In terms of your current metering situation, you will not be charged for any surplus electricity you generate. Your digital meter will record any electricity you're importing when the PV system or wind turbine does not generate enough electricity for the demand in your house. A domestic meter won't actually measure the electricity fed back to the grid unless you have specifically had an export meter fitted.
There have been rumours of certain meter models that count import and export as the same thing, but this is currently unfounded. You can do a quick check in the middle of the day by shutting off all your electric appliances where possible, and if the meter is still counting at the same rate as when everything was turned on then there is potentially an issue. If you call npower customer services on 0800 551 555 they can advise on changing the meter but its usually very rare this happens so its worth doing the check first.
If you've chosen to do the installation yourself rather then join the Feed in Tariff scheme, the best advice we can offer is try and make the most of the energy you generate. Try to plan your usage around when your PV system is generating the most (i.e. run the dishwasher or washing machine in the day). The more you use from your PV panels and wind turbine, the less you have to import and pay for.
