One bill, many tenants — the fairness problem
A building on a single bulk connection has to recover that one bill from many people who each used a different amount. The old ways don’t hold up. Manual meter-walking produces transposed digits and skipped meters; when a tenant challenges a bill, the owner has only a handwritten number as proof. Estimate-based splits — divide by area or headcount — are a guess: the resident who was away all month pays like the one who ran the air-conditioning every night.
Industry vendors estimate that manual tenant billing leaks 3–8% of total energy cost through reading gaps, calculation errors, and tenants underpaying without challenge, and that common-area energy — lobbies, lifts, parking, fire systems — is the single biggest source of disputes (EnSmart Controls, a billing vendor). The fix is the same one sub-metering has always promised: bill each user for what they measurably used, and hold the common area apart under a defined rule.
What solar changes
Rooftop solar makes the split harder, not easier. Now the building generates part of its own supply, so before you can divide anything you have to answer: how do the solar units and the utility bill combine, and who benefits from the cheaper self-generated energy? Ignore the solar and the numbers stop reconciling.
The clean way to handle it is a power ledger: total supply — the utility bill plus rooftop solar — minus what every tenant meter recorded equals line loss, surfaced every month. On a spreadsheet that figure cannot be computed at all; with a ledger it becomes visible and colour-graded, so the building is legible, not just the bill. Solar makes that accounting more important, because there is now more than one source feeding the same meters.
The deliverable is the evidence
What makes a sub-bill survive a challenge is not the invoice — it is the proof behind it. Every meter reading is captured with a timestamped photograph of the meter face and read by OCR, with weak reads routed to a person rather than billed as a blind guess. Every bill is generated from one published formula, reconciled against the single utility bill, and the month is then closed into a frozen, immutable snapshot.
The guarantees sit where a software glitch can’t reach them: the system enforces one reading per meter per month and no new readings until last month is closed, sets its own timestamps, keeps immutable paid and closed records, and attaches a name to every action. That is what turns a bill from “trust me” into “here is the photo, the formula, and the closed month.”
Where BijliBachao fits
BijliBachao builds sub-billing as two products in the WattEY family, for two different buildings. TenantBill handles commercial multi-tenant property — malls and plazas billing dozens of shops per-shop, with the power ledger and photo-verified readings. Wattey Sub-Billing handles shared living — hostels, student and worker accommodation, co-living — splitting per person and pro-rated to the days each resident was actually present, settling a leaver on the day they move out and metering the common area without billing it.
Because BijliBachao also installs the solar, the generation and the billing can be handled by one engineering house: the rooftop system feeds the power ledger as part of supply, and the split accounts for it from day one. The framing stays honest — measured, not estimated; the evidence, not just the invoice.
