Here we give historical data for PSEG Long Island's electric supply charge.
Electric supply prices vary on a regular basis. They are the price of electricity on the market. All the other costs that appear on your bill will be operating costs or state surcharges: other supply charges will include the Merchant Function Charge (costs to purchase this electricity). Your bill will also contain the delivery charges: there are the costs to transport the electricity to your home. Finally, your bill will contain the state surcharges (System Benefits Charge and Renewable Portfolio Standard amongst many others) and the state and local taxes. All these other costs will tend to be rather stable, and will not vary so much or so often.
The seasonal variation that you will see in the total cost of your electricity will mainly vary due to the market price of electricity: the Electric Supply Price.
In order for you to calculate your electric usage, you can use historical values for cost of the electric supply.
Here is a graph averaging the monthly and yearly electricity supply prices over the whole PSEG Long Island territory.