BatteriesThere are numerous possibilities for designing a power system to suit these requirements. The most basic one would be to store electric power supplied by the grid in batteries. The most widely used type for this are lead-acid deep cycle batteries, mostly sold in the absorbent glass mat (AGM) variant. They cost about 15 cents per Watt-hour at present, but can only be discharged to 50% to keep functioning without much capacity loss for a few hundred cycles. So it’s effectively 30 cents/Wh. They usually supply 12V DC power, which has to be converted to AC with an inverter to be useful for AC based equipment (which most household appliances are). Their capacity can only be fully utilized when discharged slowly within a day, so one would need 2.4kWh to run a constant 100W (enough for a laptop and some power saving light bulbs) for a day. This translates to a cost of $720 for the batteries.
An emerging alternative technology is the lithium iron phosphate (LiFePo4) type, which can be discharged several times faster without capacity loss and lasts for about 5 times as many cycles, but its initial cost is higher at about 80 cents/Wh. See this comparison between the two and a load calculator here.
Increasing demand for batteries coming from the auto industry will likely lead to further development, decreasing cost and increasing performance of new types. Notable current developments are Tesla’s Gigafactory, which holds the promise of a lower cost, albeit this kind of development in production done in one big factory runs counter to the other important goal of being able to create and refurbish batteries locally. Another interesting recent development is the entry of the LiS (lithium sulfur) type, which has a much higher theoretical energy density compared to LiFePo4.
GeneratorsAnother way to store energy are generators. These also come in several varieties. Easiest to find are gasoline fueled, which cost under $300 for a 1kW type and provide AC power. Their main obvious drawback is their dependency on fossil fuels, using about 0.7 l per hour for 1kW electric power. Propane fueled generators are also available, which are practical for mobile homes or boats, as they use the same fuel as small heaters and propane refrigerators.
More eco friendly would be to run a generator on ethanol, but most gasoline types sold don’t seem to be equipped for that. Then there is also the possibility to use a Stirling Engine, which can be driven by anything, from concentrated solar heat to burning wood. Its main drawback is its bulky size, which makes it difficult to use for mobile settings.
All generators can be used to load batteries, which makes sense when high levels of power are not constantly needed and other alternatives are not available, such as with a solar cell/battery combination in the winter.
Solar CellsStored power has to come from somewhere and if that somewhere is the grid or not locally available fuel, then the level of off-grid endurance will always be limited. Fortunately there are a few contraptions that can produce primary electric power from widely available and freely accessible sources. Solar cells should be mentioned first in this category, them being quiet, possible to operate without moving parts and using the same resource most of life on Earth depends on: the Sun.
There are 3 different types of solar power to be mentioned here. Most flexibly to use are thin film cells. These are light weight and can be bent without damaging the cells. Their main drawback is a smaller efficiency (around 10%) in converting sunlight compared to the approximately 20% of the heavier and inflexible second type, crystalline cells.
The third type to mention are concentrator mirror based solutions. There can be a Stirling Engine in the focal point of these mirrors (spherical, parabolic and Fresnel types exist, both in linear and radial forms). This configuration would create mechanical motion, which can be converted to AC electricity with a generator. As an alternativele highly efficient, heat resistant or cooled solar cells can be placed into the focal point of the mirror, which improves efficiency at the possible cost of shorter cell life due to increased radiation.
The cost of solar cells has been dropping steadily since their invention and is now becoming cost competitive with big industrial methods based on fossil fuels, nuclear power or hydroelectric dams
Wind and Water TurbinesWind turbines are another possibility for harnessing another power source that mixes well with solar, as there can be wind when the Sun is not shining and vice versa. A wind turbine is generally a less mobile solution, as it needs a supporting structure. But small wind power generators do exist even for small boats, so they don’t entirely have to be fixed to a location either. They also tend to be cheaper compared to solar on a per Watt basis, although solar cell prices have decreased rapidly in the past few years, meaning the difference is smaller than it used to be.
Lastly to be mentioned are flow-water turbines, which can also be quite small scale, used to extract mechanical power from running water sources, such as creeks, rivers, etc. Main drawback is obviously the need to be next to such a source, but when it’s present, this is an easy and affordable way to generate considerable and relatively constant power.