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Engineering & compliance · SaaS · Case study

A cable calculator that turnstwo British Standards into asubscription product.

BS7909 and BS7671 calculations electricians trust on site, a permanent free plan, Stripe subscriptions with verified webhooks, on Next.js 15.

2British Standards encoded
1permanent free plan
15Next.js major version
v17Stripe SDK
Compliance calculator · Pro live
Cable size16 mm²
Voltage drop2.1%
PlanPro
Happening now
  • Load 32 kW · 45 m · outdoorBS 7909within limit
  • SubscriptionPro · monthlyactive
  • Webhook invoice.paidverified
  • Card retryday 3scheduled
The problem

Temporary power for events and sites is governed by BS7909 and BS7671; the founder wanted a product electricians open on a phone in a field. Repetitive, error-prone, consequential when wrong.

The approach

A permanent free plan converts on limits; Stripe subscriptions with plan gating, verified webhooks, proration and dunning; admin matches Stripe. Commercial model before screens.

Architecture & AI decisions

Billing that behaves under failure

Verified webhooks update access in real time; an ORM incompatibility with the edge runtime was resolved by moving the affected routes to Node without a rewrite.

01

Plan gating in the application, proration on plan changes

Plan gating in the application, proration on plan changes

02

Dunning on failed cards

Dunning on failed cards; downgrades at period end with data retained

03

Admin subscriptions page mirrors Stripe

Admin subscriptions page mirrors Stripe

04

Standards logic as tested calculations

Standards logic as tested calculations

What shipped.

01

Standards calculator

Cable size, voltage drop
02

Stripe subscriptions

Gating, proration, dunning
03

Webhook handler

Verified, real-time access
04

Admin subscriptions

Same view Stripe has
05

Free plan

Permanent, converts on limits
06

Next.js 15

Runtime per route
07

Customer billing pages

Self-serve changes
08

Vercel

Previews and rollbacks
Result
2standards
1free plan
0manual billing steps
1runtime fix that kept billing online
Next.js 15TypeScriptPrismaPostgreSQLStripe SDK v17VercelNext.js 15TypeScriptPrismaPostgreSQLStripe SDK v17Vercel
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