Difference between on-grid, off-grid and hybrid solar systems

15 mins 23 Oct 2020

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For many, going solar means environmental responsibility, eco-friendliness and total energy independence. The first two are indisputable truths, but are you really going to become independent from the national energy grid? Well, the answer is not always yes. It depends on the solar system you choose. Let's see what the difference between on-grid, off-grid and hybrid solar systems is and which one will suit you the best.

Advantages of Grid-Tied Solar Systems

Grid-tie solar systems, also referred to as on-grid, utility-interactive, grid intertie or grid backfeeding, are popular with both homes and businesses. They are connected to the utility power grid, which is necessary to run the PV system. It allows you to export any excess solar power you generate to the electricity grid, receive credits and use them later to offset your energy bill.

1. Budget-friendly

With such systems you don't need to buy batteries, because you'll have a virtual one - the utility grid. It will require no maintenance or replacements, and thus no additional expenses. Moreover, grid-tie systems are generally simpler and cheaper to install.

2. High efficiency of 95%

According to EIA data, national annual electricity transmission and distribution losses average about 5% of the electricity transmitted in the US. In other words, the efficiency of your system will maintain at about 95% throughout its lifespan. On the contrary, lead-acid batteries, commonly used with solar panels, are only 80-90% efficient at storing energy and will even degrade with time.

95%

efficiency is maintained throughout the grid-tie solar system lifespan

3. No storage problems

Your solar panels will often generate more electricity than needed. With a net metering program, designed for grid-tie systems, you can put this excess electricity onto the utility grid instead of storing it with batteries.

4. Extra income source

In some regions, homeowners who install solar are awarded Solar Renewable Energy Certificates (SRECs) for the energy they generate. SRECs can be later sold through a local marketplace to utility companies looking to comply with renewable energy regulations. The average American home can generate about 11 SRECs per year if powered by solar, which can bring around $2500 to the family budget.

SREC price can vary from $5 to $450 apiece depending on the state you live in and the supply-demand ratio.

Equipment for Grid-Tied Solar Systems

There are only two pieces of solar equipment which should be specifically designed for a grid-tied system:

Grid-Tie Inverter (GTI) or Micro-Inverters
Power Meter

Grid-Tie Inverter (GTI) synchronizes the phase and frequency of the current to fit the utility grid (nominally 60Hz). It also adjusts the output voltage so that it is slightly higher than the grid voltage, which makes excess electricity flow outwards to the grid.

Some homeowners prefer micro-inverters, which are much smaller and go on the back of each solar panel. While micro-inverters are more expensive than string ones, they are proven to yield higher efficiency rates. Those who are suspect to shading issues should definitely opt for micro-inverters.

Finally, you'll need to purchase a power meter (often called a net meter or a two-way meter) that is compatible with net metering. This device is designed to measure power going in both directions, from the grid to your house and vice versa. Note that some utility companies may issue a power meter for free, so consult your utility before buying.

Advantages of Off-Grid Solar Systems

Many homeowners prefer to install off-grid solar systems to become completely energy independent. The thing is that off-grid solar systems aren't connected to the electricity grid, so they will continue working even if the public power grid is not functioning. However, this is only achievable with the appropriate equipment and batteries.

1. Great for remote locations

If you live more than 100 yards from the grid, you should definitely consider off-grid. Such a solar system can be much cheaper than extending power lines. For example, the costs of overhead transmission lines range from $174,000 per mile (for rural construction) to $11,000,000 per mile (for urban construction).

2. Full self-sufficiency

For some people being independent is worth more than saving money. If power failures on the utility grid are a norm for your area, an off-grid system will ensure your house is powered 24/7 whatever happens around. However, to make your system 100% secure, you should install a backup generator in case your batteries run out of charge during cloudy times.

Equipment for Off-Grid Solar Systems

Since an off-grid system is not connected to the electricity grid, it must be designed appropriately to generate enough power throughout the year. A typical off-grid solar system requires the following extra components:

Solar Charge Controller
Battery Bank
DC Disconnect Switch (additional)
Off-Grid Inverter
Backup Generator (optional)

Solar charge controllers are needed to limit the rate of current being delivered to the battery bank and prevent battery overcharging. It is crucial for keeping the batteries healthy and extending their lifetime.

No energy independence is guaranteed without a battery bank, a group of batteries wired together. They store the power generated during peak sun hours for you to use it later at night or on cloudy days.

All solar systems require AC and DC safety disconnects, but for an off-grid one you will need to install an additional DC disconnect switch between the battery bank and the off-grid inverter. It is important for maintenance, troubleshooting and protection against electrical fires.

Moreover, you'll need an off-grid inverter unless you're setting up solar panels for your boat, RV, or something else that runs on DC current. Unlike GTIs, off-grid inverters do not have to match phase with the utility grid.

Finally, you should consider buying a backup generator, as it takes a lot of money and batteries to prepare for several consecutive cloudy days. Generators can run on propane, petroleum, gasoline and many other fuel types, but diesel is believed to be the best choice.

Battery-based inverters often come with the integrated charge controller

Advantages of Hybrid Solar Systems

Hybrid solar systems incorporate the best features of grid-tied and off-grid solar systems, hence the name. Despite the fact they are connected to the utility grid, they have extra battery storage as a backup.

If you have a grid-tied solar system and drive an electric car, which you charge at home, you already kind of have a hybrid system. In this case, your car is a battery with wheels.

1. Reasonable price for such flexibility

Hybrid solar systems are less expensive than off-grid counterparts, because you can economize on a backup generator and battery bank, reducing its capacity. In case of low solar irradiation, you'll be able to buy off-peak electricity from the utility company, which is cheaper than diesel.

2. Total energy control

With a hybrid system, homeowners can take advantage of changes in the utility electricity rates throughout the day. For example, you can program your home and vehicle to consume power during off-peak hours (or from your solar panels). Moreover, you are able to temporarily store excess electricity in batteries, and put it on the utility grid when the rates are the highest.

Equipment for Hybrid Solar Systems

As you might have already guessed, hybrid solar systems are based on the components needed for on-grid and off-grid systems:

Charge Controller
Battery Bank
DC Disconnect Switch (additional)
Battery-Based Grid-Tie Inverter
Power Meter

Battery-based grid-tie inverters deserve special attention, since these devices can draw electrical power to and from battery banks, as well as synchronize with the utility grid. Such inverters differ from traditional ones because the latter must shut down completely when there is a grid outage, while hybrid inverters connected to batteries can simply switch to an off-grid mode for a while and continue to power the home.