Backup Electric Power Design Considerations, by Duliskov – Part 1


This article covers a complex area, and to keep myself focused I will break it into three sections. In the following I would like to share what I learned while researching and building an emergency power station. The content below assumes that the reader understands the basics of electricity (AC and DC), batteries, and solar power. I have no affiliation with any of the sellers of products I provided links to; the links are for your convenience only. I have no engineering degree and reserve the right to be completely wrong. However, I did build everything that I am writing about, and it works. It is possible to build the systems in many different alternative ways. My approach may not be the optimal one, but this should give you ideas.

General Advice

Building a flexible backup power station is an expensive proposition, and you would want to understand your power consumption and true needs, how the system is sized and configured, how to optimize its use, and how to build a low-maintenance system. You would benefit from building it yourself, because this will allow easy re-configuration, extension, and troubleshooting. Also, in the process of acquiring equipment, materials, and skills, you will establish valuable relationships with suppliers and professionals that could benefit your other projects as well.

You will be working with strong DC currents and high voltage AC, so be very careful and thoughtful how you approach your work. Plan ahead each step. If possible, have someone with you who can give you a hand or call help if you get yourself in trouble.

The most important decision you will have to make is what type of disruption you are planning for. It is unlikely that electronics in chargers and inverters will survive an EMP event, so prepping for that with any substantial power generating capability is basically out of question for an average person. You may wish to live off-grid, and then you need a self-sustaining system with renewable energy source. If you prepare for short duration outages, at most for a few weeks, you can make compromises and rely on a single source of energy to replenish your batteries or you may not even need a battery bank and just store enough fuel for a good quality generator. You may want to use the system only in an event or you may want to reduce your electrical bill by running your alternative energy producing equipment constantly. The easiest way to see how much energy you are currently using is to lookup your average daily consumption over a period of one year to account for AC and heating needs. I can assure you, you will not be able to generate that much energy on your own consistently! You will have to make choices. Also, do not count on the system paying for itself; emergency power backup is an insurance policy. You will be disappointed at the rate it can save you money. It may not be much, and it can be protracted over a long period of time. All those considerations are tough ones. If you have a set budget, whatever you spend on electric backup, you would not be spending on food, security, or other necessities. Make sure you have your priorities set right. You may also want to think about mobility, covering your roof with solar panels, or erecting a huge wind turbine, which will not help you if you have to abandon your compound.

Subsequently, you need to decide how much power you wish to use (power your entire house, just a fridge, or just your cell phone/radio) and what type of power (115 Volts, 240 Volts, multiple phases, or other). I do not think that very powerful generators are truly necessary unless you know for sure you will use their full power all the time. They are heavy, noisy, fuel hungry and costly. You can always add a second generator as needed, especially if they are of variety that can synch together. You can easily plot power consumption using computerized meters, that will allow you to plan for peak power (driving your inverter decision) and how much, on average, each appliance uses (determining useful capacity of battery bank).

Figure 1. Example of energy consumption of my Haier 1.5cft HLP23E Compact Pulsator Washer on Cycle 04 Quick Wash program. You could see that it only used 46 Watt/Hours, but the peak wattage was over 800 Watts. I also measured its water consumption (17 gallons on High, 9 gallons on Medium, and 5 gallons on Extra Low per each rinsing cycle).

There are on-line calculators as well, but best practice is to measure your own appliances you are actually planning to use.

The next question is how automated you want your system to be. There are transfer switches that will instantaneously engage a battery bank, in case utility power is gone, and in the meantime start your generator. When it stabilizes, the switch will transfer the load to the generator and start charging the batteries, automatically. This will ensure unattended, instantaneous failover. Alternatively, if you protect your sensitive equipment individually with an UPS, then you can get away with a manual transfer switch, because the beeping of the UPS will wake you up at night and give you at least 10-15 minutes to switch your loads to the battery bank and subsequently to a generator in the morning, or you can let your solar panels take over as the sun rises. A transfer switch is a must to protect utility workers who might be working on the lines. A transfer switch ensures that your generator cannot be fed back into the line, thereby hurting anyone. Please, never ever plug the generator/inverter into your home electrical outlets, bypassing a transfer switch. This is very dangerous. Even if you know what you are doing, you may not be around when someone else makes a mistake, costing a life. Manual transfer switches are relatively cheap. The outside power inlet box will let you connect your split phase generator to feed your critical loads safely with a single, properly-rated cable. Lastly, the budget will restrict what you can do. I suggest going slow on a tight budget but buying quality parts and tools. Start with the component you will see a return on immediately. For example, if you plan to lower your electric bill, start with solar panels, wind turbine, or hydro-power and an inverter, self-synchronizing to the grid. If you are primarily concerned about functional sump pumps and refrigerator, start with a battery bank and inverter that has a built-in transfer switch, and then add the ability to generate power later. If you want to survive a short outage but don’t have the skill to build a complex electric system, just buy a good inverter generator and focus on storing fuel and a transfer switch. Don’t forget to rotate gasoline and stabilize it. Propane can be stored indefinitely, and there are tri-fuel generators that will run on any fuel. Never store flammable fuel in the garage or basement; always store it in an outside shed, away from your living quarters.

If you are just planning to build a retreat location, consider adding DC wiring in addition to AC wiring. This will afford flexibility to run your DC appliances without any intermediate conversion of energy.

Not all energy must be centralized. For example, having small, motion-activated, battery-powered lights throughout your house or headlamps will save you from running wires in each room. Battery-powered tools will allow you to work as long as you have the ability to recharge them. Make sure you have enough chargers to charge all your batteries simultaneously, if you go that route, because it makes little sense to run a generator for eight hours just to charge several sets of AA batteries. Also, make sure your chargers like the quality of the power from your generator. (I have more on this later.)

Before you invest in alternative energy, check the availability of U.S. Department of Energy incentives and other local programs that may help offset the cost.

Electric Energy Generation

The four basic forms of electrical energy generation are solar, wind, hydro, and thermal (internal combustion, not geothermal, which is not covered here). All of them utilize at least one transformation of energy, such as chemical into thermal, thermal into mechanical, and then mechanical into electric energy. Solar converts photon energy directly into electrical power. There is loss of energy starting with capturing the initial energy fully, then subsequently at each transformation, and lastly in the electrical wires connecting the source to the load. If the energy is stored and retrieved later, there is an additional loss. The electrical power is typically converted in the end into mechanical motion, light, and heat also with less than 100% efficiency. As you can imagine 95% of the wind power hitting the turbine or solar energy may be lost in the process.

From the perspective of preparedness, solar and hydro generation are the most “concealable”, while wind turbines are visible landmarks and gas/propane/diesel generators attract attention with noise.

Wind, solar, and hydro are renewable but are not available 24/7. Generators can run day and night, as long there is fuel. If you chose to use a gas-powered generator, make sure you store ethanol-free gas. Find stations selling it using “Pure Gas” in your iPhone or Android app, and then add a gas stabilizer. Store fuel away from house and rotate it at least annually. Note that the fuel components differ in the summer and winter versions of the same grade of fuel. I don’t know which is better from a long-term storage perspective, but I store fuel that I buy in autumn.

Operationally and to extend the use of stored fuel, it is best to have multiple energy sources. For example, solar can power utilities and charge batteries during the mid-day, and a generator can supplement that and charge batteries when there is less sun and the wind picks up. Larger generators, above 3KW, can simultaneously charge a large battery bank and supply energy to utilities. Generators run most efficiently near full capacity; they consume a bit more fuel, but the energy produced will require less fuel per kWh. Therefore, it is best to run them for a short period of time– at most for a few hours– but load them with charging batteries and powering the fridge/freezer, water heaters, lights, washer, power tools, and so forth to full capacity. Then, switch to battery power, and only use the minimal load to last through the next generator cycle or solar/wind opportunity.

For any significant solar power generation, plan to cover your entire roof with panels. Consider installing a few panels on the roof of a trailer, if you have one, that will give you mobile power and better concealment. If you make panels tilt or slide out from under each other, you can significantly increase the total surface exposed to the sun, while stationary. Alternatively, install the panels on ground support, for easy access and scalability. Make sure (by taking photos in different times of the day and during different months) that the selected location does not have the house or trees casting a shadow over it. If you don’t have the luxury of time, you can use an application to simulate where the shadows of each object will be at any given moment in time– summer or winter. One, for example, is “Light Trac”, an iPhone app.

The distance between the panels and battery bank is essential, especially in case of low voltage 12-24 Volt transfer to the battery charger. The cable will dissipate part of the current as heat, wasting it, and the cables will be extremely expensive. A better option is to use higher voltage, for example 96Volt, or install an inverter at the panels and send an AC current down the line to the battery bank. Another option is to install individual self-synchronizing inverters for each panel. These inverters will synchronize frequency with each other and the utility power, so you can simply plug them into your house grid via a transfer switch and feed your house simultaneously from solar panels and utility power at the same time. The battery bank will be charged from any outlet in your house. This will make solar contribute to your energy bill without re-arranging your incoming utility lines and will be transparent to your utility company. Alternatively, it is possible to isolate a few circuits to be only fed by solar.

Wind power is noisy. If you install wind turbine on a mast connected to the frame of your house, you will hear the grinding noise throughout your house, especially with larger turbines. Smaller turbines are not going to satisfy your power needs of a larger bank, but they can supplement a solar array or generator. Specialists in wind energy say, “Go big or forget about it.” They says this because wind energy is extremely inefficient in case of small scale generators. I have never installed a wind turbine but have read many books and distilled for myself the following– don’t do wind under 5 KW; for the cost of installation and maintenance, you could buy a larger solar system and have less trouble over time.

If you are lucky enough to have a creek or river flowing through your property, you can tap into hydro power. Obviously, in summer the creek may dry up, and in the winter it may freeze up; otherwise, it is a constantly available source of power. Be mindful of the ecosystem of the creek, when deciding how much water to divert to your project. Also, make sure local regulations allow for what you are planning to do. The water/energy of the river flowing through your land may not be legally yours. Check with local authorities, before you invest in hydro generation.

Generally, there are two most important considerations in selecting the micro-hydro turbine. First, consider how high is the water column; this depends on the steepness of the terrain or the height of the dam. Twice the height gives you twice the energy the water will produce. Second, note the volume of water you can divert to turbine. Again, power output is proportional to volume. This will drive to either go with an impulse turbine, which is optimal for the high velocity low volume, or with a reaction turbine, which is low velocity of water but high volume. Unfortunately, I don’t have direct experience with hydro myself. Interestingly, the 5KW rule seems to apply here as well, based on literature; forget about it if you can’t go big.

Gas/propane generators sometimes have a 240V split phase output. It is best to load both sides equally, otherwise you will only be wasting half of the power of your generator. Be careful buying cords; the majority incorrectly only tap into one side of a 240V outlet. Buy something like this to optimally use generator power. Also, when wiring into your house using a transfer switch, ensure that the circuits you wired to one side or another are about equal under normal conditions.

Propane has indefinite shelf life versus gasoline, which has a short shelf life as mentioned earlier. Keep in mind that in winter the propane tank will have less pressure inside. If the propane is used at a high rate, the expansion cools the tank further. You may end up having access to only about half of the propane in the tank, unless you warm your tank while using it with something like this, for instance. The 120Volt heater with magnetic head does not make good enough contact with convex surface of the tank to transfer enough heat to keep it warm. I tried two 400W versions attached to a 20lb tank, and they barely raised the temperature by few degrees– not sufficient. They work well on flat paramagnetic surfaces though.

When selecting the solar battery charger make sure you pick an MPPT (maximum power point tracking) type . They will allow for fluctuation in the input voltage, while flexibly adjusting the voltage conversion ratio, so they utilize the full power of the solar panel, not wasting any energy; in other words, their peak conversion efficiency is close to 100% no matter how much sun is hitting your panels. Here is a good technical explanation of PWM vs MPPT technology. The same goes for wind turbines. However, many wind turbines come with a built-in battery charger; pay attention to what is built in, as not all are equal.

Lower quality generators may not operate at exactly 60 Hz (50Hz in other countries) and can cause clocks to run slower or faster; electronics to misbehave; or UPSes to switch to internal batteries, due to low quality of current produced. You may want to consider protecting sensitive equipment with an isolation transformer.

If your batteries have multiple types of posts, it is usually cheaper and more secure to use the ones with holes in them to connect wires. Cover the car-type posts with short pieces of garden hose that are just a bit longer that the post itself; this will prevent shorting, if you accidentally drop a wrench onto the battery.

Cover battery posts with pieces of garden hose, and use shrink tubing on cables to reduce the risk of shorts.

Some battery manufacturers provide a max torque rating for screws attaching cables. Be careful and measure because you can ruin your battery by twisting the soft lead post with excessive force. Apply shrink wrap to cover as much of the exposed conductor as possible to prevent shorts.

It is a good idea to install a 12-volt outlet right next to your batteries, for instance where you connect them to the inverter, so that you can power a 12-volt tool or light directly without power conversion back and forth.

A 12-volt outlet can be used without powering an inverter. If you have power tools, it is useful to have a car charger to recharge your tools without running the inverter, which can use as much power as you deposit into batteries.

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