The simple explanation is this:
- Solar PV works on light, not necessarily direct sunlight, hence why solar still works here in the UK
- When it is daylight, light shines on the photovoltaic panel array and the cells within the panels create a current of electricity (DC). The stronger the light, the more electricity is generated.
- This DC current flows to an inverter which converts it to alternating current (AC) which is the electricity we use in our homes and offices.
- From the inverter, the electricity flows through a meter to your consumer unit where it is connected to the rest of your electricity system
- The energy you have generated can now be used to power any of the appliances and lighting in your home or business. It is no different to the power you would have previously used from the national grid and because the power has been used without the need for fossil fuels like coal or gas, it is clean energy and better for the environment.
- If it is night time or you are using more power than your solar PV array can generate, you will simply take energy from the national grid. This is something you will not notice, and everything will continue to run smoothly.
- Any power that you have generated that you don’t use flows into the national grid to be used by someone else.
- You receive a FIT payment for the electricity you have generated regardless of whether you use or export this energy. Exported electricity will also attract a small additional payment.[link to FIT Q&A]
- Freewatt recommends you monitor your power production and usage to ensure you are using energy efficiently and will achieve the desired return on investment.
So that's how Solar PV works - if however you would prefer a more technical explanation please see below to understand how solar power works.
The science behind solar PV
Solar photovoltaic modules are most commonly made from silicon crystal wafers. When light falls on the silicon crystals the photons of light excite the crystals on a molecular level which creates a tiny voltage across the crystals (the photovoltaic affect). Because of the way the panels are produced these tiny voltages are captured using micro-busbars and essentially wired together in series to create a greater voltage across each wafer or square seen on a module. Each wafer is then usually wired together in series to create a greater voltage for the modules as a whole. This voltage usually ranges from 25 - 45 Volts.
Modules are wired together in series to create a higher voltage of between 300-650 Volts per series or string of modules. The voltage and current of an array can be manipulated to suit the particular requirements by choosing how many modules are wired together in series (to increase voltage) or parallel (to increase current). Because this direct current voltage is quite high the volt drop is minimised. We design solar PV systems to have a volt drop of <1%. This is very important to ensure you achieve the best possible return on investment from your system. The wiring required for most arrays is only about 4mm in cross sectional area, Freewatt engineered systems always use cable sizes greater than that required as a minimum. The cost increase is negligible but the benefit over 25 years can be substantial.
Depending on the designed string voltage for your particular array it can sometimes be more cost effective to run cables over a long distance on the DC side of the circuit at 450V rather than the AC side at 230V.
The DC current from the array is fed into an inverter. The inverter will convert the current from DC to AC. The inverter performs a number of tasks and is the 'brains' of the system. The inverter will monitor the incoming AC current and voltage from the national grid supply. It will then synchronise the generated AC sine wave with the incoming supply at 50 Hz. The sine wave oscillation has to be synchronised in phase in order to not disrupt the property supply or create system losses. The voltage is also manipulated so that the generated AC voltage is slightly higher than that of the incoming supply. (UK grid voltage is already always fluctuating between 216 – 253V.) By doing this it ensures the generated electricity is used within your property in preference to buying electricity from the grid.
If all of this higher voltage electricity is not used within the property it will then be exported to the grid as there will be a higher voltage within your property compared to outside. This electricity will in essence be used by other local users on the national grid.
The quality of the generated electricity has to conform to stringent UK national grid standards. This means that there is no discernible change to your supply or for any appliances used. Any inverter feeding into the grid or grid-tied has to meet Small Scale Embedded Generator (SSEG) standard G83/1. Your inverter must therefore be G83/1 certified and should have a supporting certificate.
Another function of a grid-tied inverter is its ability to turn off or self-isolate under fault conditions. Under G83/1 guidelines an inverter must turn off if the incoming grid voltage or AC sine wave is outside of UK grid tolerances. This means that an inverter will turn off in the event of a power cut. This safety measure ensures engineers are protected from working on the national grid during a power cut. When the grid becomes live again or parameters fall within acceptable limits the inverter will turn itself back on again without the need for manual switching.
Most inverters used in the UK contain transformers but transformer-less inverters are becoming more popular.
Inverters have a defined range of operating current and voltage. The array strings will be designed so that the DC voltage and current characteristics are most efficiently suited to an inverter.
The long term performance of your system is very much dependent on how well the inverter is matched to your array. In real life applications it is not necessarily better to install a lager inverter when there are several options for a specific array of modules. For example: a larger inverter may operate at higher power or light levels but will not start working as quickly in low light. In real life a smaller inverter may give a better annual generation as it would more frequently achieve the required light levels throughout the year but only reduce top end performance on the sunniest of days in the summer.