House-Senate conferees agreed yesterday in the name of energy conservation to ban decorative gas lights that burn 24 hours a day outside homes and business.

They voted to forbid any new decorative hookups and to require that existing lights be turned off in four years. There are about 4 million such lights flickering beside driveways across the nation.Turning them off is expected to save the equivalent of about 30,000 barrels of oil a day. President Carter is trying to find ways to save a total of 4.5 million barrels a day to reduce dependence on foreign oil.

The legislation contains exemptions for memorial lights, lights of historical significance and commercial uses which conform with the cultural or architectural style of the area. This would permit continued burning of lights at President Kennedy's grave and the Gettysburg battlefield and the French Quarter of New Orleans.

It may also permit continued use of existing lights in old neighborhoods such as Georgetown and Old Town Alexandria.

The outdoor gas light provision is part of the regulatory coal conversion portion of Carter's energy package which the conferees agreed on after 10 days of negotiation.

This is a backup plan to use if a tax on industrial use of oil and natural gas does not prove effective in the effort to push utilities and industry from use of oil and gas to more - abundant coal. This tax is expected to be the big oil-saver in Carter's package.

The regulatory provisions agreed to yesterday lay down a general requirement that all new industrial and power plants must be built to use coal. But plants could be exempted for a variety of reasons, such as environmental anti-coal laws or lack of a reliable coal supply at a reasonable price.

The bill also requires that all existing electirc power plants convert from gas to coal by 1990. It does not order plants to switch from oil, but this could be ordered by the Department of Energy.

The bill does not order conversion of existing industrial plants. But it em-powers the department to order conversion, by categories of industry for big plants and on a case-by-case basis for plants burning less than 300 million British thermal units of fuel an hour. In every case, exemptions could be sought.

The bill authorizes $800 million in low-interest loans over two years to help power plants install air pollution control devices to reduce pollutants from coal. It also provides $180 million to help provide public facilities in mining towns whose populations are greatly increased by the expected coal boom.

In four weeks, the energy conferees have agreed on only the two least controversial parts of the energy program coal conversion and general conservation, such as ordering local utilities to help customers insulate their homes and requiring energy standards for major home appliances.

They now move to three highly controversial areas - taxes, electric utility rate structures and natural gas pricing - where the House and Senate bills are diametrically opposed. The House approved most of Carter's proposals, while the Senate rejected most of them.

A separate conference of members from the two tax-writing committees has split off from the main body and is to begin taking tentative votes next week on the home insulation tax credtis and taxes on crude oil, low-mileage cars and industrial use of oil and gas.

The House approved all of them. The Senate approved the home insualtion credit and a modified use tax and rejected the other two. Tax decisions will have to ratified by the full conference of members from tax and non-tax committees.

Conferees on the non-tax part of the bill are to start work Monday on Carter's plan to force local utilities to revise their rate structures to save energy - for example, charging lower rates for running a disrgy - for example, charging lower rates for running a dishwasher at off-peak hours.