Residential solar is sold door to door, which means most homeowners first encounter the numbers in a form designed to persuade rather than to inform. The underlying economics are not complicated, but they are specific. Two identical houses on the same street can have materially different results because of roof orientation, consumption pattern and the tariff each household happens to be on.
Start with your own consumption, not with the roof
The first question is not how many panels fit but how much electricity the household actually uses and when. Twelve months of bills give the annual total and the seasonal shape. A household with electric heating has a winter heavy profile, which is the worst match for solar production. A household with air conditioning has a summer heavy profile, which is close to ideal.
Timing within the day matters as much as the annual total. Daytime consumption is served directly by the array. Evening consumption is served by the grid unless storage is present. A household where everyone is out until six in the evening exports most of its midday generation, and what that export is worth depends entirely on local rules.
Roof orientation and the things that cannot be changed
In the northern hemisphere a south facing roof at a moderate pitch gives the highest annual yield. East and west facing roofs produce less in total but spread production across the morning and afternoon, which can suit a household with evening load better than a south facing roof does. A north facing roof is generally not worth covering.
Shading is the factor most often underestimated. A single chimney, a vent stack or a neighbour’s maple can cut output disproportionately, because shade on part of a string affects the whole string unless optimisers or microinverters are used. A proper assessment measures shading across the year rather than glancing at the roof on a sunny afternoon, and the same discipline described in our note on what a roof assessment covers before a solar installation applies at domestic scale.
Roof age is the other practical constraint. An array installed on a roof with eight years of life remaining will have to come off and go back on, and that cost falls on the homeowner.
What the electricity is worth
This is where most of the variation in outcomes comes from. Under full retail net metering, exported electricity is credited at the same rate you pay, so every kilowatt hour is worth the same whether you use it or not, and oversizing is harmless. Under net billing or an avoided cost export rate, exports earn considerably less than you pay, which means self consumption becomes the objective and a smaller system often returns more per dollar invested than a larger one.
Time of use tariffs add another layer. Where peak rates fall in the late afternoon and evening, solar production arrives slightly before the most expensive hours, which is the main economic argument for domestic storage.
Fixed charges do not disappear. A household that generates most of its own electricity still pays connection and service charges, so a bill rarely falls to zero even when annual production matches annual consumption.
Buy, finance or lease
Paying cash produces the best lifetime economics and the simplest arrangement. A loan preserves cash and shifts the comparison to whether the monthly payment is lower than the avoided electricity cost, which it often is, though the total paid over the term is higher than the cash price. Leases and residential power purchase agreements require no capital and deliver a smaller saving, and they introduce a contract that must be dealt with if the house is sold.
That last point deserves attention. A solar system owned outright is an improvement to the property. A leased system is a contract the buyer must agree to assume, and it can complicate a sale. Homeowners who expect to move within a few years should weigh that carefully.
The incentive position
A federal tax credit for residential solar has been available for many years, and its percentage and expiry provisions have been revised repeatedly by legislation. State rebates, property tax exemptions and utility programmes add to it in some places. Because the details change and because a credit requires tax liability to offset, the only sensible approach is to confirm the current position with a tax professional rather than relying on a figure printed in a brochure. Federal consumer guidance is published through the Solar Energy Technologies Office.
Reading a residential quotation critically
Look for the assumed annual production in kilowatt hours and the basis for it. Look for the assumed utility rate escalation, and ask to see the figures with no escalation at all. Check whether an inverter replacement appears anywhere in the twenty five year projection, because it belongs there. Confirm what the export rules actually are in your territory rather than accepting a general statement. And check whether the quoted system size was driven by your consumption or by the available roof area, because under restrictive export rules those two produce very different recommendations.
A quotation that survives this reading is probably sound. One that collapses without a steep rate escalator was always a bet on energy prices rather than a calculation about your house.
Frequently asked questions
Will my electricity bill go to zero?
Rarely. Fixed connection and service charges remain regardless of how much you generate, and evening consumption is served by the grid unless you have storage.
Is a bigger system always better?
Only where exports are credited at or near retail rates. Where export compensation is low, sizing closer to your daytime consumption usually gives a better return per dollar.
Do east and west facing roofs work?
Yes, with lower annual yield than a south facing roof but a production curve spread across morning and afternoon, which can suit some households better.
What happens if I sell the house?
An owned system transfers as part of the property. A leased system or power purchase agreement must be assumed by the buyer or settled, which can complicate a sale.
Does a small amount of shade really matter?
It can matter more than expected, because shading part of a string affects the whole string unless module level electronics are fitted. A year round shading assessment is worth insisting on.