Reading an Electric Bill With Solar

A photovoltaic system on the roof changes the arithmetic of an electric bill but not its structure. The utility still reads a meter, still applies a tariff, and still sends a statement. What changes is that the meter can now register flow in two directions, and the tariff may credit one direction differently from the other. The bill that arrives is therefore a settlement between two accounts: what the array produced and what the building consumed, priced by rules the utility and the state regulator set.

Those rules are not uniform. Net metering, export compensation, non-bypassable charges, time-of-use windows and true-up schedules differ by state, by utility and sometimes by rate class within a utility. They also change, often on a schedule announced years in advance. Nothing below should be read as a description of a particular jurisdiction. It describes the categories of line item an owner is likely to encounter and what each one means, so the reader can find the corresponding line on their own tariff sheet.

Why a bill still arrives

A grid-connected array is sized to offset consumption over a year, not to make the building independent of the grid. At night the array produces nothing and the building imports. In winter production falls and consumption may not. On a cloudy week the meter runs forward for days. The utility supplies those kilowatt-hours, maintains the service drop, and reads the meter, and it charges for all three.

Three mechanisms explain most of a solar owner’s bill:

Fixed charges. A monthly customer charge or service charge is levied regardless of consumption. It covers the connection itself, metering and billing. No quantity of solar production reduces it, because it is not a per-kilowatt-hour charge.

Energy imports. Every kilowatt-hour drawn from the grid after sunset, or during a period when the array is below the building load, is billed at the applicable rate. An array that covers annual consumption in aggregate can still leave large evening imports, and those imports may fall in the most expensive time-of-use period.

Non-bypassable or fixed-cost charges. Many tariffs apply certain per-kilowatt-hour charges to all imports, with no offset from exports. These are typically the components that fund public programs, transmission and distribution infrastructure. Whether they apply to imports only, or to imports net of exports, is a tariff question.

The two directions of the meter

A bi-directional meter records energy delivered to the premises and energy received from them as separate registers, often labelled delivered and received, or import and export. The bill should show both, though the labels vary.

Import is what the building took from the grid. Export is what the array sent to the grid after serving the building’s load at that moment. Export occurs when production exceeds instantaneous consumption, which is common at midday in spring and rare in the evening.

A bill that shows only a net figure is compressing the two registers into one number. That is legitimate where the tariff compensates exports at the same rate as imports, but it hides the structure. Where exports are compensated at a different rate, or only credited against certain charge components, the two flows must be priced separately and the bill should show them separately.

Matching the bill to monitoring data

The inverter or monitoring portal reports production. The bill reports imports and exports. These are different quantities and will not match, which is a frequent source of confusion.

Production is what the array made. Export is production minus what the building consumed at that instant. A day with 30 kWh of production and 28 kWh of consumption during daylight hours might export only 2 kWh. The monitoring figure and the export figure are both correct; they measure different things.

To reconcile, compare over the same interval and account for self-consumption. Some monitoring platforms estimate self-consumption from a consumption meter or from a paired current transformer. Where no consumption monitoring exists, the comparison is approximate.

Many utilities offer an interval-data download of delivered and received energy (in the US often under the Green Button name); where one is offered, summing each register separately for the billing period gives the figures to set against the monitoring export. Discrepancies of a few percent between monitoring and the utility register are ordinary, arising from measurement accuracy, clock alignment and the treatment of intervals that straddle a rate-period boundary. A persistent gap in one direction, month after month, is worth a call to the utility.

Time-of-use periods

Where a tariff prices energy by time of day, the array’s output and the building’s load may not coincide with the cheap periods. Production peaks near midday. Many time-of-use tariffs also treat midday as an off-peak or super-off-peak period, particularly where solar penetration is high, and price the evening peak highest. In that structure, exports at midday earn little and imports at 7 p.m. cost much, so the bill reflects the timing of consumption as much as its quantity.

Whether a given utility has moved to such a structure, and what the windows are, is on the tariff sheet. The schedule of periods is not a property of the technology.

Annual true-up or reconciliation

Some tariffs settle monthly. Others accumulate credits and charges over a twelve-month period and reconcile at an anniversary date, often called a true-up. Under a true-up, monthly statements may show a running balance rather than an amount due, and the annual statement settles it.

What happens to unused credits at true-up varies. Some tariffs carry them forward, some compensate them at a wholesale or avoided-cost rate, and some expire them. The rate at which exported energy is credited, and whether that credit can offset all charge components or only the energy component, is set in the tariff.

A solar true-up bill is therefore the annual settlement of a running account, not a penalty. It is larger than the monthly statements because it is the sum of them.

What each line item means

What people get wrong

A common misreading is to treat the monitoring portal’s production total as though it were a payment. An owner sees 900 kWh produced in a month and expects the bill to fall by 900 kWh of charges. It falls by the exports plus the self-consumed production, and the self-consumed portion never appears on the bill at all. The array did its work; the bill simply does not itemise it.

The mistake is natural because the portal presents production as a single, satisfying number and the bill presents a much smaller export figure. Nothing on either document explains the gap. The gap is self-consumption, and it is usually the largest single term.

A second misreading is to assume that a credit balance on the bill is money owed to the owner. Under many tariffs, accumulated credits offset future charges within the true-up period and are settled differently, or not at all, at the end. Whether a credit is redeemable in cash is a tariff question, not an inference from the statement.

A third is to attribute a high winter bill to a fault in the array. Winter production is lower by latitude and weather, and evening consumption is higher. The bill is behaving as the tariff and the season dictate.

What to look up on the tariff sheet

The tariff sheet, sometimes called the rate schedule, is the controlling document. It is published by the utility and approved by the state regulator. The items to locate are: the fixed charge; the energy rate by period; the export compensation rate and whether it is a net-metering credit, an avoided-cost rate or a separate export tariff; which charge components exports may offset; the true-up month and the treatment of residual credits; and any demand charge.

Interconnection rules, permitting and incentive programs are separate from the tariff and are administered by the utility, the state and sometimes the local authority. All of these change, and the version in force at the time of installation may not be the version in force now. Where the bill and the monitoring data disagree persistently, the utility’s billing department can explain which register and which rate period produced a given line; work on the service panel, meter or any wiring is licensed work and is not a homeowner task.