How Do Solar Panels Work? A Simple Guide

The photovoltaic effect: sunlight to electricity

Solar panels work through the photovoltaic effect, discovered in 1839. When sunlight hits certain materials (like silicon), it knocks electrons loose from their atoms. Those moving electrons are electricity.

Each solar panel contains dozens of photovoltaic (PV) cells. These cells are made from silicon wafers treated with chemicals to create a positive layer and a negative layer, like a battery. When sunlight hits the cell, electrons flow from one layer to the other through an external circuit. That flow of electrons is direct current (DC) electricity.

Source: MCS guidance on solar PV technology and Energy Saving Trust solar panel explainer.

From DC to AC: the inverter's job

Your home runs on alternating current (AC), not direct current. Solar panels produce DC. The inverter converts DC from the panels into AC that your appliances can use.

Most UK solar installations use a single string inverter mounted near your consumer unit (fusebox). Some systems use microinverters (one per panel) or optimisers. The inverter also monitors how much electricity the system is generating.

Inverters are the component most likely to need replacing during the system's lifetime. Typical inverter lifespan is 10-15 years, compared to 25-30+ years for the panels themselves. (Source: Energy Saving Trust).

Where does the electricity go?

Once the inverter converts DC to AC, the electricity flows to your consumer unit and powers whatever is running in your home at that moment. The flow looks like this:

1. Self-consumption first. If your washing machine, fridge, lights or other appliances are running, they use solar electricity before drawing from the grid.
2. Export the surplus. If you generate more than you're using, the excess flows to the grid. Your electricity supplier pays you for this via the Smart Export Guarantee (SEG).
3. Grid import at night. When the sun sets, your panels stop generating. You draw electricity from the grid as usual.

You remain connected to the grid at all times. The system automatically switches between solar, export and import without you doing anything.

Source: Ofgem Smart Export Guarantee guidance.

What if you add battery storage?

A home battery stores excess solar electricity instead of exporting it to the grid. In the evening, you use stored electricity before drawing from the grid. This increases self-consumption from around 30-40% (without battery) to 60-80% (with battery).

Batteries typically store 5-15 kWh and cost £3,000-£8,000 installed. They add complexity: the battery has its own inverter, and some electricity is lost in the charge/discharge cycle (round-trip efficiency is 85-95%).

Batteries make more financial sense if you're on a time-of-use tariff (cheap overnight electricity to charge the battery, expensive peak electricity to avoid). Without that, payback is long.

Source: Energy Saving Trust battery storage guidance.

Do solar panels work in cloudy weather?

Yes, but they produce less. Solar panels work in all daylight conditions, including overcast days. They generate electricity from diffuse sunlight, not just direct sun.

Output on a cloudy day in the UK is typically 10-25% of the rated capacity, depending on how thick the cloud is. A 4 kW system might generate 0.4-1 kW on an overcast day, compared to 3-4 kW in full sun.

This is why UK solar panels generate around 850-950 kWh per kW per year on average, despite our climate. Bristol's typical yield is 875 kWh/kW/year. (Source: Met Office solar irradiance data).

What about snow, rain and dirt?

Rain helps keep panels clean. Most UK roof pitches (30-45°) are steep enough that rain washes off dirt, pollen and bird droppings. You don't normally need to clean panels unless you're near trees or in a high-pollution area.

Snow is rarely a problem in Bristol. Panels are dark and angled, so snow slides off or melts quickly. Even a thin layer of snow blocks generation, but snow coverage rarely lasts more than a day or two in the UK.

Heavy soiling (moss, leaves, thick grime) can reduce output by 5-10%. If you notice a sustained drop in generation, cleaning might help. Use water and a soft brush, or hire a professional. Never walk on panels.

How efficient are solar panels?

Modern solar panels convert 18-22% of the sunlight hitting them into electricity. That might sound low, but it's a huge improvement from the 6% efficiency of early panels in the 1950s.

Panel efficiency matters most if roof space is limited. A 400 W high-efficiency panel produces the same power as a 350 W standard-efficiency panel, but takes up less roof area. For most Bristol homes with typical roof space, standard efficiency panels are fine and cheaper.

Efficiency gradually declines over time. Manufacturers guarantee at least 80-85% of rated output after 25 years. Real-world degradation is around 0.5-0.8% per year, so a panel producing 400 W today might produce 320-340 W after 25 years. (Source: MCS performance data).

The components of a solar PV system

A typical UK rooftop solar system includes:

• Solar panels: 8-16 panels for a 3-5 kW domestic system. Mounted on aluminium rails fixed to your roof.
• Inverter: Usually a wall-mounted box in your garage, loft or near the consumer unit. Converts DC to AC.
• Generation meter: Records how much electricity the system produces. Often built into the inverter display.
• Export meter: If you're claiming SEG payments, your supplier may install a separate export meter or use a smart meter in export mode.
• Consumer unit connection: The AC output from the inverter connects to a dedicated circuit breaker in your consumer unit.
• Battery (optional): Stores excess electricity. Requires its own inverter if it's a DC-coupled battery, or connects after the solar inverter if AC-coupled.

Frequently asked questions

Do solar panels work on cloudy days?

Yes. Solar panels work in cloudy weather, but generate less electricity than on sunny days. UK solar panels typically produce 10-25% of their rated output on overcast days, depending on cloud density. Annual yield in Bristol is around 875 kWh per kW installed, factoring in our mixed weather. (Source: Met Office).

How long do solar panels last?

Most solar panels are guaranteed for 25 years and continue producing electricity for 30-40 years. Output gradually decreases over time, typically losing 0.5-0.8% efficiency per year. The inverter will likely need replacing once during the system's life (inverters last 10-15 years). (Source: Energy Saving Trust).

Can you store solar electricity in batteries?

Yes. Home battery storage systems can store excess solar electricity generated during the day for use at night. Battery systems typically cost £3,000-£8,000 and store 5-15 kWh. Batteries increase self-consumption from 30-40% to 60-80%, but payback is long unless you're on a time-of-use tariff. (Source: Energy Saving Trust battery guidance).

Do I need planning permission for solar panels in Bristol?

Most rooftop solar installations are permitted development and don't need planning permission. Exceptions: if your property is Listed, in a Conservation Area with panels visible from the street, or if you're adding panels to a wall. Check with Bristol City Council planning before installation if your property has restrictions.

What happens if there's a power cut?

Standard grid-tied solar systems shut down during a power cut for safety. This protects engineers working on the grid from electrocution. If you want backup power during outages, you need a battery system with islanding capability. Not all batteries support this, and it adds cost.

Can I install solar panels myself?

You can buy panels and equipment, but you shouldn't do the electrical connection yourself unless you're a qualified electrician registered with a competent person scheme. MCS-certified installation is required to claim SEG payments and some grants. DIY installation also invalidates most panel warranties and may affect home insurance. (Source: MCS certification requirements).