Connection checker: route, clipping and export limits

What the checker works out

1. The route. It adds up what the inverters could send out together: the solar inverter, plus a battery's inverter if the battery has its own. Up to 3.68 kW per phase (16 amps at 230 volts) is the connect-then-notify route; above that, the installer applies first.
2. Clipping. When the panels make more than the inverter can handle, the excess is lost. The checker takes the inverter's rating as a share of the panels' rating and reads off how much of a year that loses in Widnes, for the direction you chose.
3. The export limit. If you enter a limit below the inverter's rating, it works out how much more could be lost if nothing in the house used the power at those moments.
4. A year's output. The panels' rating times the PVGIS long-term figure for Widnes at 35 degrees, less both losses.

Six systems, worked through

Our sums from PVGIS 5.3 for Widnes: losses from the 2020 hourly series, yearly output from the long-term averages. Facing south unless stated.

Two things stand out. The fourteen-panel system makes about the same on a 3.68 kW inverter as on a 5 kW one with a 3.68 kW limit, because the limit's loss is a worst case and the clipping is real; the difference is that the larger inverter leaves room to use or store the extra at home. And an east-facing roof barely clips at all, because the sun never strikes it square on through the middle of the day.

To turn these outputs into money, set each system's cost at the North West averages on prices against the first-year sums on is it worth it?, and take the inverter rating and route to each firm using the questions on choosing an installer.

Why panels seldom reach their rating

A panel's rating is measured in a laboratory: strong light falling square on, with the cells held at 25°C. On a roof in Widnes the light is weaker or arrives at an angle for most of the day, warm panels lose a little efficiency, and wiring and the inverter take their share. In the 2020 PVGIS hourly series, after its standard 14% allowance for those losses, south-facing panels at 35 degrees never delivered more than 87% of their rating in any hour, and east-facing ones not more than 77%. That is why an inverter at 75% to 80% of the panels' rating loses so little: 0.5% and 0.1% of a year respectively, facing south.

Below that it rises quickly. At 60% the loss is 4.7%, and at half the panels' rating 10.2%. An export limit works the same way at the boundary of the house, but only on what is not used inside: a limit at 60% of the panels' rating could cost up to 4.7% if nothing at all were used at home.

What it does not cover

• Three-phase homes. The checker multiplies the limit by three; the network may still ask for the output to be balanced across phases.
• Local network capacity. Whether SP Energy Networks approves a larger system, or asks for a limit, depends on the cables in your street. Only an application answers that.
• Shade. The figures assume an unshaded roof. Chimneys, dormers and trees lower both the output and the clipping.
• Year-to-year weather. The losses come from one year, 2020, and would differ a little in a sunnier or duller one.

Related pages

• G98 and G99
• Inverter sizing
• Export limits
• Batteries and the grid

Sources for this page

1. PVGIS 5.3 hourly series for Widnes, 2020: the basis for the clipping and export-limit figures
2. PVGIS 5.3, European Commission Joint Research Centre: long-term yearly output for Widnes by tilt and direction
3. SP Energy Networks, connections: small generation and how to notify or apply
4. Halton Borough Council infrastructure plan, November 2010: which company runs the borough's network, and the Widnes Waterfront constraint
5. MCS MIS 3002 v6.0: the installation standard for solar PV