by E.R., Survival Blog:
(Continued from Part 2. This concludes the article.)
INVERTERS
Most of us are interested in running a few 120 volt AC appliances. The fridge, the furnace, the shallow well pump – standard AC devices that we want to keep alive during a power outage. For these we will require what is known as an inverter. Inverters take DC battery power and invert it into standard 120 volt AC household power.
Inverters are available in all shapes and sizes these days. You can get off-shore-manufactured modified sine wave inverters that plug into the cigarette lighter plug in your car, rather cheaply. These might even be enough to charge your phone, or laptop computer. The rule of thumb is that these devices produce about half of what they consume. For example, if your cigarette lighter plug is fused at 15 amps at 12 volts DC, the inverter can be expected to produce a little less than 1 amp at 120 volts AC.
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If you want to run other appliances, you will need a far more robust inverter. Cheap Chinese-made inverters rated at 2000 watts can be had these days for under $1,000 – sometimes far less than that. These might mostly work at the stated spec, but read the fine print before you decide to buy. And keep in mind that they are often rated for momentary maximum draw, not continuous draw. That 2000 watts stated capability might realistically be less than 1500 watts, continuous. Momentary draw duration also tends to vary, by manufacturer. For some manufacturers, momentary might refer to 15 seconds. To others momentary might be based on 15 minutes. You do tend to get what you pay for.
Figure 3: Higher-end inverters incorporate chargers for grid, generator and solar charger.
Moreover, if you intend to run appliances with electric motors, or other reactive devices, you might want to research load factors. For example, a motor that might draw 9 amps (1100 watts) continuously once running, might require 20 amps (2400 watts) to even get started. How quickly will it make the transition? Some of the better inverters can sustain such overages for a quarter-hour. Other cheaper units might fail in terms of seconds.
Also, keep in mind that the cheap inverters produce what is known as a modified sine wave. The power grid on the other hand supplies a true sine wave. Not all appliances will work acceptably given a modified sine wave. The better inverters always produce proper sine waves – but that is just the start. The higher-end inverters will automatically go into a heavy power-saving mode when they detect no 120 volt draw and will seamlessly come back on the microsecond you plug something in. The best inverters are also very heavy.
The other factor to keep in mind with running an inverter, is that you will need very heavy copper to connect the input supply power. Copper is expensive. If you want to run a 120 vac, 2000 watt, at 15 amps continuous, it could draw around 200 amps at 12 volts DC. DC resistance is significant and voltage drop can quickly become a problem – so keep the runs short as possible. We are talking heavy-duty ‘welding wire’ in which copper is a perhaps a half inch in diameter. Depending on your specific situation this might translate into a 00 American Wire Gauge (AWG). Normally these data are provided in the inverter manual, although tables also exist online which can assist you in determining the AWG based on length and your current requirements.
You can reduce your cost of wiring by, instead of 12 volts, configuring your system at a higher voltage. By placing four 6 vdc deep cycle batteries into series, you can work at 24 volts nominal. Most heavy truck appliances will be able to be fused directly to a 24 vdc system. Our DC Danfoss-based freezer can work at 24 volts. Just be aware, that if you go with a 24 volt system, you will additionally need a 24 to 12 vdc converter to power those appliances that require just 12 volts.
If you place batteries in series, in parallel, or in series-parallel, you must consider the fact that each battery must be of the same type, age, and condition. It will not work well in this configuration if batteries are dissimilar.
Very large PV systems which happen to make use battery storage are often wired based on 36 or 48 vdc. If you are thinking to power heavy-duty appliances, you will want to thoroughly research higher battery voltage systems.
Large, often government-subsidized, PV arrays are usually wired using grid-tied inverters that only work when the grid is up. While these are beyond the scope of this article, it would be interesting to read about specific experience in modifying a grid tied array to become useful in the context of a backup power system.
BACKUP POWER SYSTEMS CONTRASTED
A typical non-PV backup power system relies on generator power. The problem with generators are that they rely on fuel that might be in short supply and they can be associated with significant noise signatures. Moreover, except for some very expensive installations, they are not typically on standby when the power goes out. Most people have to haul them out of storage, only to find that they can’t find the extension cords. Then they get to deal with the fact that they have not been test-started in quite some time and the fact remains that gasoline does go bad after a while. By the time you have dealt with all the gremlins your water pipes have frozen, or in a warmer climate, your freezers will have all long thawed.
The solar backup power system, on the other hand, is always on standby and when the power fails it automatically fails over within a fraction of one cycle of a sine wave. Brownouts and power problems experienced by the grid are thus not experienced by your appliances.
The only time you would need to fire up a generator is if you have too small a battery pack, too heavy a load, or you get an extended period without direct sunlight. In any case, it buys you a significant amount of time until you get the generator set up and running. With a sophisticated inverter-charger, you can configure it to automatically start your generator when the batteries are low and to shut the generator down once the batteries are adequately charged or the sun returns – thus, conserving precious fuel.
