Two meters worth of accounting, not one
Senate File 583, codified at Iowa Code Section 476.49, requires Interstate Power and Light and MidAmerican Energy to file tariffs using either a net billing or an inflow-outflow method. Both use inflow-outflow.
Under it, energy you consume from the grid is the inflow and energy you deliver to the grid is the outflow, and each is recorded separately. They are then billed and credited according to the tariff rather than simply cancelling out.
Where outflow exceeds inflow in a period, the resulting credits carry forward to future billing periods, so a summer surplus is not immediately lost.
The most valuable outcome sits outside both categories. Electricity your household consumes at the moment it is generated never becomes inflow at all, because you never bought it, so it is worth the full retail rate without any tariff mechanics applying.
Why timing matters more than under net metering
Classic net metering makes the timing of generation and consumption largely irrelevant within a billing period, because the meter nets them into a single figure.
Tracking inflow and outflow separately changes that. Generation that coincides with household demand avoids inflow entirely; generation that does not becomes outflow and is credited under the tariff terms.
So the same annual production can produce different results depending on when it lands relative to when you use power, which is not true under a straightforward net metering arrangement.
Ask what self-consumption share the projection assumed and ask for the savings shown as separate lines for avoided inflow and credited outflow. A single net figure conceals the mechanism.
And the rate that sets the ceiling
MidAmerican residential customers in the Des Moines area have paid around 11 cents per kWh, below the Iowa average of roughly 12.7 cents and well below the national figure near 18.4.
Cheap electricity is good for a household budget and hard on a solar payback, because savings are the price of the electricity you no longer buy. That is arithmetic rather than a criticism.
It also means the margin for error is thinner here. An optimistic production estimate or an oversized design does proportionally more damage when each kilowatt hour is worth less.
Ask which rate the projection used and check it against a recent bill. A model built on a national or even a state average will overstate a MidAmerican Des Moines project.
The pieces to separate in a projection
The 30 percent federal Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, and the Iowa solar tax credit expired for residential installations completed after December 31, 2021.
Section 48E survives at 30 percent for third-party owners under leases and power purchase agreements, so ask what a provider claims and what reaches you in the rate.
What exists is the sales tax exemption, the five-year property tax exemption, avoided inflow at your actual retail rate, and outflow credits under the tariff.
Ask for those separated in the projection, with the self-consumption share stated and your own rate taken from a recent bill.