The Reagan administration once more will propose deep cuts in the civilian side of the Energy Department budget while recommending a 15 percent increase in the military part, which goes for development, testing and production of nuclear warheads, it was learned yesterday.

Under the latest draft of the fiscal 1984 budget for Energy, a copy of which was given to The Washington Post, there would be substantial cuts in the solar, fossil fuel and conservation research and development programs.

At the same time, the administration is proposing to increase department spending for "defense activities" by $1 billion to enable full-scale production of warheads for a variety of new weapons systems including the Trident, Pershing and cruise missiles.

The administration, which asked Congress for $11.8 billion for the department last year, is cutting this to $11.2 billion for fiscal 1984. The $6.8 billion earmarked for defense is 60 percent of this.

The proposed research and development cuts seem certain to cause a new furor on Capitol Hill, where Congress for the past two years has appropriated far more--particularly in the solar and conservation areas--than sought by the administration.

The fiscal 1984 budget, in what appears to be a conciliatory gesture by new Energy Secretary Donald Hodel, proposes a somewhat higher level of funding for conservation, solar and renewable and fossil energy programs than sought by Hodel's predecessor, James B. Edwards.

But while the $101 million proposed in the new budget for energy conservation is well above the $22 million sought a year ago, it also falls far short of the $279 million appropriated for fiscal 1983 by Congress.

Similarly, the $102 million proposed in the new budget for solar and other renewable forms of energy is greater than the $83 million submitted a year ago, but still only about one-third of what Congress appropriated each of the past two years.

The $138 million proposed by the administration for fossil energy research and development is up from the $107 million proposed last year, but little more than half the $217 million appropriated for fiscal 1983 by Congress.

Once again, the new budget's energy research and development expenditures are dominated by nuclear energy, with the administration seeking more than $1.5 billion for nuclear fission and fusion programs.

The new budget earmarks $270 million for a start of construction of the controversial Clinch River breeder reactor, which barely survived a House effort to kill it during the lame-duck session of Congress.

The administration also is expected to encounter opposition to the proposed increase in funding for development and production of nuclear weapons and materials.

The largest expenditure under defense activities is the $3.9 billion sought for development, testing and production of nuclear weapons.

In addition to warheads for the Trident, Pershing and air-, sea- and ground-launched cruise missiles, the budget envisages full-scale production in fiscal 1984 of the B83 strategic bomb, a new bomb that can be delivered supersonically with very high accuracy at extremely low altitudes.

The budget also calls for continued pre-production efforts in fiscal 1984 on the warhead for the MX.

The new Energy Department budget also requests more than $500 million for the Strategic Petroleum Reserve, most of which would be spent on expanding existing storage sites and developing new sites along the coast of Louisiana and Texas.

The budget also notes that the administration plans to spend $2.9 billion in fiscal 1984 to continue purchasing petroleum and filling the reserve at the rate of 220,000 barrels per day. The reserve would be used in the event of a new energy crisis.

But since this latter sum comes from a special off-budget account, it is not included in the Energy Department's budget total.