Solar export limits on commercial sites

A solar export limit is a cap the distribution network operator places on how much power a site can send back to the grid, and a capped connection reduces returns only in the half hours when the array generates more than the site is using plus whatever the cap allows. On a commercial rooftop, where the building draws a large load through the working day, most of the solar is consumed on site and never reaches the cap, so the export limit reduces the return by less than the size of the cap suggests. The energy lost is the generation above the cap that can be neither used on site nor exported, and a battery can store much of it for later use.

What a solar export limit is

When a site applies to connect solar to the grid, the network operator grants an export capacity: the maximum power the site is permitted to push back onto the network. Where the local network has room, that figure can match the full output of the array. Where the network is constrained, the operator can offer a lower export limit, or a connection with no export permitted at all, while still allowing a much larger array than the cap alone would permit, sized to the site's own use.

The limit is held to by an export limitation scheme, a control device that measures the power flowing at the connection and throttles the inverters, or disconnects them, before export rises above the agreed level. In Great Britain these schemes follow the Energy Networks Association's Engineering Recommendation G100, and the connection itself is governed by G99. The scheme is what lets a developer install more solar than the export cap alone would permit, because the cap constrains what leaves the site rather than what is generated behind the meter.

Why network operators set them

An export limit is a way of connecting solar to a part of the network that could not absorb the array's full output. Reverse power flow on a circuit designed to carry demand can push voltages outside their statutory range and load equipment beyond its rating. Rather than require the reinforcement that unrestricted export would need, which is slow and expensive, the operator offers a connection that caps what the site can send back. For a behind-the-meter project this is often an acceptable trade, because the point of the solar is to displace imported electricity, and selling power to the grid is a secondary consideration.

What a cap does to the return

The return on behind-the-meter solar comes mostly from the electricity the site no longer has to buy. Every kilowatt-hour generated and used on site avoids the full import price, including the network and policy costs added to the wholesale rate. Exported energy earns far less, whatever the site is paid for it under its export arrangement. So an export limit reduces the return only to the extent that it curtails generation the site would otherwise have sold, or holds the array below its potential in the highest-output half hours.

Whether that matters depends on how the site's demand lines up with its generation. A site that runs a heavy load through daylight hours, a factory or a distribution centre, consumes most of its solar as it is produced, and an export cap set anywhere near the load rarely binds. A site with a small daytime load and a large roof reaches the cap often, and each half hour that hits the cap is generation lost. How much a given export limit costs depends on the shape of the site's demand more than on the size of the roof.

How to size a project around a limit

Sizing solar against an export limit starts from the site's own half-hourly demand rather than the roof area. Matching the modelled generation to the measured load, half hour by half hour, shows how much of the array's output would be self-consumed and how much would hit the cap. The array can then be sized so that the curtailed energy stays small, or a larger array can be justified where the curtailment it causes is worth less than the extra self-consumption it delivers.

A battery recovers energy that would otherwise be curtailed by the cap. Surplus generation that would breach the export limit can be stored instead of curtailed, then discharged when the solar has dropped and the site is still drawing load, or into the evening when imported electricity is dearest. This shifts energy that would otherwise be lost to the cap to a more valuable moment, which is often what makes a heavily capped connection worth building on. A no-export connection, where none of the surplus can be sold, is where storage earns the most, because storing the surplus is the only way to capture any of its value. The same battery also lowers the power the site draws through its import capacity at the moments that set it.

Lupa takes a site's half-hourly data and its electricity invoice and models solar and storage against an export limit directly, so the curtailment a given cap would cause, and the extent to which a battery recovers it, are read off the site's real demand rather than assumed. For each combination of solar and storage, the platform shows how much generation the site self-consumes and how much is lost to the cap.