Swiss private equity firm Partners Group (<a href="https://www.six-group.com/en/market-data/shares/share-explorer/share-details.CH0024608827CHF4.html#/" target="_blank">SWX:PGHN</a>) has sealed a USD-2.2-billion (EUR 2.018bn) deal to acquire a portfolio of 11 natural gas-fired power plants in California as well as the company operating the assets -- Middle River Power, which is advancing a plan to add on-site battery energy storage systems (BESS) at its existing sites.