Formula
Delivered heat is the fuel's energy times efficiency, so the fuel units are the need divided by both:
units = BTU needed / BTU per unit / efficiency
Cost is those units times the price, rounded to the nearest cent. Electricity priced from watts stays on the electricity pages.
Assumptions
- You type the BTU in one fuel unit. Nothing is looked up.
- Efficiency is the share of fuel energy that becomes heat.
- The price matches that same fuel unit.
Worked example
100,000 BTU at 100,000 BTU per unit and 90% efficiency costs $1.67 at $1.50 per unit.
Input definitions
- Energy per fuel unit
- BTU in one of the units you buy, such as one therm.
- Efficiency
- The percent of the fuel's energy that heats the space.
Limitations
This is a planning estimate. The rate, sun hours, and losses are numbers you type. It is not a utility bill, a solar design, or a room-by-room load calculation.
Questions
Why is there no gas price built in?
Fuel prices move. The page multiplies the price you type and does not store a utility rate.
Is a therm always 100,000 BTU?
A US therm is defined that way. If your bill uses a different unit, type that unit's BTU instead.