conditions it is never discharged to less than 50% of its capacity. Further discharging will significantly shorten the battery life. Here, the Tri-Metric device from Bogart Engineering is an excellent investment.
- If you elect to build your own fuel-powered battery charger, be warned that most car and truck alternators are not designed to continuously output the specified current. Design accordingly or, if you are really serious, take a look at the Zena line of alternators.
- This one is for the real geeks. Many switching power adapters (the ones for IBM laptops are a good example) will run off DC as well as AC. This is because internally they rectify the input voltage and use it to run an inverter. If the unit says it will operate from say 100 to 240 volts without needing to select a voltage with a switch, it is a likely candidate. If you have a modified sine wave inverter, long wires can mean a lot of radiated hash. If you rectify the inverter output before feeding the wire, no more hash. (Note that if this does not make sense, don’t try it. Toasted power supplies can be the result.)
- Newer wall warts, the plug-in units to power radios, cell phone chargers and so much more, tend to use an inverter and switching regulator rather than a transformer and linear regulator and are therefore more efficient. One clue is that these efficient units tend to be much lighter because they do not have a big iron core transformer inside.
- If you have 12V DC in your house, look for adapters and chargers made to plug into the cigarette lighter socket in your car. Small, inexpensive and generally efficient.
- Power output of a wind generator is a funciton of the cube of the wind speed. As a rule of thumb, don’t expect much output from a wind generator unless you find the wind irritating.
- The life expectancy of low-end gas generators before needing a major overhaul is 500 to 1000 hours. Take that into account when sizing your system. If, for example, you only need to run the generator an average of five hours a month that means it should last 8 or more years but at five hours a day it won’t even last a year.
- For a stereo, you can start with a car radio/CD player, put it in a box and add speakers.
- A rule of thumb is to size your battery bank to deal with three days with no sun. This, of course, depends on where you live. For example, three months rather than three days might be a better number if you lived in Seattle.
- Charging and discharging batteries is less than 100% efficient so it is always better to use the power being produced rather than store now and use later. For example, if you cook rice in an electric rice cooker, it is more efficient to cook it at mid-day than in the evening.
- If you want to operate a normal computer directly off your battery, there are now power supplies designed to do this. While they are just a circuit board rather than a big box power supply like in a typical PC case, they have the same connections. They will be more efficient and will operate over a wide voltage range. One such unit which can operate from 6 to 32 volts can be found at http://www.itxdepot.com/xcart/product.php?productid=1475&cat=0&page=1
OK, that will do it for now. Lots more to come.