16,000 SSE customers on the Economy 10 electricity tariff could be in line for refunds because their meters were faulty.

SSE has admitted defective equipment may have led to Economy 10 customers being overcharged.

The firm is writing to customers arrange meter inspections before calculating how much they are owed.

Economy 10 users get 10 hours of cheaper electricity for three different time periods every day. (See our Economy 7 and 10 guide for more on how these tariffs work.)

Users need a special Economy 10 meter, and SSE has discovered that 16,000 were faulty. Customers could have been affected in one of two ways.

Power cut: You may have been overcharged because of a power cut. In some cases after the blackout, the meter clock moved back an hour when the power was switched back on. This problem could only have happened if the power cut was during Greenwich Mean Time (October to March) and your meter's clock was programmed to switch from GMT to British Summer Time.

If this happened to you, and after the power cut you timed your electricity usage during the cheaper periods, you may have been charged at the pricier rate. About 8,000 customers may have been affected in this way.

Wrong region: Your meter was installed with the cheaper periods set incorrectly for your region. Another 8,000 customers may have been hit by this problem. This happened in a handful of regions around the country and SSE says it'll carry out wider inspections to check switching times are correct in all regions.

Do I need to do anything to get a refund?

No. SSE is writing to affected customers to let them know if a meter inspection's required. 

How will I know if I've been overcharged? 

If an inspection shows your meter's clock is incorrect, the meter inspector will tell you on the day. SSE will also repair or replace the meter for free and recalculate your charges. Within 28 days of the inspection, a member of SSE's Economy 10 team will let you know if you've been overcharged and by how much, then discuss your refund.

If SSE discovers from the inspection that you'll be better off on a normal meter instead – where you pay one rate all the time – your meter will be changed to a standard one. 

How much will I get and how will it be paid?

You should get back what you were overcharged based on SSE's own assumption of your usage during the cheaper time periods (see below).

You'll have two options. One is that SSE will add a credit to your account, which will help either reduce any amount you owe, or offset future costs. The other is a straightforward refund, direct to your bank or by cheque.

But how will SSE recalculate what I'm owed?

SSE says it's not possible to calculate accurately what a customer's exact usage would've been during the cheaper and more expensive periods, had the meter been working properly.

Instead, it will recalculate your charges based on an assumption that you used 70% of all your electricity during the cheaper period.

SSE says its Economy 10 customers typically use 50% of their electricity during the off-peak times.

Why did this happen in the first place? 

SSE says a manufacturing fault in a batch of meters caused the problems. The supplier is in contact with the manufacturer.

About 1,300 meters around the country have been inspected so far and SSE plans to complete all meter inspections by the end of 2014.