An estimated electricity or gas bill is based on your property's previous usage pattern. The period for the estimate will either be from the previous year, or closest bill period depending on the information the network has available.
If we're unable to obtain a proper meter reading during your billing cycle, we'll send you an estimate for that period.
You can tell if you've received an estimate on your bill in the section 'Understand your bill'. There you'll see a 'Read type' listed as either 'Actual' or 'Estimate'.