Solar PV and Battery Storage
Hopefully, this guide will help you to better understand Solar photovoltaic (PV) and Battery Storage systems. Please note that manufacturers' products will be different and therefore it is always recommended that you check the specifics of any system that is designed for you.
What is Solar PV and How Does it Work?
Essentially, energy from the sun is captured and converted into electricity that we can then use in our homes. Any surplus energy can be stored in a battery which can be used later. In other words, solar energy is captured from the sun, and we then use this energy in our homes, if there is any surplus energy, this is stored within a battery. Additionally, when the battery becomes full, energy can be exported back to the national electricity grid.
Electricity in Your Home
Electricity is what we use to power our electrical items within our homes. The system that we use in the UK is called alternating current (AC) and in a domestic house it is distributed at 230 Volts (V); so that’s 230V AC. The energy captured by the solar panels is direct current (DC), which needs to be converted into AC so that it can be used in our homes (more on this later).
All our appliances (that is, anything that you plug into a socket) will have a power rating and is measured in Watts (W). For example, an average kettle will use 2000W, or 2kW (1000W is equal to 1kW, this is the same as grams (g) and Kg, the units we use with weight).
Anything that heats up uses more electricity than items that do not. So, a kettle, toaster, microwave, cooker, washing machine, tumble dryer, or hairdryer will use more electricity than say a TV, laptop, Wi-Fi router, or desk lamp. If you look at the appliances in your home, they will contain a label (or in the instruction manual) that will give you the power information. It is a good idea to make a list of all your appliances and their power ratings as this will help you work out how much power you use at any one time.
Let's discuss the difference between kW and kWh. This is important, especially when working out your electricity bill.
kW is a measurement of power.
kWh is a measurement of energy used over time measured in kW.
Let’s use the example of the kettle again. Our kettle has a power rating of 2kW, so if we turn on the kettle and left it for 1 hour, then we have used 2kWh, i.e., 2 kilowatt hours. However, if we only turn the kettle on for 30 mins, then we only used 1kWh.
If our TV is rated at 200W, then it would take 5 hours to use 1kWh
We are charged per kWh of electricity that we use. If you look at your electricity bill it will tell you what you pay per kWh.
Our electricity costs 35p per kWh and we have used 6kWh in one day.
That is 0.35 x 6 = £2.10 per day.
By breaking down what appliances are used and what the power rating of each appliance is, you can work out how much they cost to run. You can work out which appliances cost the most and then try to change how you use them to reduce your electricity costs.
Solar PV System Components
This list is by no means everything but it gives you a good idea of some of the main parts used.
Panels - These are used to capture the energy of the sun. They come in different sizes and are all rated in Watts. So, a panel can be rated anywhere from 300W to 500W. The higher the wattage, the bigger the panel. If you have 10 x 375W panels on your roof, this would be a 3.75kWp system. The ‘p’ in these units (kWp) stands for ‘peak’. This means that this system is a 3.75kW system under ‘peak’ conditions (that is when the sun, temperature and weather conditions are all optimum). In reality, this system will generate less than 3.75kW, which is why solar systems are often oversized to compensate for this.
Mounting system - This is what the panels attach to. A mounting system can typically be fixed to any type of roof or even on the ground. We only use MCS approved mounting systems in our work.
Inverter - The energy captured from the sun by the solar panels is DC. However, AC electricity is used in our homes. Therefore we must convert this captured energy. The inverter converts the captured DC electricity into useable AC electricity. The standard inverter size is 3.68kW, which can be installed without any prior permission from the electricity network. Bigger inverters are available but would require permission beforehand. The 3.68kW rating is how much energy the inverter can convert from the panels. So, if you have a 5kWp system on your roof, only 3.68kW of this can be used at any time. It is ok to install systems larger than 3.68kW as the generation from the panels will change daily and remember that the 5kWp system will only generate 5kW during peak conditions.
Battery - This allows any excess energy that is captured by the panels to be stored for later use, or to supplement the electricity used within the home if the generation from the panels is poor. Some batteries can offer an emergency power supply (EPS) in the form of a socket which can be used in the event of a complete grid power outage. You can plug in your Wi-Fi, TV, fridge or freezer. Some batteries may offer a complete backup to your home in the event of a power outage. These batteries will be able to power your entire home if there is a power outage.
It is important to remember that each battery system works differently. It is wise to decide what kind of battery service you would like to purchase. Do you need a full house backup in the event of a power outage? Would an emergency power supply socket be enough? How often do you have power cuts in your area? A standard battery installation will not provide backup power to your house, this is a more specialised installation.
Solar PV - Example 1
We have a 4kWp system on our house roof with a 3.68kW inverter. Alongside this is a 5.2kWh battery storage system. The system is currently generating 1.8kW (this is the amount of solar energy captured by the panels and passing through the inverter) but only 1.2kW is required (the property is only using 1.2kW). Assuming the battery has available capacity, the surplus 600W from the panels is stored within the battery.
The sky now becomes cloudy and the generation from the panels drops to 1kW. The property is still only using 1.2kW. The additional 200W of power now required is drawn from the battery. During this period, no electricity has been purchased from the national grid.
The battery can provide the surplus power that the house requires until it is fully depleted. Once this happens, any extra power that is not provided by the panels is purchased from the national grid.
Solar PV - Example 2
The battery is 50% full when a power outage occurs. A 5.2kWh battery at 50% capacity will contain approximately 2.6kWh. If we are using an EPS socket up to 2.6kWh worth of electricity can be used. Remembering what we discussed earlier about the difference between kW and kWh, the time that this battery will last is entirely dependent on what is plugged in.
How to Get the Most Out of Your Solar PV System
As mentioned earlier, the system is limited by your inverter. Let’s say that we have a 3.68kW inverter installed. That means we can only use 3.68kW of electricity supplied from our solar PV and battery system (if installed) at any one time. So, if we were to turn on a washing machine with a rating of 2.5kW and then turn on the vacuum cleaner with a rating of 2.5kW, we will be using 5kW. Our inverter is only rated at 3.68kW, the additional 1.32kW of electricity will have to be purchased from the national grid. However, if we were to vacuum first, and then when finished, turn on the washing machine, we are only ever using 2.5kW of electricity (although don't forget to add in your baseline-see below). If our system is generating enough or we have enough power stored in our battery, then using these appliances will be free.
This simple way of adapting your electricity usage will allow you to further lower your energy costs and this will also help repay the cost of having a solar PV system fitted much quicker.
Our Top Tips
make a list of all your regularly used appliances and their power ratings
first thing in the morning make a note of how much electricity is being used. If you average this out over a week, you will then have a good baseline value. Take this value into consideration when you use your appliances during the day (this refers to the paragraph above 'How to get the most out of your solar PV system)
look to replace any appliances that have a high kW rating with a more efficient alternative
consider the use of smart sockets so you can turn on your washing machine/dishwasher/tumble dryer during the day when your solar generation is high