Based on the following article, write a summary of about 100 words: Billions of pounds' worth of green energy projects are on hold because they cannot plug into the UK's electricity system, BBC research shows. Some new solar and wind sites are waiting up to 10 to 15 years to be connected because of a lack of capacity in the system - known as the "grid". Renewable energy companies worry it could threaten UK climate targets. National Grid, which manages the system, acknowledges the problem but says fundamental reform is needed. The UK currently has a 2035 target for 100% of its electricity to be produced without carbon emissions. Last year nearly half of the country's electricity was net-zero. Where does the UK get its energy and electricity? Wind generated a record amount of power in 2022 But meeting the target will require a big increase in the number of renewable projects across the country. It is estimated as much as five times more solar and four times as much wind is needed. The government and private investors have spent £198bn on renewable power infrastructure since 2010. But now energy companies are warning that significant delays to connect their green energy projects to the system will threaten their ability to bring more green power online. A new wind farm or solar site can only start supplying energy to people's homes once it has been plugged into the grid. Energy companies like Octopus Energy, one of Europe's largest investors in renewable energy, say they have been told by National Grid that they n Your response should contain less than 100 words.
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