Interpreting specs such as these found on many marketplace sites can be a bit of an art form, especially when no datasheets are provided.
When in doubt, the best idea is to ask the seller. If they cannot provide the information, then you have two choices: take pot luck, or find another seller!
That being said, here's how I read the information you are being provided in this case:
- Input Voltage: Dc 12V – 3A
.. is I believe the spec of the power supply that is provided, not the strip itself. In other words, capable of delivering a total of 36W at 12V.
- Lamp power: 12V 4.8 Watts
.. is most likely per meter i.e. each 60 LEDs. That is a pure guess based typical power requirements and how specs for similar LED strips are presented. You can divide/multiply this out by the actual number of LEDs you plan to use.
If you are planning to power a strip with batteries, the main challenges will be battery life and how many LEDs you have the power to drive.
Trying to use 2x9V does not seem like a very good idea. While you could probably drive at least a few LEDs with 2x9V, you will be wasting a lot of their already-limited (~500mAh) capacity in the step-down to 12V. And a step-down with resistors is going to be tough to get right. If you cut the strip and can squeeze say 0.5A out of the batteries, that's still 3W to burn off. A voltage regulator would be better if you really must.
AA or even C, D or 6V lantern batteries will be able to drive more LEDs for longer, and you can avoid losses in voltage regulation.
Most of these strips are laid out in a repeating pattern of three LEDs and their current-limiting resistors along a flexible circuit. There will be scissors marks every 10 cm or so.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Cut along the dotted line.
If it's this type of strip then the LEDs are effectively wired in series parallel along the length of the strip. You can connect the strips end to end but it should be clear from the schematic that you're actually connecting the individual strings of LEDs in parallel.
If you daisy-chain the strips then be aware that the first strip has to carry the current for all the downstream LEDs and the copper may overheat.
Alternatively wire each strip individually back to the PSU.
Figure 2. All you need to know.
- You need a 12 V supply.
- The current rating needs to be at least that required by the LEDs.
\$ 10~m \times 1.5~A/m = 15~A \$.
\$ P = VI = 12 \times 15 = 180 W \$.
Be careful daisy-chaining the strips. The start of the first one will have to carry the full 15 A, falling to zero at the end of the second. It would be safer to feed 12 V in both ends. That way each end would carry 7.5 A, falling to zero in the centre.
Best Answer
You haven't provided much info. First white thing is labelled as transformer for "LV Halogen Lamps". Most likely it's just a step down transformer which will fry your christmas LED strip during reverse polarity.
Second one is a mystery box without much info. I wouldn't go for that either.
Here is what you need to do:
Figure out what voltage is required by your LED strip - 3V, 5V, 12V or something else?
If the LED strip is 5 watts, then you can calculate the current requirement as follows:
Current = Power / Voltage required.
Ex- If LED strip needs 5V, then you will need 5W/5V = 1A of current.
Go to the market and ask for a 5V 1A(or higher) wall adapter. Modify the voltage and current accordingly. I am just giving an example.
Get something less intimidating like this:
EDIT after the question was edited:
Figure out what length of strip you want to use. I will assume 0.7 meters which will give you total power requirement:
Power = Length X Power required per unit length = 0.7 X 7.2 = 5 watts.
Voltage = 12 V
Current = Power / Voltage = 5/12 = 0.42 A
Better to go for 12V 0.5A (or higher) wall adapter or any DC power source. Make sure it's a DC power source and not just a step down transformer. If the power source is DC 12V 12W, then it will do the job without any issues.