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GarageWatts

Project Overview

GarageWatts is a home EV charging cost tracker that gives drivers an exact cost-per-mile figure based on their actual electricity rate — without guessing, without a $25/month subscription, and without relying on the manufacturer's app. It pairs with a Shelly Plug ($24.99) to automatically capture every charging session, then applies the user's real utility rate to calculate true cost per session and per mile.

Most EV owners have no idea what their car actually costs per mile to charge at home. Factory apps show kWh delivered but apply national average rates that can be off by 30–50%. GarageWatts closes that gap: plug in the Shelly, log an odometer reading once a week, and the dashboard does the rest — automatically.

$24.99 hardware cost for a Shelly Plug Gen4 — the only required purchase. Breakeven against a factory app subscription occurs in under 8 months.

Automatic session capture tracks kWh delivered, session duration, and cost in real-time — no manual entry required.

$3/month Pro tier versus $25/month for factory apps — unlocking cost-per-mile calculations, efficiency trends, and unlimited vehicle support.

The Problem

EV owners charging at home face a fundamental visibility problem: their electricity bill goes up, but there's no clean way to attribute a specific dollar amount to their vehicle. Factory apps from manufacturers like Tesla, Rivian, and GM apply national average electricity rates — figures that can be 30–50% off for users in high-rate states like California, New York, or Hawaii.

Third-party EV tracking apps exist but require manual session logging or rely on telematics data not available on all vehicles. The result is that most EV drivers are operating blind: they know they're saving money versus gas, but they can't quantify how much, and they can't optimize around time-of-use utility rates. No affordable, automatic, hardware-agnostic solution existed for home-charged EVs across all makes and models.

How It Works

Install a Shelly Plug US Gen4 ($24.99) into the 120V wall outlet used for L1 charging
The plug reports kWh consumed, session start/end times, and live power draw directly to GarageWatts
Log your odometer once a week (30 seconds) — or connect an OBDLink MX+ ($139.95) for fully automatic odometer capture
GarageWatts applies your actual utility rate to each session and calculates cost-per-mile between consecutive odometer readings
View a live dashboard with session history, monthly totals, and efficiency trends — export to CSV or PDF anytime

Technical Architecture and Decision Making

Built for zero infrastructure overhead and a sub-$10/month server cost at launch. Every technology choice was made to keep the product fast, lean, and maintainable by a solo developer without sacrificing reliability.

HARDWARE INTEGRATION: SHELLY PLUG

Local MQTT · REST API · real-time energy reporting · no cloud-lock

Shelly devices expose a local REST API and MQTT feed that reports energy consumption in real-time without routing data through a proprietary cloud. This means GarageWatts owns the data pipeline end-to-end, with no third-party rate limits or outage dependencies. The Gen4 Plug also supports scripting, enabling edge logic (like automatic session detection) directly on the device.

BACKEND: SUPABASE (POSTGRESQL)

Row Level Security · multi-tenant isolation · real-time subscriptions

Row Level Security enforced at the database layer gives each user complete data isolation with no risk of cross-account data leakage — even through a misconfigured query. Supabase's free tier supports growth to 50K MAU before any database infrastructure cost is incurred, and built-in real-time subscriptions power the live dashboard without a separate WebSocket service.

FRONTEND: NEXT.JS + VERCEL

Edge-rendered dashboard · zero DevOps · automatic deployments

Next.js with Vercel Pro handles the dashboard, marketing site, and API routes in a single deployment with global CDN. No server to manage, no scaling configuration, and no ops work during early growth — keeping focus on product iteration and customer acquisition rather than infrastructure maintenance.

OPTIONAL OBD INTEGRATION: OBDLINK MX+

Automatic odometer reads · Bluetooth BLE · cross-make vehicle support

The OBDLink MX+ connects via Bluetooth LE to the GarageWatts mobile app, automatically pulling odometer readings from the vehicle's OBD-II port. This eliminates the only manual step in the workflow, making the system fully automatic for users who opt in. The MX+ supports all OBD-II compliant vehicles (2003+ US models) regardless of manufacturer.

Business Model

Freemium SaaS with a hardware-first acquisition funnel. The Shelly Plug is a one-time purchase users make independently — GarageWatts doesn't sell or bundle hardware, keeping margins clean and removing inventory risk. The free tier provides enough value to drive activation, and the Pro tier unlocks the features most meaningful to cost-conscious drivers: cost-per-mile, efficiency trends, and unlimited data history.

Free

$0/mo

Automatic charging capture
Cost per session
Monthly cost totals
Live dashboard
90-day history · 1 vehicle

Pro

$3/mo (Billed Annually)

Cost per mile calculation
Efficiency trends
Unlimited history & vehicles
CSV + PDF export
Charging notifications
Mobile app + OBD integration

Factory App

$25/mo

Estimated rates (not actual)
Single make/model only
Limited export options
$300/year

Status & Outcome

GarageWatts is in active development with early access available at garagewatts.com. No credit card is required to register, and users can upgrade to Pro at any time. The OBDLink mobile integration is on the roadmap and will eliminate the last remaining manual step — making GarageWatts a fully automatic, set-and-forget cost tracker for any home-charged EV owner.

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