How home solar works
Rooftop solar uses photovoltaic (PV) panels, which convert sunlight directly into electricity. The panels produce direct current, and an inverter converts this to the alternating current used by household appliances and the grid. The electricity you generate is used first by whatever is running in the home. Depending on your setup and local rules, any surplus is either exported to the electricity grid, sometimes for a payment, or stored in a battery for later.
Generation depends on sunlight. It is highest on bright days and lowest at night, so a home with solar still draws from the grid after dark and in dull weather. Output also varies through the year, and in many regions it is much lower in winter, when heating demand is highest. This mismatch is a basic feature of solar and explains much of the discussion about cost and benefit.
Do not confuse PV with solar thermal panels, which heat water directly and are a different technology with a different purpose.
The environmental case
Solar generation produces no direct emissions while operating, and it can displace electricity that would otherwise come from fossil fuels. Panels do have a manufacturing footprint, from mining and processing materials to making the cells and transporting them. Over their working life, panels typically generate far more energy than was used to make them, but the exact balance depends on where they are made, where they are installed and what the grid would otherwise supply. This is the sort of question that life cycle assessment is designed to address.
How much carbon a home saves depends on the local electricity mix. Where the grid is already mostly low-carbon, the additional benefit of generating your own is smaller. Where the grid relies heavily on fossil fuels, the benefit is larger. It is also one of several renewable energy sources, and for many households a different route, such as a green tariff, may be simpler, though it carries different guarantees.
What affects suitability
Not every roof is a good candidate. Factors that matter include:
- Orientation and tilt. In the northern hemisphere, a roof facing roughly south usually collects the most sunlight; in the southern hemisphere, roughly north. East and west roofs can work, with generation shifted towards morning or afternoon.
- Shade. Trees, chimneys and neighbouring buildings can reduce output substantially, even if only part of the array is shaded, depending on the system design.
- Roof condition and structure. The roof must be sound and strong enough to carry the panels for decades. An old roof may need repairs first, which adds to the cost.
- Available space. The size of the array is limited by usable roof area.
- Local rules and permissions. Planning rules, heritage restrictions, building regulations and requirements to notify the grid operator vary. If you rent or live in a flat, you will need the owner’s agreement, and a shared roof may not be available to you.
- Your electricity use. A home that uses much of its power during the day gets more direct value from solar than one empty until the evening. Charging an electric vehicle or running appliances in daylight can help use generated power.
Cost and payback, in general terms
Solar involves a significant upfront cost, covering panels, inverter, mounting, installation and any electrical work. Costs have changed considerably over time and differ by country and installer, so this article does not give figures. A battery adds further cost and has its own manufacturing impact.
Payback is the time it takes for savings and any export income to equal the outlay. It depends on several things that vary from home to home:
- how much electricity you use and when
- the price you pay for grid electricity
- what you are paid, if anything, for exported power
- the size and quality of the system
- local sunlight
- incentives or tax treatment, which change and may not be available
- maintenance and any replacement of the inverter during the system’s life
Because electricity prices and policies change, any payback estimate involves uncertainty. Ask installers for a written estimate of annual generation, the assumptions behind it, and how much of that you are likely to use directly. Be cautious about quotes that promise guaranteed savings. Get more than one quote, check that installers hold the relevant accreditation for your country, and read warranty terms for panels and inverter.
Before you consider solar
Solar often makes more sense after the basics are done. Cutting waste through habits that reduce home energy use, and reducing heat loss, may cost less per unit of energy saved than generating more. Reducing demand also means a smaller system can meet your needs. If you are also considering heat pumps, which run on electricity, it makes sense to plan the two together.
Trade-offs to weigh
- Intermittency. Solar does not produce when you most want heating.
- Embodied impact. Panels and batteries require materials and energy to produce, and end-of-life recycling is still developing in many places.
- Lifespan. Panels tend to last for decades but gradually produce less, and inverters are often replaced sooner.
- Moving or renting. The investment may not be recovered if you move, though in some markets it can add to a property’s appeal.
- Alternatives. Community solar schemes or switching electricity supplier may suit people who cannot install panels.
Solar can be a good fit for a sunny, unshaded roof owned by someone planning to stay for years, and a poor fit for others. Weigh your own roof, usage and finances, and get independent advice before committing.