Project finance will be sought for the remainder.
The solar project is expected to generate at capacity 200 gigawatts of energy, which is far more than any solar project in existence produces.
"This is a tremendous step in human history".
If it is built, the solar park will increase almost three times the capacity of Saudi Arabia to produce electricity, reaching 77 gigawatts in 2016.
Despite being one of the world's sunniest countries, Saudi Arabia generates most of its electricity from oil-fired power plants.
Son said revenues from early stage solar parks will help fund the construction of future projects in the kingdom.
The Saudi investment is Softbank's second major global solar project. "The Kingdom has great sunshine, great size of available land, great laborers, and great engineers", "But most importantly it has the greatest vision", Son added. The project is also set to more than quadruple Saudi Arabia's solar capacity, which is now around 60GW. That's a boost for Prince Mohammed, who's been at the forefront of the Vision 2030 campaign to diversify the kingdom's economy away from oil by that year. Those efforts have included high-profile investments in technology and plans for a vast new $500 billion city.
The project is planned for the Saudi desert and at 200 gigawatts, it's going to be 100 times larger than the next biggest proposed solar farm.
The Kingdom has accelerated the pace of deals in pursuit of the diversification targets of Price Mohammad Bin Salman Al Saud.
The Public Investment Fund (PIF) invested about $54 billion a year ago according to data from the Sovereign Wealth Fund Institute.
SoftBank CEO Masayoshi Son established the Vision Fund, in concurrence with the Delta Fund set up to invest in Chinese ride-hailing firm Didi Chuxing, has funnelled $27.5 billion.
The final investment total for the 200 GW of generation, including the solar panels, battery storage and a manufacturing facility for panels in Saudi Arabia, will eventually total around $200 billion. Electricity demand in the country has risen by as much as 9 per cent a year since 2000, according to BNEF.