Switching a building between solar and the grid is easy. Proving it actually switched is the hard part.

The control plane and the product around it: the device model and claim-by-code ownership, the confirmation protocol that reads a source back from the hardware before calling a switch done, the desired-state scheduler and its reconciliation behaviour, the precedence rules between manual and automatic control, and the installable web application. He leads it as Head of Product & Platforms at BijliBachao.pk, in the Wattey product family.

That you cannot see the thing you are controlling. Every other difficulty follows from it: you cannot trust that a command was executed, you cannot assume a device was awake when you needed it, and you cannot tell whether a state you did not expect came from a fault or from a person. The three rules on this page — confirm from the sensor, reconcile to desired state, give manual intent defined precedence — are each an answer to one of those.

Because the software can be wrong, and this particular rule cannot be. Bridging two live supplies is a fault on a customer’s premises, not a defect to fix in the next release. A software check is faster to build and demonstrates identically, so choosing the interlock means deciding up front that your own system will eventually fail and designing for that day. It is the clearest example on this page of a judgement call rather than an implementation detail.

Because the source material has none. The product brief this chapter is built from states no device count, no install base and no uptime figure, and no customer is named anywhere in it. Rather than estimate a figure to make the page look substantial, the chapter argues from the engineering — which is the stronger case for this product regardless. If a verified fleet figure arrives it will be added with a date attached.

STS is transfer-switch equipment, so it has a real standards home. The device is designed to the intent of UL 1008 (the safety standard for transfer switch equipment), and its single-source behaviour is aligned to the ATSE model of IEC 60947-6-1, which connects the load to one source at a time. The telemetry runs on MQTT, an ISO/IEC 20922 standard. The interlock serves the same objective as anti-islanding standards like IEEE 1547 — a local source must never back-feed the grid — but meets it structurally, through a physical break-before-make interlock. STS is aligned to these standards; it is not certified or Listed to them, and the page says so plainly.

Because a building on rooftop solar has two supplies and can only safely use one at a time, and the decision is made all day, every day. Switch late and you burn expensive grid power while the sun is up; forget to switch back and you leave a dead load on solar at night; get it wrong badly enough to bridge both and you have a back-feed hazard. Pakistan added roughly 17 GW of solar imports in 2024 and runs two-to-eight-hour load-shedding, so millions of sites now make this switch by hand. STS makes it scheduled, confirmed, and safe — which is the same problem anywhere a solar boom meets an unreliable grid.

They are three parts of one account of a site’s electricity, and Ali built all three. WattEY measures what the site draws from the grid and what it costs under the tariff. Solar Performance Cloud inspects what the site’s own solar generates, string by string. STS controls which of the two sources is actually carrying the load — and until that switching is recorded, neither of the other measurements can be attributed to the right source.

Nearly all of it. Strip out the supplies and this is a control plane driving a physical actuator it cannot observe directly, over a network it cannot rely on, alongside a human who can overrule it — with one invariant important enough to be enforced below the software. That is the same shape as fleet or robotics control, industrial process automation, and any orchestration system that reconciles a desired state against a real one. The judgement about which guarantees must be physical is the part that generalises.