Bulb is the biggest green energy company

This claim is supported by data shared by Ofgem, the energy regulator, in their quarterly report 'Electricity supply market shares by company: Domestic(GB)' (as of October 2018). This report shows the market share of energy companies over time. All suppliers with a market share of 1% or above are shown individually. Suppliers with less than 1% market share are grouped together and listed under 'Small Suppliers'.

There are only three green energy suppliers listed as having a market share of 1% or above: Bulb, Co-operative Energy and Green Star energy. These suppliers are understood to be 'green energy companies' as they supply 100% renewable electricity. Ofgem defines a 'green tariff' as 'those tariffs with at least 100% renewable electricity'. You can find a full list of 100% Renewable Electricity suppliers here.

Of the three green energy suppliers in the report, Bulb hold the biggest market share (3%). The data shows that Bulb have seen the fastest growth of any supplier, moving from 0% market share in Q1 2017 to 3% market share in Q3 2018.

Q1 2017Q2 2017Q3 2017Q4 2017Q1 2018Q2 2018Q3 2018
British Gas22%22%21%20%20%20%19%
SSE15%14%14%14%14%14%13%
E.ON14%13%13%13%13%13%13%
EDF12%12%12%11%11%11%11%
Scottish Power11%11%10%10%10%10%10%
npower9%10%10%10%9%9%9%
First Utility3%3%3%3%3%3%3%
OVO2%2%3%3%3%3%4%
Co-op Energy2%1%1%1%1%1%1%
Utilita2%2%2%2%2%2%2%
Utility Warehouse2%2%2%2%2%2%2%
Green Star1%1%1%1%1%1%1%
Bulb0%0%0%1%1%2%3%
Octopus0%0%0%0%1%1%1%
Small Suppliers6%7%8%9%8%8%8%

This chart shows the domestic electricity market shares for each of the six large suppliers, medium and small suppliers.

The full report is published on the Ofgem website.