Need help understanding which products are best for your needs? Call us on 01684 607002
Home > Blog

Sizing an Off-Grid System

Working out your power needs and creating a load profile

Whether you have a caravan, boat, motorhome or even a small building you want to power from solar, you need to start with a load profile.

All solar (PV) off-grid projects start with the load profile as without an accurate understanding of the power requirements of the installation, power demand at any one point and the time of year you will be using the power it is impossible to design a robust and functional system.

All solar systems will be different, one size will not fit all, and each project will be designed individually based on the load profile. It’s not as hard as it seems. Just take a methodical approach and take it step by step.

What is a load profile?

The load profile is a detailed analysis of all the equipment you intend to run via the battery. Once we understand the power going out (being used), we can then determine the solar panel size in watts needed to replace the energy used. The load profile considers all energy used in a 24hr period.

We would also consider a certain amount of autonomy (backup power), so the connected equipment can function on battery reserve if we have a period with little or zero solar influence.

We will be doing a blog on battery sizing for your solar projects later, but if you need help now then contact us

Calculating your panel size

Portrait of Kevin, our new technical director

To understand the power consumption, we would look at each appliance or piece of equipment individually.

If an appliance has a power consumption of 20W (this can normally be found on the data plate at the back or underneath an appliance) and it runs for 3hrs in a 24hr period, we have a total power consumption of 60W (20W x 3H = 60W)

We then add all the individual equipment together to give the total load profile. This load profile will then enable the calculation of the solar panel and battery needed for the project.

For example:
Lights @ 10W / 2h per day = 10W x 2h = 20W total
Fan @ 60W / 4h per day = 60W x 4h = 240W total
Phone charger @ 5W / 2h per day = 5W x 2h = 10W in total
Radio @ 10W / 6h per day = 10W x 6h = 60W in total
TV @ 65W / 1h per day = 65W x 1 = 65W in total

Total energy used is: 20W + 240W + 10W + 60W + 65W = 395W per day.

On top of this calculation, we need to also look at other devices that might be used to run the above appliances for example an inverter used to power mains devices from a battery. An inverter will incur at least 20% system losses, so it must be calculated into the load profile.

Using the above example loads, the TV and fan are both mains devices running from an inverter. So, we need to add to our daily power:

Fan's total power needed = 240W x 1.2 (+20%) = 288W (an extra 48W)
TV’s total power needed = 65W x 1.2 (+20%) = 78W (an extra 13W)

Giving us a new total of 395W + 48W + 13W = 456W per day.

Keeping that load in mind, we can now work out the size of solar panel needed to replenish that amount of power back into the battery during the following day.

Year round solar power

Solar panels will give approximately 5-6 hours of their rated designed wattage energy in the summer months per day.

However, in the winter it can be as low as 0.8 hours per day. Some days may be higher or lower but if we base our calculation on these figures, we can be sure the system will work all year round.

Using these figures, we can calculate the solar panel needed to recharge a battery. Using the total daily requirement from above at 465W per day.

456W / 6 = 76W rated solar panel as a minimum but we would always say to purchase a solar panel above this rate i.e., an 80W or 100W unit.

Use the numbers 2 or 4 depending if its nearer summer time or winter time.

BUT, and this is a big 'but'; if you required the same power in the winter: 456W / 0.8 = 570W rated solar panel(s). (That's 6 x 100W panels!!!)

As you can imagine, winter is where solar isn't at its best, but with more understanding and using lower power rated products, it can still work for you.