A billing engine is the system that turns usage and subscription data into charges. It rates events, applies pricing and discounts, manages balances and invoices, and tells the payment processor what to collect.
A billing engine is the part of a monetization stack that decides what a customer owes. It sits between the product, which generates usage and subscription events, and the payment processor, which moves money. Everything between those two ends, rating, pricing, balances, and invoicing, is the billing engine's job.
How Billing engine works
A billing engine ingests events (usage, subscription changes, plan purchases), applies the pricing model to rate them, and produces an amount owed. Along the way it handles discounts, proration, tax hooks, credits, and entitlements, then either issues an invoice or debits a balance, and hands the payable amount to a payment processor.
Engines differ mainly on timing. An invoice-based engine aggregates events over a cycle and rates them at the end. A real-time engine rates and applies each event as it happens. The same core responsibilities apply; what changes is when the charge is computed and whether a balance is checked before usage occurs.
Billing engine examples
A subscription product uses a billing engine to charge $49 per month, prorate mid-cycle upgrades, apply a coupon, and generate the monthly invoice. The engine computes the amount; a payment processor charges the card.
An AI product uses a billing engine to rate token usage, draw it down against a prepaid balance, and trigger an auto top-up when the balance runs low. Here the engine also performs real-time billing: it checks the balance before each request rather than at month end.
Billing engine vs Payment gateway
| Billing engine | Payment gateway | |
|---|---|---|
| Job | Rate usage, price, invoice, manage balances | Authorize and capture card payments |
| Output | An amount owed and an invoice or debit | A settled transaction |
| Examples | Subscription, usage, and hybrid logic | Stripe, Adyen, Braintree (processing) |
| Relationship | Tells the gateway what to collect | Executes the collection |
Benefits & when to use it
A dedicated billing engine matters once pricing outgrows a flat monthly charge. Tiers, usage, hybrid plans, proration, credits, and entitlements are hard to maintain in application code; a billing engine centralizes that logic so pricing can change without redeploying the product.
Teams reach for one when they adopt usage-based or hybrid pricing, support multiple plans, or need to separate billing logic from the payment processor so they can keep or swap the processor independently.
FAQ
What is the difference between a billing engine and a payment gateway?
A billing engine decides what a customer owes: it rates usage, applies pricing, and produces invoices or balance debits. A payment gateway moves the money by authorizing and capturing card payments. The engine tells the gateway what to collect.
Does a billing engine replace Stripe?
Not necessarily. Stripe provides both a payment gateway and billing features. A separate billing engine can handle rating, balances, and entitlements while still using Stripe (or another processor) to actually collect payment.
What is a real-time billing engine?
One that rates and applies each usage event as it happens and can check a balance before usage runs, rather than aggregating events and rating them at the end of a cycle.
How Credyt handles Billing engine
Credyt is a real-time billing engine for AI products. It rates each usage event the moment it occurs, draws it down against a customer's multi-asset wallet, and authorizes or blocks based on live balance, all before handing collection to a payment processor like Stripe. Because it sits in front of the processor rather than replacing it, teams keep the PSP they already use. Explore Credyt →