Add the values of items with same name in the table

I have heater, lamp, tv and so on in a table. the table records their details. There will be some items with more than one entry. such as below.

---

device watt watt used

heater 220 2000

heater 220 2000

heater 220 2500

tv 1000 3000

tv 1000 5000

---

I wan to total up all watt used for heater and tv respectively for display in the pie chart, such as heater=6500 W, tv=8000 W. Also, there are many more items that can be added. so how can i do this so that it total up the watt used of every items in the list only.

Thank you