We've been contacted by owners of solar PV panels who have experienced their electricity-usage meters (otherwise known as supply or mains meters) running backwards.
This means that whenever the solar panels are exporting to the grid - because the panels are generating electricity that's not being used in the home - the mains electricity meter starts turning backwards. The result is lower electricity bills for the householder, as the reading will be less than expected.
This seems to happen mainly with analogue meters that don't have a built-in 'backstop', as the flow of electricity goes the other way round (out instead of in). As more meters are upgraded, or smart meters are fitted, meters running backwards are becoming less of a problem - but it's still something worth watching out for.
One in four Which? members who have solar PV have noticed their electricity-usage meters running backwards, according to our survey of 1,300 members in January 2013.
However, the numbers of those affected may actually be higher. This is because a significant number of respondents - around 20% - told us they didn't know whether or not their usage meter is running backwards, with several explaining that they never look at their meters.
Some households we spoke with in 2013 noticed that their meters were running in reverse, but incorrectly believed that this was simply because they were generating their own electricity.
Some people were warned by their solar PV installer that their meter might run backwards. But for most of those who noticed, it was unexpected.
If your electricity supply meter runs backwards, it will show a false low reading - leading to an inaccurate bill. Some meters have even been showing negative readings, leaving households in credit.
Because of this financial advantage, some households may have been tempted to stay quiet about the problem, getting free electricity on top of the savings they are making from their feed-in tariff.
Others wrongly thought that meters were supposed to go backwards if you have solar PV.
'I thought this was normal'
As more than 900,000 solar PV systems have been installed in the UK, this could mean a large amount of electricity is being, or has been, consumed for free - often unknowingly.
This might potentially lead to increases in electricity costs if suppliers seek to make up for unplanned losses.
What's more, solar PV customers, many of whom have been unaware that their meters are running backwards, are sometimes being landed with unexpected electricity bills retrospectively once their supplier finds out that their meter is going backwards.
The bill is worked out from an estimation of what their usage would have been if their meter had been working properly.
More than two thirds of respondents to our survey said that they were unclear about who should upgrade the meter when the current one runs backwards.
It should really be the responsibility of the solar PV installer to check that the meter is fit for purpose and for the installer (if it's the same company as the supplier) or the household (if the FIT installer is different to the supplier) to then inform the supplier if it needs upgrading.
But because this problem wasn't initially expected, it wasn't made part of the procedure from the start.
Regulator Ofgem told us: 'Where a customer suspects that the installation of electricity-generating equipment has affected the operation of their import-supply meter and it's no longer accurately measuring the electricity supplied to them, they should contact their import supplier at the earliest opportunity.
'Once the supplier is aware that the meter may not be suitable, it is its responsibility to take any necessary steps to ensure the meter is appropriate.'
In cases where the FIT installer is the same company as the electricity supplier, internal communications should - although often don't - ensure that the supplier becomes aware that a household has an unsuitable meter.
However, it's difficult for an energy supplier to find out that this problem is occurring if the installer hasn't notified the supplier that the meter will need to be upgraded.
In such cases, the supplier can only find out by doing a standard meter check - normally undertaken every two years minimum - or by analysing discrepancies in a household's monthly bills.
More than two thirds of our survey respondents with meters running backwards told us that they had informed their supplier. Of these:
A small number were charged by the supplier to have their meter upgraded, although we're assured by the Big Six suppliers that this isn't their policy.
We asked some larger energy suppliers to tell us their view on the meter problem:
British Gas told us: 'As the majority of installs are not completed by British Gas, we therefore rely on the customer to inform us that their meter is running backwards. However, as the customer notifies us every three months of their meter reading, we're normally able to identify at that stage if the meter is running backwards.'
EDF said: 'The initial responsibility for identification of meters that have no 'backstop' lies with the surveyor/installer of the microgeneration system. They should advise the customer on this. However, installer knowledge in this area when the FIT was introduced was limited. Microgeneration Certification Scheme installer training, accreditation and quality-assurance procedures have all been updated since introduction to include checking for meters suspected of having no backstop.'
SSE believes it's the responsibility of both the energy supplier and the customer: 'SSE has a list of compliant meters that we check at the registration stage of FITs. If the customer has an incompatible meter, then we exchange it for them. In addition, we have a two-year meter-read obligation. It is also suggested that consumers check their meters when they receive an estimated bill.'
So if you notice your meter running backwards, should you tell your supplier or not?
It might be tempting not to, as you will benefit from lower electricity bills. But beware that if the supplier finds out, it has a right to charge you retrospectively for up to a year for the electricity you have used but have not been charged for.
Energy suppliers are deciding whether - and how much - to bill for energy usage retrospectively on a case-by-case basis. Most say they understand that, in most cases, the customer is initially unaware of the problem. And so they might, for instance, seek to recover only a proportion of the costs, worked out by looking at usage patterns after the meter has been upgraded.
However, once you have told your supplier that your usage meter is running backwards, the company has a period of one year in which to issue a bill for past usage. After this period, you don't legally have to pay a bill issued retrospectively.