Our GAAP results for the first quarter were also $1.28 per share versus $0.88 per share in the first quarter of 2020.
Beginning was out nearly $2 billion of Clean Energy Future programs, which have moved from approval to execution.
Last week, the New Jersey Board of Public Utilities voted unanimously to award a continuation of the full $10 per megawatt hour zero-emission certificates, I'll just call them ZECs from now on, for all three New Jersey nuclear units, that would be Hope Creek, Salem number one and Salem number two through May of 2025.
I congratulate PSEG Nuclear for being recognized by as an industry leader in operational reliability, one of only two nuclear fleets across the industry with no scrams over the past 365 days.
During the quarter, the BPU also approved PSEG's 25% equity investment in Orsted's Ocean Wind project.
PSEG eagerly encourages an advocate for a national approach to accelerate economy wide, net zero emissions even sooner than 2050 in a constructive manner that expands green jobs by investing in clean energy infrastructure.
Following the BPU approval of our $700 million AMI proposal in January, we have begun implementation of the 4-year program.
Last month, FERC promulgated a new proposed rule to limit the 50 basis point RTO return on equity incentive to a 3-year period.
Stakeholder meetings are being conducted to consider a solar financial incentive program that will permanently replace the Solar Renewable Energy Certificate, or SREC program, and the Temporary Transitional Renewable Energy Certificate, or TRAC program, which was instituted in 2020 upon the state attainment of 5.1% of kilowatt hours sold from solar generation fee.
So to wrap up my remarks, we are reaffirming non-GAAP operating earnings guidance for the full year of 2021 of $3.35 to $3.55 per share.
We are on track to execute PSEG's 5-year $14 billion to $16 billion capital program through 2025 and had the financial strength to fund it without the need to issue new equity.
Over 90% of the current capital program is directed to PSE&G, which is expected to produce 6.5% to 8% compound annual growth in rate base over the '21 to '25 period, starting from PSEG's year-end 2020 rate base of $22 billion.
As we've noted previously, PSE&G's considerable cash-generating capabilities are supported by over 90% of its capital spending, continuing to receive either formula rate last based or current rate recovery of and on capital.
As Ralph mentioned, PSEG reported non-GAAP operating earnings for the first quarter of 2021 of $1.28 per share versus $1.03 per share in last year's first quarter.
PSE&G, as shown on Slide 15, reported net income for the first quarter 2021 of $0.94 per share compared with $0.87 per share for the first quarter of 2020, up 8% versus last year.
Results improved by $0.07 per share, driven by revenue growth from ongoing capital investment program and favorable pension OPEB results.
Transmission capital spending added $0.02 per share of the first quarter net income compared to the first quarter of 2020.
On the distribution side, gas margin improved by $0.03 per share over last year's first quarter, driven by the scheduled recovery of investments made under the second phase of the Gas System Modernization Program.
Electric margin was $0.01 per share favorable compared to the first quarter of 2020 on a higher weather-normalized residential volume.
O&M expense was $0.02 per share unfavorable compared to the first quarter of 2020, reflecting higher costs from several February snowstorms.
Depreciation expense increased by $0.01 per share, reflecting higher plant in service, and pension expense was $0.02 per share favorable compared to the first quarter of 2020.
Flow through taxes and other were $0.02 per share favorable compared to the first quarter of 2020.
Winter weather, as measured by heating degree days, was 4% milder than normal, but was 18% colder than the mild winter experienced in the first quarter of 2020.
For the trailing 12 months ended March 31, total weather-normalized sales reflected the higher expected residential and lower commercial and industrial sales observed in 2020 due to the economic impacts of COVID-19.
Total electric sales declined by 2%, while gas sales increased by approximately 1%.
PSE&G invested approximately $600 million in the first quarter and is on track to fully execute on its planned 2021 capital investment program of $2.7 billion.
And as of March 31, PSE&G has recorded a regulatory asset of approximately $60 million for net incremental costs, which includes $35 million for incremental gas bad debt expense.
With respect to subsidiary guidance for PSE&G, our forecast of net income for 2021 is unchanged at $1.410 billion to $1.470 billion.
In the first quarter of 2021, PSEG Power reported net income of $161 million or $0.32 per share, non-GAAP operating earnings of $163 million or $0.32 per share, and non-GAAP adjusted EBITDA of $321 million.
This compares to first quarter 2020 net income of $13 million, non-GAAP operating earnings of $85 million and non-GAAP adjusted EBITDA of $201 million.
The expected increase in PJM's capacity revenue improved non-GAAP operating earnings comparisons by $0.03 per share compared with last year's first quarter.
Higher generation in the 2021 first quarter added $0.01 per share due to the absence of the first quarter 2020 unplanned [on one average.
] And favorable market conditions influenced by February's cold weather benefited results by $0.03 per share compared to last year's first quarter.
We continue to forecast a $2 per megawatt hour average decline in recontracting for the full year recognizing that the shape of the annual average change favors the winter months of the first quarter.
The weather-related improvement in total gas send out to commercial and industrial customers increased results by $0.04 per share.
O&M expense was $0.03 per share favorable in the quarter, benefiting from the absence of first quarter 2020 outages at Bergen two and Salem 1, and lower depreciation and lower interest expense combined to improve by $0.01 per share versus the quarter ago -- or the year ago quarter.
Generation output increased by just under 1% to total 13.3-terawatt hours versus last year's first quarter when Salem Unit one experienced a month-long unplanned outage.
PSEG Power's combined cycle fleet produced 4.7-terawatt hours, down 8%, reflecting lower market demand in the quarter.
The Nuclear fleet produced 8.2-terawatt hours, up 3%, and operated at a capacity factor of 98.8% for the first quarter, representing 62% of total generation.
PSEG Power is forecasting generation output of 36 to 38 terawatt hours for the remaining three quarters of 2021 and has hedged approximately 95% to 100% of this production at an average price of $30 per megawatt hour.
Gross margin for the first quarter rose to approximately $34 per megawatt hour compared to $30 per megawatt hour in the first quarter of 2020, which contained one of the mildest winters in recent history.
And this winter's temperatures were 12% cooler on average and resulted in better market conditions compared to the first quarter of 2020.
Power's average capacity prices in PJM were higher in the first quarter of 2021 versus the first quarter of 2020 and will remain stable at $168 per megawatt hour -- per megawatt day through May of 2022.
In New England, our average realized capacity price will decline slightly to $192 per megawatt day, beginning June 1.
However, Power's cleared capacity will decline by 383 megawatts with the scheduled retirement of the Bridgeport Harbor Unit 3, achieving our goal of making Power's fleet completely coal free.
Over 75% of PSEG Power's expected gross margin in 2021 is secured by our fully hedged position of energy output, capacity revenue set in previous auctions and the opportunity to earn a full year of ZEC revenues and certain ancillary service payments such as reactive power.
The forecast of PSEG Power's non-GAAP operating earnings and non-GAAP adjusted EBITDA for 2021 remain unchanged at $280 million to $370 million and $850 million to $950 million, respectively.
For the first quarter of 2021, Enterprise and Other reported net income of $10 million or $0.02 per share for the first quarter of 2021 compared to a net loss of $5 million or $0.01 per share for the first quarter of 2020.
For 2021, the forecast for PSEG Enterprise and Other remains unchanged at a net loss of $15 million.
With respect to financial position, PSG ended the quarter with $803 million of cash on the balance sheet.
During the first quarter, PSE&G issued $450 million of 5-year secured medium-term notes at 95 basis points and $450 million of 30-year secured medium-term notes at 3%.
In addition, we retired a $300 million, 1.9% medium-term note at PSE&G that matured in March.
In March of 2021, PSEG closed on a $500 million, 364-day variable rate term loan agreement, following the January prepayment of a $300 million term loan initiated in March of 2020.
For the balance of the year, we have approximately $950 million of debt at PSEG Power scheduled to mature in June and September.
$300 million of debt scheduled to mature at the parent in November and $134 million of debt at PSE&G scheduled to mature in June.
As Ralph mentioned earlier, we are affirming our forecast of non-GAAP operating earnings for the full year of 2021 of $3.35 to $3.55 per share.

Summary:
Our GAAP results for the first quarter were also $1.28 per share versus $0.88 per share in the first quarter of 2020.
So to wrap up my remarks, we are reaffirming non-GAAP operating earnings guidance for the full year of 2021 of $3.35 to $3.55 per share.
We are on track to execute PSEG's 5-year $14 billion to $16 billion capital program through 2025 and had the financial strength to fund it without the need to issue new equity.
As Ralph mentioned, PSEG reported non-GAAP operating earnings for the first quarter of 2021 of $1.28 per share versus $1.03 per share in last year's first quarter.
As Ralph mentioned earlier, we are affirming our forecast of non-GAAP operating earnings for the full year of 2021 of $3.35 to $3.55 per share.