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 (4%). The data shows that Bulb have seen the fastest growth of any supplier, moving from 0% market share in Q1 2017 to 4% market share in Q1 2019.

Q3 2017Q4 2017Q1 2018Q2 2018Q3 2018Q4 2018Q1 2019
British Gas21%20%20%20%19%19%19%
SSE14%14%14%14%13%13%13%
E.ON13%13%13%13%13%13%12%
EDF12%11%11%11%11%11%11%
Scottish Power10%10%10%10%10%10%10%
npower10%10%9%9%9%8%8%
OVO3%3%3%3%4%4%5%
Co-op Energy1%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%1%1%2%3%3%4%
Octopus0%0%1%1%1%2%2%
Small Suppliers8%9%8%8%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.