What is the electricity volatage in Beijing? What is the reliable prepaid simcard for mobile phone?

The Electricity in China(https://www.travelchinaguide.com/essential/electricity.htm) is generally 220V, 50HZ, AC. If you travel to China and wish to bring electric devices for use during your stay, a transformer, which can be bought in China for CNY100-200, is necessary. Most of the hotels in China have both 110V and 220V electrical outlets in the bathrooms, though in guest rooms usually only 220V sockets are available.
As the shape of a socket varies between countries, a portable plug adaptor may also be necessary. It can be purchased from travel stores (such as Franzus) or electronics stores (i.e. RadioShack, Best Buy) in your local countries. Also you can buy it in China.
After arrive at the airport, you can buy reliable prepaid simcard for mobile phone, it may be a little expensive. You can also go to downtown to buiy it in the China mobile phone business halls.