How Many Solar Panels Do I Need?

How many solar panels do I need?

Solar panels are a great way of reducing energy bills while lowering your carbon footprint. But before you can reap the rewards of solar power, you need to establish how many solar panels you need to provide 100% of your electricity requirements.

The number of panels required will depend on a range of factors including the size of your home or office, the number of people living or working there and the average number of sunshine hours your property is exposed to each day.

While this may seem like a complicated process, we’ve broken it down into four easy steps for you to follow. Complete the following steps in order and you’ll get a pretty accurate answer to how many panels you need to install.

Step 1. Calculate your daily power consumption

Before you do anything else, you need to know how much power you are currently using each day. Fortunately, this is easy to work out because your electricity bill will show you how many kilowatt-hours (kWh) you used during the last billing period.

To get an accurate figure take an electricity bill from the summer and winter months then add up the total number of kWh used. To get your daily power consumption figure you need to divide the total amount by the number of days in the billing period.

Let’s look at an example:

In June, our electricity bill shows that we used 320kWh, while our bill for December shows consumption of 400kWh. We add these two amounts together to get 720kWh and then divide by the number of days in the billing period. In this case 60 days. 720kWh / 60 = 12kWh per day.

Note: If you are billed quarterly then take two quarters (summer + winter), add them together and divide the amount by 180.

Step 2. Work out the average daily sunshine hours for your home

Next, you need to work out how much sun your home is exposed to each day. Fortunately, the Met Office has calculated this for us so there’s no need to stand outside all day counting. You can download the latest data here. But we have included a summary for 2019 below.

Month	Daily Sun Hours
Month: January	Sun Hours: 1.7
Month: February	Sun Hours: 4.4
Month: March	Sun Hours: 4.2
Month: April	Sun Hours: 5.9
Month: May	Sun Hours: 6.6
Month: June	Sun Hours: 5.6
Month: July	Sun Hours:6.8
Month: August	Sun Hours: 6.7
Month: September	Sun Hours: 5.4
Month: October	Sun Hours: 2.6
Month: Novemeber	Sun Hours: 1.7
Month: December	Sun Hours: TBC

The UK saw an average of 4.7 sunlight hours during 2018.

Because the number of sunlight hours varies according to the month it’s a good idea to get an average for the year. So add up the number of sunlight hours for each month and divide by 12. In our example, we’ll use the above data which is the average number of daily sunlight hours the UK received in 2018 (4.7 hours).

Step 3. Calculate the daily output of your chosen system

Once you know how much electricity you use and how many sunshine hours your home is exposed to each day, you can work out how many solar panels are needed. The exact number of panels required will depend on the wattage of the panels you install. In the UK solar panels range from about 250 watts to 400 watts per panel.

The following formula will help you work out the output of each panel:

Solar panel watts x average hours of sunlight x 0.75 = daily watt-hours

You may ask what the x 0.75 is for? This helps to account for variables we have not factored in such as the amount of shade the panel receives and the direction they are facing. These can dramatically impact the efficiency of the panel, so it’s as well to account for them in your workings out.

For our example, let’s assume we want to install mid-sized 300-watt panels. The formula looks like this:

300 watts x 4.7 hours x 0.75 = 1057.5 daily watt-hours

Step 4. Divide total daily watt-hours by your daily power consumption

Now it’s time to work out how many panels you need to generate enough electricity for your requirements. To do this simply divide the total daily watt-hours, calculated in step 3, by the total amount of electricity used, calculated in step 1.

The final formula looks like this:

12 / 1.06 = 11.32

So for our example, we need to install around 12 panels to meet 100% of our electricity requirements.

Note: A higher watt panel can reduce the total number of panels required. For example, if we installed 350-watt panels this would reduce the total number of panels required to 10.

Conclusion

While this may seem complicated at first, it’s pretty easy once you start working through the data. You must do the maths at this stage so that you know how many panels are required. This will not only help you work out the cost of installing the system, but it will also ensure you generate enough power to meet your needs daily.

Judging the exact number of panels is often a juggling act between the amount of available space, the power required and the efficiency of the panels themselves. Higher watt panels are typically more expensive, but they will reduce the amount of surface area required allowing them to fit nicely on the roof.

If you don’t have enough roof area to install the required number of panels, there are still options open to you. You can install a ground-mounted solar panel system in the garden or opt for a partial system to provide a percentage of your electricity needs. Typically such systems provide up to 70% of a homes power requirements.

Whichever option you choose you will reduce your electricity bill and lower your carbon footprint. You will also qualify for the Government’s Smart Export Guarantee (SEG) scheme which pays an export tariff for any surplus electricity passed back to the National Grid.