grid-tie and off-grid

Choosing between grid and off-grid

“Grid” and “off-grid” are the two main types of solar powered system. In addition there is a “grid-interactive system” and a lesser known “grid fallback system”. The naming- “grid”, “off-grid” would be able to suggest you something about their meaning. Now let’s clear up the confusion if any.

Grid connected setup

This setup starts with the sun over the roof illuminating your solar panels, the solar cells in the solar panels convert the light energy to electrical energy. In this setup you own a solar panel system which is installed over the roof and have it connected to the grid power. The reason it is done this way is because you can use the solar energy in the morning or whenever the sun is available and during the night time you can use the grid electricity.

The grid electricity is AC(Alternating Current) which is of high voltage and low current. The voltage can be between 100-240V(high voltage) depending on the country you live in. The low voltages are necessarily as low as 12V-24V.

1. Inverter

The electrical energy produced by the panel is DC (Direct Current) in nature, but the electrical energy used by the wall socket is AC in nature. Hence there is a need to convert DC to AC, the inverter takes care of this part of  the conversion.

An interesting thing about the “alternating current from the wall socket” and electrical appliances is that, some of the commonly used electronics like laptop require “direct current”. But our input is alternating current, won’t it harm the laptop? The small box-like thing which comes with the charger wire connecting the wall socket and laptop plug-in port, finally has some use other than being a paper weight.

The box converts the AC to DC again and reduces the voltage to around 12V to keep the laptop safe. The box contains rectifier– the opposite of inverter(since it converts AC to DC) and a step down transformer to reduce the voltage..

Today’s inverters have an efficiency of around 95% meaning, the energy loss in converting the AC to DC is 5%.

The inverter is linked to monitors which keeps you informed of the System’s performance. The inverter is also linked to a meter to measure the energy generated(in kWh) for the calculation of  profit by the use of solar system.

2. Fuse Box

The electricity from the inverter passes through the fuse box and is available to the electric appliances in the home. The fuse box is also connected to the national grid.

3. Grid

Any surplus electricity the system produces is sent to the grid, which is distributed to other users in the grid. This whole process happens when we have Sun over our head. Then what about night, when we don’t have our source?  In the night since we have our home connected to the grid, we start using up the electricity from the the grid. Therefore, in the morning the system produces, in the night we use from the grid.

Any difference in the electricity we produce in excess in the daylight with that of what we use from the grid in the night will be profit we earn(Price/kWh)- available only for the countries which have “feed-in tariff” scheme.

Downside of  grid-tie setup

In grid-tie setup, during power-cuts the house power supply is hampered(even the solar power). The reason being, any power produced by the solar panel during the power-cut, when fed back into the grid may prove fatal to the workers trying to fix the problem. That’s a disadvantage of grid-tie setup.

Off-Grid setup

Off-grid setup or standalone setup can have different variations of implementation, I’ll provide you with the more informative setup. The significant difference between grid-tie and standalone setup is that standalone contains a battery bank to store the energy produced and is not connected to the grid whereas grid-tie do not have battery, but is connected to the grid.

The off-grid starts with the same note as in grid, where the sun illuminates the solar modules,electrical energy is produced.

1. Combiner Box

The solar modules are connected to the combiner box, to output the correct system voltage.

 2. DC Disconnect

This setup takes care of system shut-off with circuit breakers(commonly called fuse) and ground-fault protection. All parts of the system gets disconnected on a code. The batteries, collectively called as battery bank which are connected to the DC disconnect, get charged up in the day and can be used in the night or on cloudy days. The regulator connected to the DC disconnect,  prevents the over-charging of the batteries connected to it.

3. Inverter

Inverter converts the on-coming DC input to AC output to supply the household electrical appliances. It works the same as said in the grid-tie system. Generator can be connected to the inverter if necessary.

4. AC Disconnect

AC input from the inverter and generator( if used) are combined here using circuit breakers.

5. AC Panel

Electrical loads are connected here for use.

Off-grid setup is suited for people in rural areas, where connection to the grid is improbable or expensive.

Downside of  Off-grid setup

 Going completely off-grid wouldn’t be the recommended choice until you have the excess solar panels to power the home for the whole year.

In the months of summer, a home with adequate solar panel will work like a charm, but those solar panels may not be sufficient enough to sustain in the winter. In grid-tie system, you can be sure to watch the Christmas special episode of your favorite TV show.

The second down side being any excess power produced can’t bring you a fortune, since the setup is grid free and doesn’t support the “Feed-in tariff” scheme. But few countries such as Australia can benefit, since they have the Feed-in tariff scheme available to off-grid systems.

Grid-interactive setup

In this setup, our system is connected to the grid, as well as contains a battery bank.

Connected to grid- solves the problem of inadequate power supply during the winter.

Contains battery-This setup solves the problem of power interruption during a power-cut. The energy produced by the solar panels continue to power the home during power-cuts with the help of the battery. This setup contains “protected circuits” which continue to power the house during power outage.

If your area of residence is prone to extended periods of power outage, designing a generator into the grid-interactive system is an option.

This setup can produce fortune by selling the extra units produced by the solar panel to the grid, it qualifies for the “Feed-in tariff” scheme.

Downside of Grid-interactive setup

The investment costs of this system is higher than any other system. The extra battery bank and charge controller needed adds up 12%-20% of the cost of a standard grid tie system.

Grid-fallback setup

This setup is similar to grid-interactive system, contains battery and is connected to the grid. The solar panels charge the battery. When the battery is fully charged, the power from the battery is used until it is emptied, after which the grid utility is used to power the house again. The solar power is connected to only the essential appliances.

Battery provides the energy-> battery is used up-> grid provides energy-> solar array powers the battery-> battery provides the energy happens in a cycle.

This setup is the most environment-friendly way to solar power a house. It’s cost-efficient too, this complete setup costs less than $700.

Well, every setup has it’s own downside, it’s a matter of perspective and the purpose of the solar power which helps in taking the right decision.