There's a bit involved here but it can be done.

1. Note how many lights in the set (eg 300)

2. Look at the transformer - it should state the output voltage (eg 36 V)

3. Within the set of lights, there will be several groups. For example, a set of 300 lights might consist of 20 groups each of 15 bulbs. You can work this out by noting how many bulbs go out when ONE bulb is removed.

4. Divide the transformer voltage by the number of bulbs in each group. So for example, if it's a 36V transformer and there are 15 lights in each group then (36/15) that's 2.4 volts per bulb (this is a fairly common size). You'll need to do the math for your set of lights however - DON'T assume anything.

5. Now look at the transformer again. It should also state an output power rating in Watts (W) or alternatively VA. The two measurements can be considered interchangeable in this situation, although strictly speaking they are not the same thing (but treat them as the same here). Your transformer might be rated at, for example, 60VA or 60W (60 Watts).

6. Now divide the W or VA rating by the TOTAL number of lights in the entire set (eg 300). This will give you the power rating in Watts per bulb. So for a 60W transformer and 300 lights (60/300) it would be 0.2 Watts per bulb.

7. In practice, you'll find the actual bulbs used are generally slightly lower power than the maximum the transformer can supply but will ALWAYS be the correct voltage as worked out above. So in this example your bulbs would be 2.4V (that's certain) and probably about 0.1875 Watts which is a standard size.

I've got about 15,000 Christmas lights running right now and bought a heap more this week. Happy to help with any problems - been doing this since 1992 so have learnt most of the tricks.

If you're going to get serious then I recommend annually servicing the lights when they're not installed and doing so sitting at a work bench inside. A volt stick and an earthed piece of metal nearby will come in handy and will save heaps of time finding faulty bulbs but you'll need to do a lot of experimenting to get this working well. Also a multimeter, soldering iron and a simple bulb tester are useful. Plus small screwdrivers, pliers, side cutters, long nose pliers, blowtorch, heat shrink and crimp links. And a hot melt glue gun comes in handy too.

No need for all of that if you're just putting up a few sets of lights though. But it takes on another dimension when you start rounding the number to the nearest 1000, running extra power circuits to power them all and end up with a traffic jam out the front. At that point productivity when servicing the lights becomes important (well, it does unless you have lots of spare time).

I service mine between January and October each year - takes somewhere around 75 hours of work all up. Then I spend a week putting them all up in late November.

Biggest light fixing tip I can give is to make sure both the socket and the bulb's contacts are clean. Use a small flat blade screwdriver and a hard surface (side of a pair of pliers works nicely) to gently scrape any oxide from the copper on the bulb leads. For the socket, again use the screwdriver but just do it in situ with the bulb out of the socket. (Only clean bulbs you have removed for some other reason - don't take them out just to clean).

Also if you're getting a large number of lights, be sure to label the lead and the transformer for each set with the exact same label so you don't have hassles working things out next year. And be sure to put the bulb size on the label - that will save lots of messing about.