How do offline earnings work in idle games?

The best-loved part of idle (or "incremental") games is that progress keeps going after you close them. Opening the game the next day to a "while you were away, you earned..." message is one of the genre's core pleasures. But how is that number calculated, and why is it limited in most games? This article explains the general logic, with examples from our own game, Crystal Depths.

Does the game really run while you're away?

In most browser games, no. When the tab closes, the game stops. It only stores the time of your last save. When you come back, it calculates the time in between and gives you what you would have produced, all at once:

offline earnings = time away × output per second × offline rate

That's why the game doesn't need an internet connection or a server for this; the math happens entirely on your device. Crystal Depths works this way too: your save lives only in your browser.

Why is there a cap?

Almost every idle game limits offline earnings to a certain time. In Crystal Depths the cap is 8 hours. The reason is simple: without a cap, someone who doesn't open the game for a month would blow past the whole balance in one go, and the rest of the game would stop making sense. The cap also gives players a rhythm: if you check in two or three times a day, you miss nothing.

Practical tip: checking in before the cap fills keeps your earnings flowing. Opening the game once in the morning and once in the evening is ideal for an 8-hour cap.

Why is the rate lower?

Many games don't give full output while you're offline, only a share of it. In Crystal Depths that share starts at 50%. This does two things:

Keeps active play worthwhile: triggering Resonance, catching geodes and buying upgrades while you're at the screen should move you faster than waiting.
Gives you something to work toward: upgrades that raise the rate and the cap become long-term goals. In Crystal Depths, the Deep Sleep charm raises the rate to 100% and the cap to 24 hours.

How to make the most of offline earnings

Spend before you leave. Offline earnings are based on your output at that moment. Buying every producer and upgrade you can afford before closing directly raises what you'll earn while away.
Spend as soon as you're back. When you return with a big offline haul, buy new producers right away. Crystals sitting in the bank produce nothing.
Know your cap. With an 8-hour cap, staying away for 20 hours is no different from 8.
Prioritize permanent upgrades. Things that raise the offline rate or cap (like Deep Sleep) are the best purchase for players who check in rarely.

Are offline earnings fair?

Because earnings are calculated from your device's clock, moving the clock forward could in theory work. That's one reason games add a cap and ignore a clock that goes backwards. In a single-player game this hurts no one. But in competitions with leaderboards and prizes, earnings have to be calculated on a server. That's why the monthly seasons coming to Crystal Depths will run on the server, with the same rules for everyone.

Quick summary

Offline earnings = time away × output × rate, up to the cap.
Spend before closing and right after you return.
Check in before the cap fills; two or three times a day is enough.
If you play rarely, prioritize permanent upgrades that raise the rate and cap.

To see it in action, open Crystal Depths, buy a few producers, close it and come back in a few hours.