If you are installing panels
to for financial reasons, you will want to size your system to maximise self-consumption of the solar energy that is generated.
The reason for this is that the amount you buy power for (import) and the amount you sell power for (export) are wildly different; the average price to import power is c.11-14p per kW/h wheres the price you are paid for exporting power is c.3-5p per kW/h.
With this in mind having a higher self-consumption rate will shave your off the time it takes for the solar system to pay for itself.
As a rough rule-of-thumb we tend to size arrays so that they have a minimum ‘utilisation rate’ of 80% (80% of the power consumed on site, 20% exported to the grid). The reason for this is that whilst the value of the utilised power is fairly easy to predict – energy prices are only going one-way, after all – the export tariff is typically guaranteed for only 12 months at a time. As a result, whilst we do want to build in some headroom for future expansion/energy storage
, we do not want to rely too heavily on the export rate to justify the business case as this payment is likely to reduce overtime as the grid is overwhelmed with small-scale generators looking to export their power at the same time.