For decades, judging a home’s energy performance meant asking one question: how much does it use? Labels such as energy performance certificates still ask only that.

But homes are changing. Add solar panels, a battery or an electric car, and a house starts to generate, store and sell electricity. Meanwhile, what that electricity costs depends more and more on the clock. Since June 2026, Ireland’s largest suppliers have had to offer tariffs that change every half hour. In the UK, households pay about 26p for each unit they buy, but solar owners can be paid as little as a few pence for each unit they sell.

The question has shifted from how much energy is being used to when is it being consumed? Every building is now an energy system. The clearest example of what that means is a long way from Europe.

In Pakistan, an estimated 7.3 million households, nearly one in five, now have solar panels. In the year to June 2025, panels on homes, shops, farm sheds and small factories produced the equivalent of 46% of all electricity sold through the national grid. Three lessons stand out.

First, nobody paid them to do it. There was no rooftop programme and no subsidy. Squeezed by rising tariffs and power cuts, people bought cheap panels imported from China: nearly 54 gigawatts by March 2026, roughly the capacity of the entire grid.

Second, most of it was invisible. By mid-2025, an estimated 38 gigawatts of solar was installed, generating 51 terawatt-hours of electricity in 2025, enough to power every home in the UK for more than six months. Yet only 6.8 gigawatts was registered where the regulator could see it. Overall electricity sold through the nation’s grid system fell 11% in three years, and planners could not see what was replacing them.

Third, when the rules caught up, the Pakistani government changed the deal mid-boom. In February 2026, the regulator cut the price paid to new solar customers for exported power by more than two-thirds. After a backlash, those who had already applied kept the old terms. But the households who could never afford panels are left paying for an electricity supply through the nation’s grid infrastructure that fewer people use.

Pakistan is an extreme case, but the direction is global. Electricity is being priced by when it is used, not just how much. In the US, California has paid new solar owners about a quarter of the retail price for exported power since 2023. Australia, which has more midday solar than it can use, now offers households three free hours of electricity at around lunchtime.

The message is the same everywhere. Solar power is worth most when you use it yourself, or when the grid needs it, usually in the early evening.

Measuring when, not just how much

If timing matters this much, we need to measure it. As part of my PhD research, I developed a time-of-use score that rates a home on when it draws power from the grid. Electricity used in the 5pm–7pm peak counts least; electricity used in the after midnight hours, when the grid is quiet, counts most. For homes with solar, a grid interaction score does the same for the power still bought after the panels and batteries have done their work.

We applied the score to newly built, A-rated (highly energy-efficient) apartments that we monitored for a full year. Almost all of them fell into the lowest time-of-use class. On paper these were among the most efficient homes in the country, yet they leaned on the grid at the worst times. Much of their load, such as hot water, washing machines and dishwashers, could easily have moved.

You don’t need a research project to run your solar-fitted home more efficiently as an energy system. Three rules cover most of it.

Use the sun first. Run the dishwasher, washing machine, electric car charger and hot water around midday. A solar diverter can heat your water automatically from surplus power.

Keep off the grid at teatime. Between about 4pm and 7pm, let a battery or a delay timer cover what it can. This is when power is dearest, and the grid is under the most strain.

Buy the rest when it is cheap. In winter, solar may not cover all your needs. Use night or “boost” rates in the small hours to heat water, charge the car and top up the battery.

Then automate. Spain has offered households hourly prices since 2014, yet only 40% of those customers checked the cheapest hours before running big appliances. Timers and smart controls do it every day.

A colleague in our research group took this one step further. Rory Monaghan, a professor in energy systems engineering, wanted to know which electricity supplier and which plan really suited the way his household uses power. So he put his consumption and the available plans into a spreadsheet and ran an optimisation. The results were very different from what you would expect from the suppliers’ adverts. Electricity deals are confusing even for sustainability researchers.

Energy labels across Europe are being redesigned under the revised Energy Performance of Buildings Directive. This is the moment to rate buildings on how they behave, not just how they were designed: what they use, when they use it, and how they work with the grid.

Pakistan shows what happens when nobody is counting. Millions of buildings can remake an energy system faster than regulators can see it. Better to measure that change, and help every home run itself well, than to discover it from falling grid sales and have to rewrite the rules in a hurry.