Net Zero Carbon
Solar Electricity to offset Fossil Gas
Most sites consume electricity and gas. Solar can capture electricity on site and, if the site is efficient enough, excess solar-generated electricity can be put back into the grid to offset the locally-consumed gas. If most heating is done via solar or electric (heat pump), this may be a milenium-scale sustainable option: I'm assuming some amount of biogas will be available even after we stop using fossil gas.
Soren's goal for his home's energy upgrade is to move towards this "net zero carbon" concept, starting with electricity and gas consumed on-site (post-upgrade, though pre-upgrade is also interesting) and expanding after that to on-site water and possibly off-site transportation.
Calculations
The details are critical here. Assume that your (old) gas plant is only 33% efficient and an additional 10% of the output (3.3% of the input energy) is lost in transmission. That means using gas locally is much more efficient, but it also decreases how much electricity you need to put back into the grid to offset any gas used.
| 1 therm of gas used locally w/N% efficient grid supply |
local kWh delivered / needed to offset |
| 15% ("the grid" is half coal w/2x the carbon/kWh of (old?) gas) |
|
| 30% (all old gas plants) |
|
| 54% (60% combined cycle gas) |
|
| 75% (modern gas, all hydro, wind) |
|
Only the all-gas grids can be considered directly equivalent, but as the grid evolves (hopefully towards more-sustainable) during the lifetime of a new (hypothetical) heating system + solar electrical offset supply, you want to have made the choice that gets closest to net zero carbon for the lowest cost.
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