Not One Hole in an Old Roof: 40 kW Ballast-Mounted Solar with DG Sync at Liberal Traders, Okhla

Why this roof needed a different approach

• An old building: the roof is a concrete slab on the second floor of a building that has stood in Okhla for decades. Nobody could be sure how the slab would take anchor bolts, and a leak into the floor below was not a risk the owner would accept.
• No drilling allowed: the condition was absolute. No holes, no chemical anchors, no core cutting.
• A DG that runs: Okhla sees power cuts, and the site runs on a 100 kVA diesel generator when the grid fails. A standard solar inverter shuts down the moment the grid goes, and cannot simply be left running with a DG.
• Net metering: the owner wanted surplus daytime generation credited against the bill, which means the plant had to pass BSES Rajdhani's technical checks and be connected through a net meter.

Ballast instead of bolts

A ballast-mounted structure is held in place by weight. Precast concrete blocks sit on the roof surface and the galvanised steel frame is fixed to the blocks, not to the slab. Nothing penetrates the roof, so the waterproofing stays exactly as it was.

The blocks were sized so that the array stays put in wind, and spread so that the load on the old slab is well within what it carries every day. The frame carries the modules at a low tilt to keep wind uplift down, which is what lets ballast do the job that bolts would normally do.

Running solar with a 100 kVA DG

When the grid fails and the DG starts, an on-grid inverter normally switches off, which means the solar sits idle for exactly the hours when the site is burning diesel. Letting it run freely is not safe either: if the factory load drops, solar can push power back into the generator.

At Liberal Traders a DG synchronisation controller sits between the inverter and the generator. It lets the inverter run while the DG is on and holds solar output back so that the generator always carries a safe minimum share of the load. Every unit of solar produced during a power cut is diesel that does not have to be burned.

Net metered with BSES Rajdhani

MGetEnergy handled the net metering application with BSES Rajdhani Power Limited from start to finish: feasibility, the single line diagram and component list for approval, the installation certificate, and the completion certificate that triggers the net meter installation. The plant exports surplus daytime power to the grid and the owner is credited for it on the electricity bill.

What we installed

• Solar array: 40 kW of 380 Wp mono PERC modules on the second floor roof
• Structure: galvanised steel on precast concrete ballast blocks, no drilling into the slab
• Inverter: Luminous three-phase grid tie string inverter, wall mounted indoors
• DC and AC distribution boxes by Efab Power Control, with Type II surge protection and L&T breakers
• Polycab copper DC cable and armoured AC cable, with a dedicated earthing and lightning protection system
• DG synchronisation controller for the 100 kVA generator
• BSES Rajdhani net meter

Maintained under a five year CAMC

• Module cleaning and inspection
• Ballast blocks and structure checked for movement and corrosion
• Inverter fault logs, ventilation and cooling
• DG synchronisation controller checks
• Earthing, surge protection and net meter checks

What this means for other old buildings in Delhi

Okhla, Naraina, Mayapuri, Wazirpur and the older estates of Delhi are full of buildings where the landlord or the structure will not allow drilling. That does not rule out solar. A ballast-mounted structure, a DG synchronisation controller and a properly handled net metering application turn an old roof into a working power plant without touching the slab.

Frequently asked questions