Energy Payback Calculator Formula
Understand the math behind the energy payback calculator. Each variable explained with a worked example.
Formulas Used
Energy Payback Time
epbt = payback_yearsEnergy Return on Investment
eroi = annual_production_mwh * system_lifespan / embodied_energy_mwhNet Lifetime Energy
net_lifetime = net_energyNet Energy Ratio
net_pct = net_energy / embodied_energy_mwh * 100Variables
| Variable | Description | Default |
|---|---|---|
embodied_energy_mwh | Embodied Energy (Manufacturing)(MWh) | 20 |
annual_production_mwh | Annual Energy Production(MWh) | 12 |
system_lifespan | System Lifespan(years) | 25 |
payback_years | Derived value= embodied_energy_mwh / annual_production_mwh | calculated |
net_energy | Derived value= annual_production_mwh * system_lifespan - embodied_energy_mwh | calculated |
How It Works
How to Calculate Energy Payback Time
Energy payback time (EPBT) measures how long a system must operate to produce the energy used in its manufacture.
Formula
EPBT = Embodied Energy / Annual Production
EROI = Lifetime Production / Embodied Energy
Typical Values
Worked Example
A solar system with 20 MWh embodied energy producing 12 MWh/year over a 25-year lifespan.
- 01Energy payback: 20 / 12 = 1.7 years
- 02Lifetime production: 12 * 25 = 300 MWh
- 03Net energy: 300 - 20 = 280 MWh
- 04EROI: 300 / 20 = 15x
Frequently Asked Questions
What is energy payback time?
Energy payback time (EPBT) is the time a system must operate to generate the energy consumed during its manufacture, transport, and installation. Lower is better.
Do solar panels produce more energy than they use?
Yes, by a large margin. Modern solar panels pay back their embodied energy in 1-3 years and then produce clean energy for 22-27 more years. Their EROI is 10-20x.
What is EROI?
Energy Return on Investment is the ratio of energy produced over a lifetime to energy invested in manufacturing. An EROI above 5x is considered good for energy sources.
Ready to run the numbers?
Open Energy Payback Calculator