Solar Energy Offset Calculator
A solar offset tells you how much of your yearly electricity a solar array covers and how much grid carbon it avoids. This calculator takes your annual solar generation, your annual consumption and your grid emission factor, then reports the offset percentage, the remaining grid electricity you still draw, and the CO2 avoided. The grid emission factor is an editable input so you can use the figure for your region. Estimate generation from your system size and local specific yield, or enter a measured figure.
Solar offset formula
Offset % = (generation / consumption) * 100
Remaining grid use = max(consumption - generation, 0)
CO2 avoided = generation * grid emission factor
Tonnes = CO2 avoided / 1,000
CO2 avoided uses total generation, since every solar kWh displaces a grid kWh whether you consume it directly or export it. Remaining grid use is the net annual shortfall you still buy, floored at zero.
What affects your offset
- System size and local specific yield set how much you generate.
- Roof orientation, tilt and shading change real output versus the nameplate rating.
- Offset is an annual energy balance; timing means you still import and export through the year.
- Grid carbon intensity sets how much CO2 each generated kWh avoids.
Solar offset: frequently asked questions
What is a solar offset percentage?
It is the share of your annual electricity consumption that your solar system's production covers: annual solar generation divided by annual consumption, times 100. A 100% offset means your panels generate as much over the year as you use, though timing differences still mean you draw from and feed back to the grid at different times.
How much CO2 does solar avoid?
Each kilowatt-hour of solar generation displaces grid electricity, avoiding its emissions: solar kWh times the grid emission factor (kg CO2 per kWh). The factor depends on your local generation mix and is a user-editable input here, sourced from the US EPA eGRID database or your provider.
How do I estimate annual solar generation?
A common approximation is system size in kW times the local specific yield (kWh produced per kW of capacity per year), which depends on sunshine, orientation and shading. Enter your expected annual generation directly if you have a quote or monitoring data; otherwise use a sourced specific yield for your area.
Can offset exceed 100%?
Yes. A system sized larger than your consumption can generate more than you use over the year, giving an offset above 100%. Whether you are credited for the surplus depends on your local net metering or export rules.
Official sources
- U.S. Environmental Protection Agency: eGRID emission factors.
- U.S. Department of Energy, NREL: PVWatts solar production estimator.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.