The Optimal Solar installation

Buying solar is just putting money up for something that is a variable right? what is the actual optimal solar system setup here in the UK?

The main problem for solar in the UK is that its feast and famine, but the averages change depending where you are and there are local factors such as which way your facing your solar panels. in truth there is no year round optimal solar installation or kit and anyone who makes the claim will be very uneducated. one of the biggest myths is a south facing roof. if you hear this from anyone you know your on a losing path.

The sun in the UK is not that bad, its not great either, but solar does work and its a lot more complex than you would think. We can split solar into summer and winter as the solar yields are so much smaller in winter and therefore to gain benefit from them is more difficult. You may have read more of the post about solar and our calculations, we use 275 days of the year as these are what we consider “sunny days” where as the other 90 days

It’s crucial to understand that “optimal” is relative to your specific energy consumption, roof space, and future plans (e.g., electric vehicle, heat pump). However, here are common benchmarks for a well-balanced system for an average UK household:

1. How Many Solar Panels (kWp)?

The average UK household (2-3 people) uses approximately 2,700 kWh of electricity per year.

• Optimal Solar PV System Size: For most UK homes, a system between 3.5 kWp and 5 kWp is considered optimal.
  • A 4 kWp system is very common as it often falls within “Permitted Development Rights” (meaning less planning hassle) and can broadly cover the average annual electricity consumption.
  • Reasoning: Going significantly larger than 4-5 kWp can lead to diminishing returns, as the rate you get paid for exported electricity (Smart Export Guarantee) can decrease for larger systems, and you might generate more electricity than you can realistically use or store, especially in summer. however for independence then a larger system would be required to meet the winter power needs.
    
• Number of Panels:
  • Assuming modern panels with an output of 400Wp (watts-peak) per panel, here’s a general guide:
    • 3.5 kWp system: Approximately 9 panels (3500Wp / 400Wp per panel = 8.75, so 9 panels).
    • 4 kWp system: Approximately 10 panels (4000Wp / 400Wp per panel = 10 panels).
    • 5 kWp system: Approximately 12-13 panels (5000Wp / 400Wp per panel = 12.5, so 13 panels).
• Physical Dimensions: A standard residential solar panel is roughly 1.75m x 1.1m. So, a 4kWp system (10 panels) would require around 28.5 square meters of roof space, plus spacing.
• General production: is around 4 times the complete array size -35% for roof installations and normal conditions.

2. How Much Battery?

Battery storage is key for maximizing self-consumption in the UK. You want enough capacity to store your excess daytime generation for use in the evening and overnight.

• Average Daily Consumption: The average UK household uses around 7.4 kWh per day.
• Optimal Battery Size: For a typical 3-bedroom UK house with a 4kWp solar system, a battery with a usable capacity of 5 kWh to 10 kWh is generally recommended.
  • A 5 kWh battery is often sufficient to cover evening and night-time usage for average homes.
  • A 10 kWh battery offers more resilience and can be beneficial if your consumption is higher, or if you want to maximize off-grid potential for longer periods (e.g., during cloudy spells or potential future EV charging).
  • 15kWh Battery is more cost effective and lowers the DOD giving extended life to the battery.
    
• Consideration: Depth of Discharge (DoD): Batteries aren’t meant to be fully discharged. Lithium-iron batteries typically have a high DoD (e.g., 80-90%). So, a 5 kWh battery with 80% DoD provides 4 kWh of usable energy.

3. What Size Inverter?

The inverter converts the DC electricity from your panels into AC electricity for your home.

• Inverter Sizing Rule of Thumb: The inverter’s AC output rating (in kW) is generally sized lower than the total DC peak power (kWp) of your solar panels. This is known as “inverter oversizing” or “DC-to-AC ratio.”
  • A common recommendation is for the inverter’s AC rating to be around 75% to 85% of your solar array’s DC kWp. in certain installations, however it is better to cover the full power needs with the inverter.
  • For example, if you have a 4 kWp solar array (10 x 400Wp panels):
    • Your inverter might be sized at 3 kW to 3.5 kW. (4 kWp x 0.75 = 3 kW, 4 kWp x 0.85 = 3.4 kW).
  • If you have a 5 kWp solar array (12-13 x 400Wp panels):
    • Your inverter might be sized at 3.5 kW to 4 kW.
      
• Why Inverter Oversizing?
  • Panels Rarely Produce Peak Power: Solar panels only hit their peak power rating (kWp) under ideal conditions (Standard Test Conditions – STC), which are not often met in the real world, especially in the UK.
  • Maximizing Efficiency: Inverters are often most efficient when they are working closer to their maximum capacity. By having a slightly smaller inverter, it can operate more often in its sweet spot. For those with larger inverter needs, two parallel inverters work best.
    
  • Cost-Effectiveness: It can be more cost-effective to have a slightly smaller inverter and “clip” (lose) a tiny bit of power on the very brightest days, rather than buying a larger, more expensive inverter that is oversized for most of the year’s production. Withstanding PV is only one side of the requirements.
  • UK Grid Limits: Historically, there have been grid connection limits that make larger inverters more complex to install without additional permissions. While regulations can change, it often influences the practical sizing.

Important Note: These are general guidelines for an “optimal” balance for the typical UK household. A professional solar installer will conduct a detailed site survey and energy audit to provide the most precise recommendations tailored to your specific circumstances, including roof orientation, shading, and your exact electricity consumption patterns.

Optimal average system

At RenewSolar we see a lot of homes and installs from large to small. For the most part the question is should you spend more on solar, or more on the battery? Here they tend to go hand in hand, but this depends on if you are grid charging. You could say you do not need solar panels at all.
~We would suggest a point around 5kw inverter and a 15kwh battery. We would say a summer array of 3kwp and a winter array of around 9kwp for more independence from grid power.

A mixed use array tend to clip for both summer and winter peaks or drops in the sun and lower sun time, meaning that in summer the peak power is not there, but the sun time is much longer. In the winter the sun hours are much shorter, the suns lower and weaker, so you cannot get much from solar. therefore if you bias a winter array the losses from the summer sun position is countered by the longer sun hours. But you should think about grid or generator charging.

Catch or caught…
Working with the grid or most hybrid systems are connected to the grid and export power. This is not a blanket condition and if you work in parallel with the grid then your going to pay more for that system than one which is attached.
A 6kw attached inverter costs around £460, a parallel inverter is around £1,500

The 2027 take over your investment/money….
The government is planning to use your money and equipment to benefit themselves; Here’s a breakdown of the current trajectory and what it implies:

The Core Idea: Smart Meters as the Foundation

• Smart Meter Rollout: The UK has been rolling out smart meters with the aim for most homes and small businesses to have them by the end of 2025. These meters provide near real-time data on energy consumption and, crucially for solar, export. This data is fundamental for a “smarter” grid.
• Benefits of Smart Meters: They enable accurate billing, remove the need for manual readings, and, most importantly for solar owners, facilitate participation in Smart Export Guarantee (SEG) tariffs (conditions apply).

Evolution Towards More Dynamic Control (Not Direct Government Control of Your Panels):

Instead of the government directly “controlling” your power, the evolution is focused on incentivizing and enabling smart, flexible energy usage through market mechanisms and technology. This primarily involves:

1. Time-of-Use and Agile Tariffs:
   • Smart meters allow energy suppliers to offer time-of-use (ToU) tariffs where electricity prices vary throughout the day. This means cheaper electricity during off-peak hours and more expensive electricity during peak demand.
   • Agile tariffs (like those offered by Octopus Energy) take this further, with prices changing as frequently as every half-hour based on wholesale market conditions.
   • Implication for Solar: These tariffs encourage solar owners with batteries to:
     • Self-consume: Use their solar-generated electricity directly when it’s sunny, reducing expensive imports.
     • Store excess: Store excess solar in their battery when generation is high and export prices might be low, then use that stored energy when grid prices are high.
     • Charge from grid: Potentially charge their battery from the grid during very cheap off-peak times (e.g., overnight) and discharge it during peak times to sell back or reduce expensive imports. This is often called “arbitrage.”
       
2. Smart Energy Appliances and Devices:
   • The government is moving towards requiring smart functionality in certain energy-consuming devices like EV chargers and heat pumps. This “smart mandate” for energy smart appliances is intended to allow these devices to respond to price signals or grid conditions.
   • Implication for Solar: This means your EV could automatically charge when solar generation is high or when grid electricity is cheapest. Your heat pump could likewise adjust its operation to optimize for renewable energy availability or lower prices.
     
3. Demand-Side Response (DSR) and Flexibility Services:
   • This is where the concept of “control” comes closest. The grid needs to balance supply and demand in real-time. With more intermittent renewables (like solar and wind), this becomes more challenging.
   • How it works: Aggregators (companies that manage energy for many individual homes/businesses) can offer “flexibility services” to the grid. They might offer incentives for you to slightly reduce your consumption or export power from your battery during periods of high grid demand or low renewable generation.
   • Your Choice: Crucially, this is generally opt-in and provides financial incentives. You wouldn’t be forced to relinquish control, but you could choose to participate to earn money. For example, if there’s a surge in demand, your battery might automatically discharge to the grid for a short period, and you’d get paid for it and then you would have to pay for to refill your battery or use power from the grid.
     
     
4. Terms and Conditions: Any agreement for a third party to control your battery would be outlined in a contract or service agreement. This contract must clearly define:
   What control they have: e.g., can they discharge your battery to the grid, or only influence when it charges?Your override capability: You must retain the ability to override any automated control. If you need your battery for personal use (e.g., for self-consumption or backup), you should be able to tell the system to prioritize your needs over grid service’s
   Compensation: The financial benefits you receive for participating in these services. This is the incentive for you to allow your battery to be used.
   
   
5. Economic Incentives: The core driver for you to allow your battery to be controlled for grid services is financial gain.
   Payment for Services: If an aggregator discharges your battery to help the grid, you would be paid for that service. This payment would be designed to compensate you for the energy used and to make it worthwhile for you to participate.
   Optimized Charging: If your battery is discharged, the smart system would typically aim to recharge it when electricity is cheapest (e.g., overnight on a cheap off-peak tariff) or when your solar panels are generating, minimizing your cost to refill the battery. The system’s purpose is to benefit you financially, not penalize you.

Why is this happening?

• Grid Stability: To integrate more renewable energy efficiently, the grid needs to become much more flexible and responsive. Knowing when and where power is being generated and consumed (and having the ability to influence it) helps maintain balance.
• Decarbonization: Maximizing the use of clean, homegrown power (like solar) and reducing reliance on fossil fuels, especially during peak demand, is central to net-zero targets. however we have discovered that this is meaningless.
• Consumer Benefits: The theory is that this smart system will ultimately lead to lower overall energy costs for consumers by optimizing energy flows, reducing the need for expensive peak generation, and enabling new revenue streams (like selling stored energy back to the grid).

In essence, the UK’s approach for 2027 and beyond isn’t about the government seizing direct control of your solar panels. It’s about building a highly interconnected and intelligent energy system where smart meters, flexible tariffs, and smart devices allow for dynamic, incentivised responses to grid conditions, ultimately leading to a more efficient, stable, and greener energy supply for everyone.

We should note that we have been aware of the public private funding of solar installations was “incentivized” to avoid the profit losses of the network installation of the hardware which the grid needs. Rather than installing at their cost, the home owner works and pays for the required hardware keeping the billions made in profits for the companies share holders. unfortunately the government mandates have been drawn to this for many decades as all “public services” should be fully funded by the public and this has been the same conduct in my view.
For example the MCS has released its 2025 Q1 earnings report, with revenue of 148.77M, and Ofgem £24B for 2025/26. Eon 9.8B euro, SSE set out its results for the 2024/25 financial year, the Group has confirmed: Adjusted operating profit of £2,419m

How and why battery storage is not paid for by the company opperators is unknown to myself as I am sure it is to you.

How to avoid control and company profiting?

You should look into grid attached, therefore being independent and also less tied to the rules and limits. The limits could be the space that you have in order to be independent. if you retain a grid connection, take a look at no standing charge tariffs, therefore you can limit the cost and have the grid available when you need it.
Maybe off grid is a good idea, you may want to consider water systems too.