Off-grid energy supply is a task that has to be analyzed on three levels:
- producing the energy from any off-grid source, preferably renewable
- reducing energy use,
- storing it for future when supply exceeds demand.
True off-grid life means using only free renewable (sometimes referred to as alternative) energy sources. But solar photovoltaic panels and wind turbines supply you with power only when the solar/wind power is available. There are only two ways to simple deal with this problem. But neither of them is a good one.
The first is to buy a conventional energy source, like a diesel genset, to produce enough electricity to satisfy your needs. It’s a very easy solution, but I don’t suggest it. Diesel genset is not environmentally friendly, as it uses crude-derivative diesel fuel. You might use biodiesel or straight vegetable oil, but whether those fuels are fully renewable, is a questionnable matter. Genset with internal combustion engine is a good way to supply enough power when you need it, as it’s relatively easy to increase the output of this machine when you need more electricity. So you don’t need to store energy or reduce energy use — you just buy larger genset with equivalent turndown ratio.
The second solution is to buy renewable energy source, such as photovoltaic panels or wind turbine, that’s able to supply enough power for your need at all times! So if you need up to 10 kilowatts (kW) of electricity, you buy a turbine that’s able to supply 10 kW at even very light winds. Oh, and despite that, you would also have to limit your apetite for energy to sunny daytime or windy hours/season… I must be honest — except for the places where you have constant winds it is not cost-efficient way to make your home off-grid…
That’s why we need to take into account all the three levels.
Save as much as you can!
The first thing you need to do is to think about how much electricity you can save. You take a look at all your electrical devices, like kitchen stove, microwave, washing mashine, dishwasher, power tools, electric heaters, air conditioners, computers, tvs, lightbulbs, etc, and to see if some of those devices may be replaced with more efficient ones.
Start with all the light fixtures, replace the incandescent light bulbs with compact fluorescent lightbulbs. They are the easiest to replace and will generate substantial savings — each CFL uses about 20% of the energy used by the incandescent bulb…
The next step would be to identify all the devices that use the most electricity. Getting a portable (plug-in) wattmeter would be great, as it not only shows the amount of power used by the measured device (in a given time), but also can calculate the amount of energy it used in a given period — within a day, a week, or so. You just plug the device via this wattmeter, wait for a week, use the device normally, and then read the wattmeter.
When you identified which devices are reponsible for the most significant energy use, think if you can replace the device with more energy-efficient one, or get rid of it at all! For example, check if you can sell your electric heater and replace it with wood-firing furnace. Can you replace the air conditioner with solar chimney connected to ground heat exchanger? Can you get a solar water heater instead of your electric water heater? Can you identify and kill all the phantom loads?
You must remember that in off-grid conditions it’s easier to save one watt of electricity than to make it on your own!
Produce enough to fulfill all your needs
When you can’t reduce your energy use any further, calculate the amount of electricity you need daily in summer and winter. You can assume that the amount you need on during any other season lies somewhere between those two, so one will be minimum and the other — maximum. Which is which would depend on your climate, life style, etc.
Then take a look in a good solar/wind database and see how much energy you can make with a given photovoltaic panel or wind turbine. You should probably find a map that shows the solar irradiation or wind speeds, use this data to see how big wind turbine you need to buy (or make), and how many PV panels you need to get. If you live in Europe, use this great PV potential estimation tool.
In most cases you can’t be sure that each day you will get the desired amount of electricity. So I suggest you to get two turbines of the calculated power and two sets of PV panels, so they can produce more energy when wind / sun is at its peak, to be stored in batteries. You might even increase the output of turbine / PV further, as you need to take into account the efficiency of storing electricity in batteries!
Think not about the power all your devices use and solar panels / turbine supplies. Think about how much energy you use daily / weekly, and how much energy can be supplied from your source in the average day or week. Remember about the seasonal changes of weather!
Store for a week or so
Depending on your local weather / climate conditions, you might need to store enough energy for a week without a wind / sun or so. If not, I believe that storage capacity for at least 2 days is a must. With anything less you might end up with going to sleep early in the evening when you use all your energy produced that day…
You can reduce the size (and cost!) of your battery bank easily. I think that this is the place where a small genset might become handy — and reasonable. In such a case you would not need to buy a large genset, able to supply all the energy you use, but a smaller one, that will be enough to charge your batteries in 10 hours of continuous work. If you like doing it yourself, try using the waste heat from this genset to heat your home. ;)