It took eight years after getting solar panels installed for one family to solve the mystery of why they weren't saving as much as expected on their electricity bill. But they're unlikely to be the only ones affected.
The electricity meter in Jon Doyle's home does something unexpected. When his solar panels generate more electricity than is being used, the excess goes into the national grid and he is charged for it.
He shouldn't be but this happens because his meter treats all electricity the same, whether it is flowing into his home or out of it.
And Jon estimates he has overpaid by more than £2,000 for electricity as a result.
The Doyle family had solar pv panels fitted on their new home before they moved in. But despite their best efforts, their electricity bills remained higher than they expected.
Jon said: 'We invested as much as we could in trying to reduce our consumption, including a heat pump tumble dryer and modern appliances. We changed our behaviour to use power when the solar is on.'
A summer holiday in 2018 finally revealed the answer. After six weeks away during a sunny spell, the electricity meter showed that consumption had shot up. No one had used power at home but the solar panels had generated a lot of electricity.
Using two energy monitors, Jon found that the amount of electricity his home used from the grid apparently increased when his solar panels were generating.
His energy supplier, Pure Planet, confirmed this after it installed a 'check meter'. It said: 'We identified the cause of the billing issue as a very unusual configuration with his electricity meter that predated his Pure Planet membership. This means energy exported by his solar panels and energy drawn from the grid is wrongly counted cumulatively'.
It stopped charging for electricity when it confirmed the problem and will install a smart meter to solve it.
It's tricky to calculate how much Jon should be refunded for the electricity that has been exported to the grid and which he has paid for. Exported electricity is not measured by Jon's meters.
When he took his case to the Energy Ombudsman, it said: 'due to the issue with your meter it is not possible to prove exactly how much energy you have exported as this is not recorded'.
So it decided that Pure Planet should refund 50% of the electricity generated from when Jon became a customer until it removes the check meter later this year, plus an additional refund if the check meter's data shows that more electricity than this was exported.
'We're pleased that we were able to help Jon out with his issue from Siemens, and that he's still a customer with Pure Planet', said its co-founder Steven Day.
But this isn't the end of Jon's problems. He switched energy supplier most years since installing his solar panels so faces the same process with each supplier to get a refund for overpayment.
The electricity meter in Jon's home in Leeds is a Siemens SA2A100. Siemens told us that these were commonly installed in the 1990s but it stopped making them in the late 1990s.
'The meter is behaving as it was designed,' it said. 'These meters were developed at a time when households were not likely to have microgeneration capabilities but there were a large amount of fraudulent situations'. So the meters add up exported and imported energy to prevent someone cheating them.
The Office for Product Safety and Standards (OPSS) said that some meters can be affected by microgeneration (such as solar panels) and so aren't compliant with the Electricity Act when it is installed.
'A small number' of meters were affected, it explained, where the meter had fraud detection and microgeneration was installed at the home.
Suppliers 'have removed the majority of meters which are not appropriate, however some may well exist'.
It's very tricky to tell whether your electricity meter is at fault if you're not seeing the savings you would expect from your solar panels on your bill.
When we asked Which? members whether this sounded familiar, several of you got in touch, including one whose home seems to use more electricity in summer than winter despite running appliances while the solar panels are generating.
Another said their electricity bills did not drop as expected after installing solar panels.
A change in how much electricity you use, or the performance of the solar panel system could also be the cause if you're not seeing the savings you'd expect.
Switch off all your appliances for a couple of hours while your solar panels are generating. Then check how much power your generation meter has recorded, and how much power your home has used (according to your electricity meter). With everything switched off, you should be using very little power from the grid.
Contact your energy supplier if you are worried and share your findings. If possible, use online contact methods as you may face a long wait on the phone at this time while energy firms are dealing with coronavirus-related queries and may also have reduced staff.
If your meter is inaccurate, your energy firm must replace it.
Suppliers may base their estimates for your correct bill on your previous usage, taking into account the size and type of your solar panel system, seasonal variation, and after a review when the meter has been exchanged. The biggest six committed to Energy UK to do this in 2014.
Maximise the amount of solar electricity you use. This will mean that you use less electricity from the national grid, and reduce your bills.
To do this, consider when you use your appliances and how much electricity they require to run.
If you are home during the day, you can time using your washing machine, dishwasher or tumble dryer for when your panels are generating the most, for example. Some appliances have delayed-start features so you can set them up to run during the day while you're out.