Before you buy a single component, you need to figure out how much power you need at present. Off-grid systems are modular, so don’t worry if you want to add more later, just start with what you use now.
Begin by evaluating the appliances in your home. Record how many hours a week you use the appliances and how much energy each consumes, in terms of watts. For instance, a lamp with a 40-watt bulb (an extremely inefficient light bulb these days) that is on for 5 hours a day will use 40 watts for 35 hours a week, or 1,400 watt-hours a week. Try and be as thorough as you can, and when in doubt, overestimate.
If your estimate comes out very high, and you can’t afford such a system yet, look into ways of either conserving energy or of finding more efficient appliances. Get 10 or 15 watt light bulbs. A laptop uses a lot less power than a desktop. There are fridges made especially for solar power that use little power. Don’t use electricity for heating (whether cooking, water or your rooms), as this is a very inefficient energy conversion. And so on.
Once you get your typical energy consumption for a week, you can design your system to generate enough power in a week to keep your batteries fully charged. Your battery bank should be able to last you one week without any energy coming in, in case of clouds or malfunctions. If this ends up being too costly for you, you could setup your battery bank to last only a couple of days without incoming energy, and then get a small back-up generator to charge your batteries during a cloudy week. You can even convert the generator to use woodgas, instead of gasoline, for further savings.
Say you want enough battery storage to cover the above 1,400 watt-hours a week. At 12 volts (volts x amps=watts or watts/volts=amps), your battery bank will need a capacity of at least 117 amp-hours. Therefore, getting two 12 volt batteries with a 90 amp-hour capacity each, would meet your needs and give you a buffer.
Your sources should be able to keep your batteries fully charged all the time. Lead-Acid Batteries should never go below 80% capacity to maximize their service life. In a general day, your sources should be able to completely recharge your bank. So, for the above example, we will need to produce 117 amp-hours at 12 volts in a week, or 16 amp-hours a day. Sources will be rated in watts, but their outputs can vary. A 120 watt solar panel is producing 15-18 volts and 6-8 amps at peak performance. It will produce less in the morning and evening, but in our area, we can get 25-30 amp-hours average daily from a 120-watt solar panel.
So, now we are producing more than we are using. Is this a problem? Not really. This is actually good, and the system should be designed to use this extra energy for something useful. Water heaters work great for this as a diversion load for your system. When your batteries get full, a diversion controller can turn on the water heater and uses the energy coming in from the source. The controller and diversion load should be designed to handle the maximum output of all the sources. Diversion loads can be stacked to come on at different intervals, giving your system a sliding scale, depending on how much power is coming in at the time. If you don’t have any diversion, or dump, loads, make sure that you have a charge controller to prevent your batteries from overcharging, which can be very dangerous.
Once the usage, storage, and source capacity have been determined, you will need to decide how to use the energy. What appliance will be on 12 or 24 volt DC and which ones will be run through an inverter?
We use DC for our lights, fridge and freezer. We also have DC outlets throughout the house for any appliances that have a 12 volt plug. The advantage of using DC is that it runs directly off your batteries without the need of an inverter. Not only is this more efficient, as any energy conversion suffers a small loss, it is also more reliable, as inverters can malfunction. Be warned that DC appliances often cost more, as they are not as popular.
Furthermore, DC wire is more expensive. Not only does it have to be stranded, but also you often have to use larger wire sizes. The reason for this is that DC experiences a line loss, in that its power decreases the further it has to travel. The thicker a wire is, especially stranded, the more expensive it is.
AC wire is cheaper, as are the appliances that use it. Furthermore, there are many appliances that you can’t really find in DC, especially in regular stores. So for the most part, you will be using AC within your home, and for this you will need an inverter (as well as a backup if possible).
Your inverter should be sized 25% larger than your biggest draw. Inverters are rated in watts and have a surge rating, but most users have found that surge ratings are useless. In the above example, the lamp is pulling 40 watts, so we would need at least a 50-watt inverter convert to 120 volts AC. You should place the inverter as close to the batteries as possible and should use a big battery cable to connect the two.
There are many ways to generate power for an off-grid system, some of which we deal with in other articles.
The three most common would probably be solar PV, wind generators and a generator (which can be converted to use many fuels that you can produce at home). The best system would be a combination of the three.
Hybrid systems are a good idea, as you are guaranteed power, no matter what the weather. Even if you won’t need all three all the time, it’s reassuring to know that a backup is available.
Solar PV is one of the easiest, and the most popular, sources for the off-grid home. This technology has been steadily getting cheaper over recent years, and is super simple to install. Simply attach the panels to a sturdy mount and wire them into your system. However, if you want to be extra efficient you can use a mount that tracks the sun. For more information about solar as an electrical source, you can look at our solar article.
Wind can be a great option for the DIYers. You can build your own machine, customizing it for the types of wind you most commonly experience, whether light and constant or strong and gusty. You can make either vertical or horizontal axis machines, though the latter is the more widely used. Unless you have almost constant wind, like on the coast, wind is more likely to be part of a hybrid system. It is harder to do than solar, but can give you power, even when it’s dark or cloudy. If you’re interested in getting your feet wet in wind power, check out our Chispito and Wind Tower how-tos.
As for generators, they work very well as a backup, for when there is neither sun nor wind. You need to find one that will be able to charge your batteries. Gasoline is expensive and unsustainable, but there are many ways to convert a small generator to something more in tune to off-grid living. A charcoal wood-gasifier would be an excellent option, or using biodiesel on a diesel generator.
Once you have figured out what source your electricity has come from, you will need to figure out how to store it.
Storing the electrical energy that has been converted by your source is the hardest part of the home-energy process. Most systems use lead-acid batteries, which can not be constructed very easily in your average home workshop. Still, they are efficient compared to the cost, and along with conservative energy use, deep cycle batteries can play a very important role. Lead-Acid battery systems can be any voltage, but most people use either 12 or 24 volts. If you have the money, buying a set of Edison batteries would be the way to go. They are far more expensive, but are said to last a hundred years or more.
So, what is a battery? A battery converts the electrical energy from your generator or solar panel into chemical energy by means of a specific chemical reaction. When you need to use electricity, the battery reverses the chemical reaction and releases electricity. Batteries come in all shapes and sizes, but for most small home systems, deep cycle lead acid batteries are used. These batteries can be found in most cities, and have many applications including golf-carts, forklifts, and telephone lines. Lead-Acid Batteries are rated in Amp-Hours, which means they contain a certain amount of time at a particular electrical draw. A 200 amp-hour battery with a 20-amp draw will be discharged in 10 hours of use. So, if you had a lamp that pulled 2 amps, you could light a room for 100 hours.
Most batteries in this class come in either 6-volt or 12-volt sizes. Both are fine, although you have to wire two 6-volt batteries together in series to bring them up to the 12 volts required by most appliances. Then, you wire the 12-volt batteries in parallel to give you more amp hours. Some people use a 24-volt system, in which case you would need to wire smaller batteries in series to reach this voltage. We prefer a 12 volt system.
Because batteries are expensive and are not the most environmentally friendly component, you will want to make them last as long as possible. Unless you have a closed battery, checking the water level regularly is vital, and should be part of your general maintenance schedule. Another key factor in the lifespan of your batteries is not leaving them drained too low for too long. We have learned the hard way, and it has cost us dearly. Now, we try not to drain our system below 12.0 volts (a battery is full at 12.6V), and so far we have been very pleased with the performance of our battery bank.
One thing to note when reading the level of your batteries is that you will only get a true voltage reading when there is no power coming in. For example, if there is no wind or sun, and hasn’t been for a while, your voltage reading will tell you exactly what your batteries are sitting at, 12.6v being the maximum. However, when the wind is blowing, your voltmeter might read anywhere up to 14.6. This is because you are reading an average of what your batteries are sitting at and what is coming in. The closer the batteries are to full, the less they act as a voltage buffer, and the voltage rises faster.
Should your batteries continually read low, even though you have power coming in, the chances are that you have a bad battery. Don’t despair, it’s not usually a problem with the whole bank. You might be able to see which one is bad by putting a voltmeter to each one while they are still hooked up to each other, but your readings will be more accurate if you disconnect them. You could always try the first, easier option, and if you still can’t tell which is bad, unhook them. When you find which has gone bad, remove it from the system and your voltage should immediately increase.
It is important to include electronic controls for your home energy system. Electricity coming from the source should be regulated against overcharging your batteries, and you should also have some kind of alert (light, noise or switch) to tell you that you are draining your batteries too low.
Regulating your sources can be done in several ways, most commonly, using a charge controller. The charge controller can read when your batteries are full, and subsequently cuts out the source, so that no more power comes in. Then, when the level of the batteries drops a little, the charge controller will switch the source back on. Of course, if the charge controller cuts out the source, there is effectively a bunch of power that is being wasted (not being used). To counteract this, you can use a diversion, or dump, load, which uses up the excess energy when your batteries are full to power something like a hot water heater or air compressor.
Manual cut-off switches are essential for safety purposes. If you need to work on any part of the system, it helps to have a way to easily shut off any electricity coming in.
Your system should always be fused between the source and the battery, and also between the battery and the appliance. That way, if something should happen, the fuse will blow, interrupting the circuit, and your components will remain unharmed. Fuse your system as close to the batteries as possible.
You will also want a voltage meter to check the state of your batteries and an ammeter on each source to show how many amps your source is generating. Proper system health can be achieved with the proper controls, regulators, and metering.
