What does it mean to be "off grid"?
The term “off grid” means just that—power and energy are generated from sources other than an electrical utility or power company. Off-grid solar systems incorporate components required for a standalone electrical power system. This includes energy generation from photovoltaic solar panels, energy storage usually in battery form and power generation utilizing an inverter/charger. Off-grid systems can also incorporate secondary energy generation from a fuel-powered generator and sometimes hydro-electric and wind power generation and more rarely fuel cell electric generators.
Off-grid systems are often the first choice for energy production where the cost of utility power connection is prohibitive or not available, such as remote homes or cabins. Small scale off-grid solar systems are also used to power remote scientific or monitoring equipment, lighting systems, even electric fences or gate operators.
Before purchasing an off-grid power system a few basic pieces of information are needed to select the proper system components.
Energy – Energy is a physical property that allows us to do work, such as heating things, moving things, producing light, etc. Energy is expressed in watts per unit time or more commonly watt hours. Watts are a measurement of power, whereas energy is power per unit time. An example of this would be lighting a 100-watt light bulb for one hour—the bulb uses 100 watts of power while it is on and if you leave it on for one hour, you have consumed 100 watt hours of energy in the process.
When designing an off-grid power system, how much energy the system needs to provide is based on all the appliances, lights, fans, pumps—any device that uses electrical power and energy to operate. To calculate your watt hours/day energy consumption, you would tally up all the devices you want to run on electric power and how long you use them in a 24-hour period. A shorthand way to do the same calculation would be to look at an electric utility bill or statement and take either the highest monthly energy figure and divide by days/month or a yearly figure and divide by 365 days/year. This yields a reasonably accurate daily energy consumption figure you can use to design a solar power system.
Power – Power is a physical property related to energy and also allows us to do work. Horsepower is a measurement of power as is watt. In fact, 1 HP is equal to 745.7 watts. When putting together an off-grid system you need to know how much power the electrical appliances might need at any one time. For instance, if you are using a microwave oven and a hair dryer at the same time, these two appliances might require a total of 2600 watts of power—1000 watts for the microwave and perhaps 1600 watts for the hair dryer. In this example you might need an off-grid inverter that can supply 3000 watts to cover the two appliances running simultaneously.
This is an important consideration when designing an off-grid system, because enough power needs to be available to run the kinds of appliances the off-grid user requires. Some appliances require very high power. For example, electric water heaters need about 4000-8000 watts and central air conditioners may require around 17,000 watts. This power figure relates directly to the size or power rating of an off-grid inverter system.
Battery Storage – Power can only be produced when the sun is shining on the solar panels, but it’s needed at all times—at night, when it’s cloudy, etc. That’s why off-grid systems require an energy storage system, using batteries. The chemical reactions that take place in batteries store and also release energy when and where it is needed. Off-grid battery system size and scale is also directly related to energy and power discussed above. If the battery system is too small, the energy will be depleted too quickly.
Traditional battery storage for off-grid systems has been based on lead acid batteries—a technology that has been around about as long as there have been batteries—but more and more off-grid power systems are now using lithium battery technology. Which battery chemistry you choose involves design, environment and budgetary considerations.
Load Management - When on an off-grid power system, sometimes the limitations of the system require you to use energy and power differently than if you were getting your electric power from a utility grid. Depending on the size of your inverter, you may want to shift appliance use so you don’t run out of power from the off-grid inverter. For example, you might want to run the dishwasher now but wait to wash your clothes until after the dishwasher cycle ends. You might also want to run some appliances like an air conditioner when the sun has fully charged your batteries and you still have sunshine on your solar array. The batteries can remain essentially fully charged and the solar panels will run the air conditioner purely off sun power.
Backup Power – Increasingly some off-grid systems are being incorporated to run essential loads when utility grid power is unreliable. A small off-grid based system can be integrated into a utility grid-powered home to run appliances like refrigerators, lighting, heating and air conditioning, medical equipment, communications equipment ,etc. that will get you through a power outage. This off-grid system is not directly “grid-tied” but CAN use power from the utility grid to supplement energy generation and storage.
How do I select the panel that’s right for me?
Most often selecting an effcient and cost effective panel (sometimes called a module) is going to be the best solution for most off-grid applications. A typical full size residential solar panel measures roughly 40 inches wide by 66 inches long and is often composed of 60 (or 120½ cells) cells per module. Power ratings for panels of this size range from 300 watts to 375 watts, and in some cases even more power is possible. The output power is often driven by effcacy. Nonresidential size modules below 300 watts are manufactured in much smaller numbers and are usually more expensive (meaning they have a higher price per watt). More powerful panels (350-500 watts or more) are typically used in commercial power generation systems. These panels consist of 72, 96 or even 120 cell modules. Bi-facial panels are modules that can absorb energy from both sides. These panels produce extra power and energy from reflected light that bounces off light-colored roof surfaces such as snow. Consider a panel that’s going to provide the power and life your system will need.
What are some of the different ways to mount panels?
Solar panels require a safe and secure method of mounting so they can harvest power and energy from solar radiation and withstand environmental conditions like high winds and heavy snow loads. Take care to select a mounting setup or combination of mounting options that will provide adequate solar exposure for your system.
A common way to mount solar panels is to put it on the flat surface of a building’s roof. Roof mounts use parallel rails secured to the roof system with feet secured to roof trusses or cross members with the solar panels set on top of these rails and secured with a clamp-type system. Roof mounting solar panels has the advantage of using existing flat roof area. However, roof mounts may not optimize the solar panel angle, and thus reduce the potential energy production of the array.
Top of pole mounts use a gimbal that is attached to the top of a vertical steel pipe. The solar panels are then secured to several rails that are attached to the gimbal. Top of pole mounts can attach one panel or up to as many as 12 solar panels on a single pole. These mounts can be custom fabricated to suit optimal tilt of the solar panels, from completely horizontal to nearly completely vertical. Top of pole mounts are more easily cleaned without the need to climb a roof and they also shed snow very effectively.
Ground mounts involve a lattice of vertical and horizontal steel poles with the solar panels secured to parallel rails which are usually aluminum. The panels are arranged row and column style and the entire array can be angled towards the southern sky, optimizing energy production. Ground mounts can be cleaned easily like top-of-pole mounts and can also be cleared of snow more easily than roof mounted arrays. These mounts have a limited tilt range. They are a cost effective solution for very large solar arrays, with the only limitation being the available ground space.
How do I choose an inverter?
IInverters used for off-grid or standalone applications are designed to work independently of the electric grid. These inverters provide maximum independence from the grid or utility company and allow a solar system to be active regardless of the presence or status of the grid. While these types of inverters do not require a connection to the grid, they do require a connection to a battery bank in order to operate properly. Most of these inverters do not accept solar power directly like a grid-tied inverter, but rather they rely on an external charge controller (separate component) to regulate energy flow from the solar panels to the battery bank. The off-grid inverter then discharges the battery bank to power your loads. For off-grid type applications, things like output wattage, surge capacity and sine wave type are factored in when choosing an inverter as this is often the primary source of AC power for that system.
Hybrid (Grid-Tied with Battery Back Up)
A hybrid inverter, also referred to as grid-tied with battery backup, incorporates the best of both grid-tied and off-grid type inverters. A hybrid inverter can sell excess solar production back to the grid like a grid-tied inverter (if grid connected) and can also easily be connected to a backup battery bank like an off-grid inverter. Some hybrid inverter systems are all-in-one, while others are configured with individual components (i.e., separate inverter, charge controller, battery monitor, etc.). There are some hybrid inverter options which do not require batteries to operate, meaning the system can start as a simple grid tie and then batteries can be added later on (this will not work if there’s no grid). This is usually the preferred inverter type for anyone that wants battery backup during a grid outage. It’s also what most people use for things like peak-load shaving, selfconsumption and time of use. For some applications, these inverters can also be used for off-grid since they are also battery connected inverters.
Inverter Recommendations for Modern Systems
Inverters can be found with two types of output, pure sine wave and modified sine wave. For most modern appliances, pure sine wave is going to work best. Pure sine wave inverters have a wave form most similar to the grid.
Off-grid inverters usually have fixed battery input and output voltage range specific to common nominal battery voltages (most commonly 12V, 24V and 48V). Input voltage considerations should be considered during the initial design, as most off-grid inverters cannot change or adjust their input voltage. Most projects are best suited with 24V or 48V inverters.
The AC output voltage of the inverter is just as important. In North America most household appliances use 120V AC. Some larger appliances like dryers, HVAC, water heaters, etc. require 240V AC. All off-grid inverters output 120V, but some can output 240V to power heavier loads. Also known as a split phase, there are off-grid inverter options capable of outputting 120/240V AC so that loads requiring either voltage can be operated, just like in most grid-tied homes. Some inverters which are only capable of 120V output can be stacked with another unit to produce 120/240V. In most cases, inverter AC output voltage is also fixed, so if heavier loads may be added in the future to an otherwise small starter system, a split phase inverter can make things easier later on.
Most off-grid inverters are actually inverter/chargers, meaning they can utilize an external AC power source for pass through to loads and/or battery charging during times of high consumption or insuffcient solar potential. A generator back up is highly recommended for most off-grid applications, and thus it’s also a very important consideration to make sure the system incorporates an inverter/charger. The inverter/charger can pull power from the generator and use that to run the loads, plus charge the batteries if they are low.
Sizing an Inverter
To size an off-grid inverter, you should consider the unit’s output power rating and sure capacity. Calculating the maximum continuous AC load for the system is a good starting point in choosing the right size inverter. One very important note regarding off-grid inverters is that their power rating has nothing to do with the size of the solar array or battery bank. It is common to oversize an off-grid inverter past the initial demand for future use in case you think your power needs might increase in the future. If going the inverter/charger route, then it’s especially important that the unit be compatible with whatever type of battery is used in the system.
What battery types are there?
The renewable industry is dominated by three common battery types. Flooded lead acid batteries are lead batteries with a liquid electrolyte solution. They have a low upfront cost but require various levels of maintenance and can be sensitive to abusive high demanding applications. Absorbed glass mat batteries (or seal lead acid) are lead acid batteries with the electrolyte absorbed in a fiberglass mat. These are sealed recombinant batteries—they do not require maintenance nor do they off gas hydrogen. However, they are sensitive to over discharge and partial state of charge just like flooded lead acid batteries. Lithium batteries have an obvious high upfront cost, but they are extremely safe and offer the best levelized cost of all solutions for most RE applications. They are extremely resilient to abuse, can accept fast charging/discharging and aren’t affected by partial start of charge. They also can be expanded over time, which is ideal for most dynamic growing applications.
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