interactive solar

The Art of Solar, Selecting the Right PV System for Your Home or Cabin

by Cliff on 01/17/2011

The fun part of solar design is helping customers figure out which solar system is right for their situation. To that end, we are going to start a series of basic solar information articles to help families and business owners start thinking about going solar. Once you start to understand some of the basics of solar, it becomes easier to make the switch and start putting more money in your household budget each month.

Let’s start out by the toughest solar system to design which is a utility connected solar system with a battery backup or utility interactive solar system. A utility interactive solar system is not only the most complicated, but it is also the most expensive. Why? Because you not only need the equipment to interact with the utility company but also a battery bank. All of that needs to match up with the family’s expectation. Battery interactive solar systems are popular for homeowners and small business where a back-up power supply is required for critical loads such as computers, refrigerators or water pumps.

The art of solar comes into play when we are working with clients on a battery interactive solar system first defining realistic load profiles. Load management is a function of the owner more than the solar company because not only must they decide what is important to power with the grid goes down, they must have the understanding and discipline to control energy demands.

You can begin that process by helping the home or business owner to choose what is important to run by only solar power. Another way to look at it is, when the grid goes down, does the customer want to run the entire household electrical demands, or would they settle for a small percentage of the power to get by on. Not too many people want to admit how much TV their family watches which is why education on battery demands couple with solar product is so important for a utility interactive system. Utility active solar system designs starts with the customer deciding on a critical load or how much power “can” they live with. Key here, don’t overpromise.

Battery based utility interactive solar systems start like any other type of home solar installation, but that is where the similarities end. The power that comes off the solar panel goes into an interactive inverter that is not only monitoring the solar panels but also the battery system. Battery interactive solar systems have many of the components of grid-tied and off-grid solar systems, but they operative by servicing the on-site loads, then send excess power to the utility grid while keeping the backup batteries fully charged. When the power goes down, it switches to draw electricity from the batteries in a fraction of a second.

Battery based interactive systems consist of solar panels that produce electricity, an inverter that monitors the grid and battery bank, a charge controller that prevents the batteries from overcharging (a bad thing), a set of batteries to store the power and last but not least, disconnects and critical load breaker box (s).

Mounting the solar panels

One of the really cool things about solar is that it is so versatile. You can mount solar panels on a pole on the ground like in the top view, put them on your roof which is the most popular, or put them on the top of a tree if you so desire (though not recommended).

Charge Controllers

Battery interactive solar systems are wired just a little different that regular grid tied solar. With a battery interactive solar system the solar panels are a slave to the batteries first, and then power the inverter which powers the house. With a grid tied system they just connect to the inverter.

Solar panels are wired to a charge controller that makes sure the battery bank gets fed the proper amount of electricity at a rate they can handle. You mess up this part, and you can damage the batteries which are very pricey. (Broken batteries also need to be match when replaced.) The most popular solar battery storage is a flooded lead acid battery. The batteries have vented caps which discharge hydrogen during charging and though all batteries must be vented, these need it the most. Absorbed glass mat (AGM) batteries are just like flooded lead acid batteries, except the electrolyte is being held in the glass mats, as opposed to freely flooding the plates. For the purpose of this information, I am going to keep battery selection simple but there are many more choices.

Inverter

It is often said that the solar inverter is the brains of the solar system. An inverter changes low voltage dc (Direct Current) into 120 volt ac (Alternating Current) for regular household appliances. Inverters can range from the cheap Black and Decker from local hardware store, to very expensive and popular Outback inverters with built-in battery charger and automatic transfer switch for connecting to the grid or a generator.

Fuses, Breakers, and Disconnects

For safety and code compliance there needs to be a way to disconnect the solar panels and utility power source and isolate the components.

Battery Monitor

You need a way to monitor the battery performance which can also give you some sense of security and peace of mind. No matter how much energy you have stored in the batteries the homeowner controls when it runs out and it can always run out.

How Much Will A Battery Backup Utility Interactive Solar System Cost?

The simple answer is a lot of bucks. It is not uncommon for a grid tied system to have a return on investments in 8 – 9 years with a 15% payback. If you are motivated to purchase a battery backup grid interactive solar system return on investment is not your motivation. These types of solar systems can cost you $14 – $16 a watt. A regular grid-tied home solar system installed by licensed solar contractors will run you around $6.50 a watt and up.

One last tip. Always use properly licensed solar installers for your safety and protection. Having said that, if you have the money and are motivated to be ready for any power outage emergency, a grid interactive battery backup solar system may be the right move for you.

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