FAQs

Solar (photovoltaics) cells are small electrical devices that can convert the energy from solar radiation into usable electrical energy. You can think of them as modules, which a solar panel on your roof consists of. They are connected in a specific way to obtain desirable electrical parameters of the photovoltaics panel, such as voltage, current and power. They produce direct current (DC), which later can be used to power various DC devices such as LED lighting and mobile phone batteries, or can be converted to alternate current (AC) by solar inverters and used in households.

Solar cells are most commonly made with silicon which is doped with various elements such as selenium or germanium to increase their efficiency. Depending on the silicon structure we can divide solar cells into three main types:

• mono-crystalline
• poly-crystalline
• amorphous

In mono-crystalline cells, silicon atoms create just one uniform crystalline structure. They are the most efficient solar cells on the present market, but also the most expensive ones. They are the most common types of solar cells used in the energy industry.

Poly-crystalline solar cells consist of many different smaller silicon crystals which are connected to each other. Their efficiency is slightly lower than in the mono-crystalline cells.

Amorphous cells are the least efficient, although the cheapest solar cells out of the most commonly used. The silicon within the amorphous cells does not form crystalline structures.

The basic components of a solar cell are two semiconductors – a positive (p-type) and a negative (n-type) one. On top of the cell, there is a negative electrode that holds negative charges and on the bottom of the cell, there is a positive electrode that holds positive charges.

Solar cells work on the principle of a phenomenon called the photovoltaic effect. In simple words, when light strikes the solar cell, it gets absorbed into the structure causing electrons in the cell to move from the crystalline structure of the semiconductor to the top negative electrode. The electron holes (positively charged “holes” which form when the electron leaves its place) move to the bottom positive electrode. This results in a potential (charges) difference between the top and the bottom of the cell which is called voltage. After connecting both of the electrodes to a DC device, an electric current starts running through the connected wires powering up the connected device.

An off-grid solar system is a photovoltaic system that is not connected to the electrical grid. The system can generate and store energy on its own and provides it for further use by the consumer. As it is not connected to the grid, the energy generated by the solar panels needs to be stored to provide electricity to the user at all times (even at night when photovoltaic modules do not generate any energy).

The most basic components of an off-grid solar system:

Electricity generator

The role of the electricity generator in an off-grid solar system is fulfilled by the solar panels, which can generate electricity from the Sun.

Charging & discharging controller

In order to prevent the battery from overcharging and over-discharging, a solar controller must be used. It controls the charging voltage of the batteries, battery level and the voltage and current parameters of the solar panels so that they can provide the maximum power output that is possible.

Energy storage

The most common energy storage solutions consist of battery packs (mainly Lithium-Ion, Lead Acid or Gel batteries). They are used to store the energy generated by the solar panels to use it when solar panels do not generate enough of it.

Load

All of the electrical appliances that use electricity in the system are called load. Load is also referred to as the sum of energy that is consumed by the appliances.

Inverter

An inverter is used in off-grid systems that require AC Voltage output. It converts DC voltage to AC voltage which then can be used in houses in the same way as the grid electricity. They are most commonly used in households or summer cabins, where electricity is provided to the electrical sockets in the form of AC voltage. In off-grid systems that run solely on DC voltage (e. g. solar lighting systems) an inverter is not necessary.

Energy meters

An energy meter might be added to the system to measure the energy consumption of the system. It is not an obligatory component in off-grid systems.

Why off-grid solar systems?

Off-grid solar systems are commonly used in solar lighting or solar pumping locations where it is difficult or inconvenient to run an AC power cable to. They are a great source of power for sign lighting, as they generate and store enough energy to enable lights to operate at night without inconvenient cable connections.

These systems are perfect for homeowners that live in remote areas and the grid is far away from their households. Everyone can generate electricity on their own without having to connect the house to the electrical grid, which in some cases might not be possible. They are commonly used in summer cabins, as it is easier and more cost-efficient than connecting the cabins to the electrical grid.

They make a phenomenal solution for campers, yachts and other means of transport that need electricity but are mobile, and connecting them to the grid would be difficult. Solar panels are even used on the International Space Station, space rovers, and space shuttles as they can generate power in space or on other planets!

The electrical grid is a set of wires, devices and installations. It is used to transmit, process, and distribute electricity generated at power stations and supply it to customers.

The electrical grid consists of:

• Power stations. Power stations are industrial plants that produce electricity from different primary energy sources, such as coal, natural gas, and oil.
• Electric power transmissions. Electric power transmissions carry electrical energy over long distances (high voltage).
• Substations. Substations transform voltage from high to low, or in reverse.
• Electric power distributions. Electric power distributions provide electricity to individual customers (low voltage).

Electrical grids are designed to supply AC current to their customers at largely constant voltages and frequency. In the United States, the nominal voltage and frequency of power delivered to customers are 120V and 60Hz. In Europe, the nominal voltage and frequency of power delivered are 230V and 50Hz.

The phrase 'you can't manage what you don't measure' is a good introduction to why proper energy metering is a crucial step for an efficient energy management system to work successfully. If you want to save money by decreasing your energy bills with the help of proper energy management, you will have to collect data. This data will tell you more about your household's profile in terms of energy consumption.

Energy metering is more than just a tool that allows electricity suppliers to provide their customers with bills at the end of each billing period. Traditional energy metering devices have to be supplemented with a smart energy meter combined with data loggers, and a proper gateway which set up all together allows the successful withdrawal of the data. This is the foundation of the next step - the energy management system.

Without the right hardware and its configuration meeting the requirements of a particular household's energy profile, it would be impossible to establish the right strategy eventually leading to significant savings on the energy bills.

Net energy metering (also referred to as NEM) is a billing mechanism that allows a customer to store the energy generated by their solar system in the electric grid. This approach lets the consumers use this energy when their solar panels are underproducing by withdrawing it from the energy company at a reduced rate.

Utility companies allow their customers, who produce energy with solar systems, to store their excess in a Virtual Energy Storage System (VESS). This is a tool, which lets them combine that factor with the energy consumption in the net metering. It lets customers store the produced energy in the form of a credit to be used when their household is not producing it.

The amount of energy in the Virtual Energy Storage System is calculated on a monthly interval. At the end of a month, the amount of credits is reset and can be build up again over the next month period. This allows customers to use the power generated from their solar system during the day, at night when the sun goes down.

The credit of the produced solar energy is stored for 12 months. At the end of that period, the amount of the energy stored in the grid is lost and the calculation of the credits resets. This approach allows a customer to use the surplus of solar energy generated over summer during less sunny winter months (withdraw the energy).