The majority of “green homes” which use renewable energy are not off-grid, independent propositions. Like traditional homes, they are attached to the grid and have utility providers. The difference is that they try to reach a net-zero or net-positive situation.
A net-zero home is one in which the net cost or net power use is zero, usually measured over the period of a year. A net-positive home is even better, generating sufficient power that it has enough for itself and some spare.
Ideally, everyone would love to install enough solar panels and have a system to generate all their own power throughout the year and reduce their grid-based, non-renewable power use to zero. Unfortunately, the space and homeowners budget required to achieve may be restrictive. To produce enough power for an average family home would require roughly 20 solar panels depending on the amount of solar irradiance available and how close the home is to “average”.
Consequently, the choice is usually between net-zero cost and net-zero use.
With both net use and net cost systems, enough energy is produced that there is an excess at peak times and a shortfall at the opposite point. In other words, in the middle of summer, the system produces more power than the home needs and sends the excess to a utility company; in the middle of winter, the system doesn’t produce enough and power is taken from the grid to cover the difference.
For a net-use system, the amount of energy produced is calculated and compared to the amount of energy used for the whole year. The excess produced and sent to the utility company should match the amount taken from the grid. Thus, the net amount of power used is zero.
For a net-cost system, the cost of the energy produced and used throughout the year is calculated. The value of the excess power produced and sent to the utility company should match the value of that taken from the grid during shortfalls. Thus, the net cost of power used is zero. To begin the calculation to size the right system for you, take your 12 most recent utility bills and add up the monthly kWh then divide by 365 X 1,000 to change the kW to watts and to get your daily average watt energy requirements. For most areas of the country you can divide your daily average watts by 6 in the southern states, and 5 in the norther states to get the approximate system size. That is not all the calculations you have to do, but it will get you in the ball park so you can start to make some decisions about the right size system that fits your budget, ground and roof space and other considerations.
In real terms, most people find that the choice is moot. The so-called sweet spot to maximize return on investment is usually around the 90% off-set range. The calculations are still important as part of the overall return on investment but the limiting factor is usually budget for the average home owner. However, with the ability to grow systems with even a small initial investment, solar is now accessible to every homeowner in America especially those who are willing to invest a little brow sweat and do-it-yourself. (DIY).