How do I write the value of a single property of a object?

This is how my current script looks like:

$cpu = Get-WmiObject win32_processor | select LoadPercentage
logwrite $cpu #this fuction writes $cpu into a .txt file

The output of the file is:

@{LoadPercentage=4}

I want it to be only the number so that I can make calculations.

That is a pretty simple fix. Instead of selecting the LoadPercentage when running Get-WmiObject, just select the property when calling your function. This will write only the number to your log file.

$cpulogpath = "C:\Monitoring\$date.csv"
function logwrite
{
    param ([string]$logstring)
    add-content $cpulogpath -value $logstring
}

$cpu = Get-WmiObject win32_processor #don't select the property here
logwrite $cpu.LoadPercentage #select it here