House-Senate conferees finally reached agreement yesterday on the last of three major energy bills President Carter asked for 10 months ago. This one would promote development of synthetic fuels to reduce reliance on oil.

Conferees had agreed 2 1/2 months ago on the main point of the bill, a $20 billion synfuel fund and creation of a corporation to administer it. They have been working since on a long list of conservation programs added by the Senate.

The conferees agreed to allow utilities to lend money to customers to insulate their homes, and agreed to set up an energy bank to subsidize these loans, including loans to install solar heat.Grants of up to 50 percent of the cost could be made to help the poor insulate their dwellings.

One of the last acts of the conference was to agree on use of $1.45 billion during the next two years to encourage production of gasohol and production of other fuels from wood and waste products. A proposed program to encourage development of wind-produced energy was split off to be handled as separate legislation.

The goal of the synfuels program is to produce the equivalent of 500,000 barrels of oil a day by 1987 and to increase the fund by up to another $68 billion in the mid-1980s. The nation consumes close to 20 million barrels of oil a day.

The other two energy bills already approved by Congress are the so-called "wind-fall" tax on U.S. crude oil and the bill setting up an Energy Mobilization Board to cut red tape for big energy projects.