Are you registered for VAT already?
If not, registration thresholds are €35,000 pa for services and €70,000 pa sales of goods. For the higher threshold to apply 90% of sales must comprise sale of goods. Therefore if yuour sales come 50% from goods and 50% from services then the lower threshold applies.
Where you are registered and service and supply of goods applies then the higher (21%) rate applies if the value of the goods comprised in the total charge exceeds 2/3 rds of the total charge to the customer ( the two-thirds rule) , otherwise the lower (13.5%) rate may be used.
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Generally if you are talking about high value items with a small installation charge, it will be 21%. However it is open to the trader to charge separately 21% for the goods and 13.5% for the service as two transactions.