4 kW solar system - why are they still popular?

 

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Unless you have limited roof space, a 4 kW solar system is unlikely to be the best choice for your home. By following bad advice regarding system sizes and output limits, you could cost yourself £000’s without knowing it.

We explain below.

Performance of different sized solar systems

4kW solar system performance 4.42kw
4kW solar system performance 5.11kw
4kW solar system performance 6.57kw
4kW solar system performance 7.3kw
System sizeCostAnnual generationLifetime savingsNet savingsDifference in savings
4.38 kW£80704298 kWh£22,831£14,761£0
5.11 kW£83674912 kWh£24,259£15,892+£1131
6.57 kW£89635898 kWh£26,505£17,542+£2781
7.3 kW£92606310 kWh£27,425£18,165+£3404
4kW solar system table

 

 

“By following bad advice regarding solar panel system sizes and output limits you could be costing yourself £000's without knowing it”

Thomas Moyes - Director of Leoht Ltd

*For illustration purposes only. Assumed usage 5000kWh per year. For a bespoke model request a free solar design

 
 

History of a 4kW Solar system.

Why is a 4kW solar system such a popular option? To answer this question, we must look back to when the Feed-in Tariff was first introduced. The highest level of support under the old Feed-in Tariff subsidy was in the 0-4kW band. At the time, solar panel sizes maxed out at roughly 250W per panel, which made it nice and simple as 16 panels x 250W =  4 kW solar system.

Therefore, the largest system often sold to residential customers would be a 4kW solar system as it didn’t make financial sense to go any larger and have the subsidy rate cut in half.

Today, modern panels of similar size go up to 400W, a 16-panel system today would be 6.4 kW.

As the Feed-in Tariff subsidy came to an end in 2019 and is no longer needed, there is no reason why you should limit yourself to installing a small 4kW solar system.

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When should I install a 4 kW solar system?

  • If you have limited roof space where the maximum amounts to 10 panels or 4kW.
  • Budget constraints – a 4kW solar system will allow you to install a cheaper inverter as there is less power to handle and less panels are needed.

It will always be more cost-effective to go for a larger system as the marginal cost of adding a panel to the installation is much lower. You’ll also generate more energy.

 
 

Why are 4 kW solar systems still so popular?

  • The public is much more familiar with a 4 kW solar system – it’s much easier to sell something that a customer is already familiar with.
  • Productisation – creating a standard offering for your salespeople to sell.
  • 4 kW solar system capacity is often confused with the grid’s 3.68 kW AC output limit, the two are unrelated and shouldn’t be mixed up but they often are.
  • You can use a cheaper inverter as there is less power being generated/converted and no AC clipping (when DC generation exceeds the AC rated output of the inverter, the power is ‘clipped’ either the inverter uses the Maximum Power Point Trackers to lower the Wattage or the power is lost as heat).

Is a 4kW solar system the maximum size for homes? What is the 3.68 kW grid limit?

3.68 kW is the AC output limit per phase before permission is required from the DNO (District Network Operator). The goal should be to maximise the amount of time your solar panel system’s inverter is outputting 3.68 kWh.

A 4 kW solar system’s inverter will output 3.68 kWh when irradiation (light) is above 920Wh/m2.

A 7 kW solar system’s inverter will output 3.68 kWh when irradiation is above 525Wh/m2.

To maximise the output when not seeking permission and where roof space is not limited, a Huawei hybrid inverter combined with Huawei optimisers on each panel can handle a 10 kW solar system.
A 10 kW solar system's inverter will output 3.68kWh when irradiation is above 368Wh/m2.

The average irradiance in the UK is 101Wh/m2.

Sacrificing some generation through AC clipping during the middle of the day in summer months, when irradiation is at its highest, during this time generation is likely to exceed usage anyway, to maximise production throughout the rest of the year (90% of daylight hours) when generation will likely be more useful is the advice that we would give to everyone considering installing a 4 kW solar system.

Unless you have limited roof space a 4 kW solar system is unlikely to be the best choice for your home. By following bad advice regarding system sizes and output limits, you could cost yourself £000’s.