Home Solar Power

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A solar inverter (photovoltaic inverter)  is a type of electrical inverter that is made to change the direct current (DC) electricity from your solar panels or wind turbines into alternating current (AC) for use with home appliances. Some inverters are designed to be connected to the power utility company’s grid.

Three Basic Types Of Solar Inverters

Stand-alone inverters: Used in independent solar energy systems or any energy system that is completely off the grid. The inverter is designed to draw  DC energy from batteries charged by solar panels or wind turbines, and change it to AC power.

Most stand-alone inverters also incorporate integral battery chargers and charge controllers to replenish the batteries. The charge controller regulates the input from the solar panels, regulates the battery output, and handles charging the batteries. Normally these do not interface in any way with the utility grid.

Grid Tie Inverters: Many solar inverters are designed to be connected to a utility grid and they contain special circuitry to precisely match the voltage and frequency of the power supplied by the utilities grid.

The inverter takes the electricity generated by your renewable energy system and sends it to the power distribution panel, from there the power may be used by  appliances within your home, or if not needed it will redirected to the utility grid.

This redirected energy is used by the other utility customers, and you receive some form of compensation for putting excess power into the grid. When there is no energy generated, utility power is pulled from the grid to provide power to your home.

Grid-tie inverters are designed to shut down automatically for safety reasons as required by law, upon the loss of the utilities power supply to protect the utility workers who are repairing the system.

Battery Backup Inverters: These are special inverters which are designed to draw energy from your battery bank, manage the battery charge via an on board charger and charge controller. The DC power is converted to AC power for your appliances and they export excess energy to the utility grid.

Unlike a standard grid tie inverter, these inverters are capable of supplying energy to your home during a utility outage, and are also required to disconnect from the grid during power outages.

No matter what type of system you use, an inverter is an integral part of it.

Here is a video, keep in mind that a small home system will be less complicated.

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A “grid-tie” solar energy system is designed to sell power back to the utility company and can be used with or without batteries.

Batteries Or No Batteries

If you are using batteries the power generated by your solar panels will be used to charge your batteries and any left over electricity generated by your  solar power system is directed onto the power grid. It adds to the overall total of kilowatt hours that can then be used by someone else.

With a battery less system if the grid supplied power goes out due to a storm or other malfunction, the user has no power for lights, furnace, refrigerators, etc, which could be powered by batteries for a limited amount of days.

There is a difference in cost between a battery less grid tie system and one with battery back-up. A battery-less system requires an inverter and a solar array or other renewable power source.

In addition to these two components, a battery grid-tie system requires several batteries, a charge controller for efficient battery recharging, breaker panels, circuit breakers, and enclosures to house the components.  A grid-tie system with battery back-up adds 10%-20% more cost over a stand-alone grid-tie system.

If you are not using batteries, any and all left over electricity generated by your residential solar power system and not used in your home, is directed onto the power grid, where it adds to the overall total of kilowatt hours available, and can then be used by everyone.

These systems will offset your utility usage, with the correct size system it will earn you a credit during the day that you would consume at night. These systems are easy to install and since some do not have batteries for back-up, the lack of batteries in these systems means no battery maintenance or replacements to worry about.

Grid-Tie systems are part of your overall solar system, the number and type of solar panels will determine how much energy you can produce, as will your geographic location. The DC power generated by your panels goes into an inverter.

Inverters

Inverters work by taking the DC power from the source, such as an array of solar panels or wind generators and converts it to AC power so it can be used by your appliances and fed into the grid.

The inverter must also synchronize its frequency with that of the grid (e.g. 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage.

Grid-tie disconnects allow you to stop the flow of electricity between your solar power system and your electrical system. This provides for the safe maintenance of electrical and utility systems.  Grid-tie disconnects are also designed to quickly disconnect from the grid if the utility grid goes down.

This is an NEC requirement that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy it produces from harming any line workers who are sent to fix the power grid.

Selling Power Back To Your Utility Company

Ideally you want the utility company to buy back any excess electricity that you produce at the same retail rate that you buy electricity from them at. This is called net metering and is the simplest way to setup a grid-tie solar system.

In such a system you only have one utility kWh meter and it is allowed to spin in either direction depending on if you are buying or selling energy. If your solar power array produced enough electricity, your utility meter would begin to run backwards, and you earn credits on your electric bill.

In a non net metered system, the utility company will require that you install a second kWh meter to record any excess energy that you sell back to them and they will only pay you the wholesale rate.

To find out if your state offers “net metering” or any other incentives for installing a renewable energy system, Click Here.

How Much Storage Is Needed

If you are having a system installed, buying a kit, or just making your own home built system and you plan to store electricity you will need to know how large of a capacity you need.

For those of you planning to build your own, I recommend buying the earth4energy manual, it is more detailed than this article, and along with basic panel construction gives you sources of inexpensive or even free batteries, and how to wire them for your needs.

Many renewable energy systems like solar panels or wind turbines incorporate batteries as part of the system. They are not mandatory as you can simply sell your excess power back to the electric company when your solar system is making more power than the house can consume.

Getting Off The Grid

A grid tie system can be used to connect you to the electric company and is used to off set your electric bill. The main advantage of a grid tie solar system is there is no need for a solar battery bank. The disadvantage is having to buy power on cloudy days.

But to get completely off the grid you will need batteries, the energy stored in the batteries can then be inverted from DC to AC to power your home. The batteries recommended for doing this are deep cycle batteries. To ensure you have enough reserve capacity to provide the electricity you need, you will need to size your battery bank properly.

How Much Energy Do You Use?

Various conditions affect the battery bank size. The first thing you’ll need to know is the amount of energy that you consume. This involves the different applications you use, appliances, electronics, etc, and how much power they consume per day.

Keep track of this information on a list, you will need this list for sizing other components of your entire system as well. Your final tally should be expressed in Watt-hours (Wh) per day.

Look at all the applications you use and find the listed watts, this is time consuming but it has to be done. You take the power used, times the amount of time it is used, to get the total kilowatt hours (kWh) per day. Once you know the kWh per day just multiply that number by 1,000 (kilowatt hour=1000 hrs) to determine the Watt-hours per day. Example: 4.8 kWh = 4,800 Wh.

A helpful site that lists averages and has a handy calculator to do the work for you is on the Alternate Energy Resource Network.  If you can only find the amps or volts on your applications it will convert them too.

Temperature Considerations

Battery life and capacity are affected by temperature, batteries perform best in moderate temperatures. Plan your storage space accordingly. The temperature standard for most battery ratings is around 70°-80% F.

While you do not have to make this exact try to be close and avoid extremes. Cold temperatures will reduce battery capacity while high temperatures tend to shorten battery life. It is highly recommended to find a way to store your batteries properly.

How Many Days In Reserve

Next, you must determine the how many days of battery back-up that you want to have on hand for cloudy days, etc. In practical terms around three days worth will give you the power you need without an unduly large system.

How Much Discharge Of The Batteries

It’s recommended that you never discharge your batteries below 50% of their capacity, some battery manufacturers recommend even greater discharges, but a battery’s life can be shortened with frequent discharges of more than 50%. The lower you can keep it the more you will will extend your batteries life.

The Calculation

Identify total daily use in Watt-hours. As an example say 8,000 WH a day

3 days storage 8,000 x 3 = 24,000 Wh

How much discharge you will allow, converted to a decimal value. Divide total by this value.

40% discharge 0.4
50% discharge 0.5
60% discharge 0.6 and so on.

Example 50% discharge: 24,000 / 0.5 = 48,000 Wh

(optional cold weather calculation)
60 1.1
50 1.2
40 1.3

Multiply your results from by one of these factors.

60° F. = 1.1 48,000 x 1.11 = 52,800 Wh

48,000 Wh compared to 52,800 WH, a difference of almost 5,000 WH.  From this you can see the adverse effects of cold and the amount of storage you will need.

What Is Your Systems Voltage?

You need to know this, it is usually 24V or 48V. One typical panel will put out 12 volts, more panels wired in series or parallel increases this to 24 or 48 depending on how they are wired, this is one of the considerations when you plan and install your entire system.

Divide results by system voltage. Result is the minimum Amp-hour (Ah) capacity of your battery bank.

48,000 / 48 = 1,000 AH
48,000 / 24 = 2,000 AH

You now know the Amp hour capacity that will give you the storage you need.

Batteries

The batteries you need must meet both your system voltage requirements and the AH capacity you calculated. For example if you want 1,000 AH, you could have four 12V batteries in series (48volts), each with a capacity of 1,000 AH or more. Or eight 12V batteries wired in two parallel series (48 volts each) and so on.

There are many options to meet the amp hours you need, consult with your battery supplier about what options are available and best for you.