Making sense of figures

SIR, – As I know to my embarrassment, a well-placed typo can make nonsense of what you are attempting to say. 
The average household in the UK is expected to use about 5000 kWh of electricity per annum. This is just the same as 5 mWh. So Mike Wilson’s controversial AD plant (The SF September 10) would expect to produce 67 mWh (not 67 kWh). 
However, this does not quite square with the 15,000-20,000 HGV tractor trailer additional annual movements. 
Assuming that slightly more than half of them (8000) are taking raw material to the digester and that on average each load contains about four tonnes of dry matter, this material should end up producing about 4 mWh of electricity per load. That comes to about 32,000 mWh (or 32 gWh) per annum. 
In the alternative energy parlance, that is enough to power about 6400 homes. 
If he meant that the power output was 67 kWh, that would still only be an annual total of about 587 mWh (about 118 homes powered).
I can understand somebody being upset at the prospect of an additional 50 HGV/tractor and trailer units passing almost every working day, but it helps if the figures make sense.