What return did my DeGiro portfolio actually make?
The obvious way to answer this -- what it's worth now, minus what you've put in -- is also the way most likely to mislead you, and the more deposits you've made, the worse it gets.
The obvious calculation, and why it isn't a return
“I've put in €20,000 and it's worth €23,000, so I'm up 15%” is arithmetic, not a return. A return has to answer a question with a timeframe in it — 15% over what period, earned by which money? — and “everything I've ever deposited” is not a period. It's a pile of deposits made on different dates, each of which has had a different amount of time to grow.
Put €10,000 in on day one and it grows 15% by year end: a plain 15% return, no argument. Add another €10,000 the week before year end and the same €23,000 balance now means something closer to a 30% return on the money that was actually exposed to the market for most of the year — the second €10,000 barely had time to move. Total value minus total deposits can't tell these two years apart, because it never looked at when the money arrived.
What a real return has to account for
A return that means something either removes the effect of cash-flow timing (a time-weighted return, TWR) or prices it in on purpose (a money-weighted return, XIRR) — see the TWR vs XIRR guide for what each one actually answers, and a worked example of the two disagreeing on the same account.
Either way the ingredients are the same: every deposit and withdrawal, dated, plus a daily value for the account in between — not just a start and an end balance. Two numbers can't tell a steady year from a lucky, or unlucky, one.
What the export already has
Transactions.csv and Account.csv between them carry every trade, deposit, withdrawal, dividend and fee with its own date — everything a real return calculation asks for. What they don't do is the calculation itself: daily valuation, chain-linking sub-period returns, and solving for a rate for XIRR. That's the part this tracker does with the two files once you upload them, and it's also entirely doable by hand for anyone who'd rather check the arithmetic than trust it.