Various programs will tell you how much some things consume, if there are sensors for it.
For example HWInfo64 will probably be able to tell you the power consumption of the cpu, the video card (if not integrated in cpu), maybe even the monitor power consumption is reported (led driver for the backlight)
A laptop would be somewhere in the 30-50 watts.
The actual lcd screen anything between 5w or less and maybe 20-30w if you run it at full brightness, ssd is mostly idle (1w or less), processor it depends on how much processing power you need and how you configure it in power management
If you want extremely accurate results, you could measure it as it comes out the battery (ex solder wires to the battery connector in laptop, solder the wires to the battery contacts, add a multimeter in series to measure current and use another to measure voltage ... then you can multiply voltage with current to get the power consumed at that moment in time.
Anyway... a 12v 7Ah battery is quite small, and would most likely keep a laptop consuming 30w working for around 10-15 minutes, maybe even less, because the inverter is not 100% efficient and the adapter of your laptop is also not 100% efficient converting the AC back to DC, so you have two conversions that result in losses.
Here's a typical 12v 7Ah datasheet, has some charts on the second page :
https://media.digikey.com/pdf/Data%20Sheets/Zeus/PC7-12.pdfYou don't want to let the battery discharge to less than around 10.5v (or around 1.75v per cell) if you want it to charge and discharge for a lot of cycles, so look at those lines (highlighted in the pic below)
Look for a value closest to the 30w I went with ... you can see
26.41 in the 10 minutes column.
That's about how much a 7Ah battery would provide 30w constantly under the voltage drops to around 10.5v and you should stop discharging the battery