Audit a solar quote with 12 months of utility bills

A solar quote should begin with the homeowner's own twelve-month record, not a national ‘typical bill.’ One bill can hide heating or cooling season, a rate change, a vacant month, a true-up, or a temporary credit. Twelve monthly statements reveal the load pattern the proposed array is supposed to serve.

Build the source packet before opening the proposal

Download twelve consecutive bills in their original PDF form and preserve the rate-schedule page or tariff that was effective during the period. Add any annual true-up statement, current utility distributed-generation rider, and twelve months of inverter production if the home already has solar. The source packet should make every input reproducible without relying on the installer's portal.

About the quoted excerpts: each one was compared word for word with the linked document on the date shown on it. That check covers the quoted passage only. It does not review your eligibility or confirm that the rest of the document is current.

Worksheet 1: annual load and the monthly shape

1. Record billed kWh for each month. Sum the twelve values for annual grid consumption. Do not average first and round.
2. Record total bill dollars. Keep taxes, credits, late fees, prior balances, and one-time adjustments in separate columns so they do not masquerade as energy prices.
3. Mark the rate schedule. Note any month in which the tariff name or effective date changed.
4. Explain outliers. Record weather, vacancy, new equipment, an EV, a pool, a heat pump, or a partial billing period rather than deleting the month.
5. Preserve seasonality. Compare the seller's monthly production table with the household's monthly load, not only annual totals.

If the household plans an EV, heat pump, addition, efficiency retrofit, or occupancy change, put that in a separate future-load scenario. Do not silently inflate historical consumption or pretend the future load already appears in the bills. DOE likewise tells homeowners to consider planned changes such as an electric vehicle.

Worksheet 2: average price is not the avoidable rate

Dividing annual bill dollars by annual kWh produces an observed average price. It is useful as a check, but it may include customer charges, taxes, prior-period adjustments, and other amounts that solar does not avoid. The EIA uses the same revenue-divided-by-sales concept for its published averages and explicitly warns that those averages are not utility rates.

Build the avoidable import price from the tariff and bill line items. Separate fixed customer charges, minimum bills, demand charges, non-bypassable charges, and one-time adjustments. For a time-of-use tariff, keep each time period separate; a single annual cents-per-kWh value can hide that solar output and household demand occur at different hours.

Where to look on the statement

Bill layouts and line names differ by utility, so match each worksheet row to a line by what the line does, not by its exact wording. If a line is unclear, the tariff sheet named on the bill is the document that defines it.

• Monthly kWh (kwh_01 to kwh_12). The usage or meter-reading lines for that billing period: the kWh billed for that period, not a year-to-date total or a chart. If the bill shows several time-of-use periods or tiers, record the total and keep the period lines as notes.
• Rate schedule (tariff). The rate, schedule or plan name printed with the account details. If none is printed, ask the utility which residential schedule the account is on.
• Fixed charges (fixed_01 to fixed_12). A flat amount per month or per day that does not change with the kWh used, such as a customer, basic service or base charge.
• Avoidable import price. The charges that are priced per kWh. If the bill separates supply or generation charges from delivery charges, look at both groups; the tariff decides which of them an exported or self-consumed kWh actually offsets.
• Bill total (bill_01 to bill_12). The amount for that period's service. Keep taxes, prior balances, late fees, credits and one-time adjustments as separate notes so they do not distort the average price.

Illustrative bill arithmetic: fixed charges and exports

Assume twelve bills total $2,400 on 12,000 kWh, including an $18 monthly customer charge. The illustrative all-in average is 20¢/kWh. Excluding $216 of annual fixed charges leaves $2,184, or 18.2¢/kWh, as a simplified variable-price input before any time-of-use split.

Assume the proposed 10 kW system produces 10,000 kWh, with 4,000 used while produced and 6,000 exported at 5¢. The illustrative energy values are $728 from self-consumption and $300 from exports. This site's calculator gives $1,028 in year 1, rather than the $2,000 from valuing all production at the all-in 20¢ average.

The two corrections can be separated in this illustration. Valuing all 10,000 kWh at the 20¢ all-in average gives $2,000. Removing the fixed charge but still valuing every kWh at the 18.2¢ variable price gives $1,820. Splitting production 40% on site and the rest exported at 5¢ gives $1,028. So $180 of the illustrative gap comes from the fixed charge and $792 from the lower export price. Each step can be entered in this site's calculator by changing the import price, export credit and on-site share.

For the illustrative $30,000 cash price, zero degradation, zero escalation, no incentives and no incremental annual costs, the model returns no break-even within 25 years. At year 25 the unrecovered cost is $4,300. Plain cost divided by year-one value is 29.2 years, an extrapolation outside the model horizon, not a reported break-even result. The existing $216 customer charge was excluded from the avoided price; do not subtract it again as an added solar cost. A new solar-specific charge would be a separate incremental cost.

These bill and quote values are assumptions, not a customer story. A real tariff may have tiers, hourly prices, minimum bills, demand charges, taxes linked to usage or credit rollover. Keep unknown items blank in the worksheet, document simplifications, and test a lower-value case rather than treating missing information as zero.

Worksheet 3: check the seller's production model

The proposal should state system size in DC kilowatts, panel count and model, inverter model, roof-plane tilt and azimuth, shading or obstruction assumptions, system losses, first-year annual production, monthly production, degradation, and the weather dataset or modeling tool. If two quotes show different annual production for the same roof, compare those inputs before comparing payback.

• Run a low-production case. Reduce the seller's annual output or use a separately generated roof-specific estimate and observe whether the project still meets the household's threshold.
• Check monthly fit. Annual production equal to annual load does not prove exports are small; production can peak in months or hours when the home uses less.
• Keep DC and AC units straight. Panel nameplate capacity and inverter output capacity answer different questions and should not be substituted without explanation.
• Ask what is guaranteed. A modeled number, a performance guarantee, and a workmanship warranty are different documents.

Worksheet 4: compare cash price before monthly payment

Write the cash price first. Then list loan principal, annual percentage rate, term, payment changes, dealer or origination fees, prepayment terms, total of payments, and any balloon or tax-credit re-amortization assumption. A low first payment is not a low system price. The energy return should be tested against cash price and financing cost separately.

For a lease or power-purchase agreement, record the starting payment or energy price, annual escalator, term, production guarantee, maintenance duty, transfer requirements, buyout formula, and end-of-term options. For a battery, record usable capacity, round-trip assumptions, warranty throughput, backup reserve, and the exact tariff spread the battery is expected to capture.

For the export-credit arithmetic behind self-consumption and battery questions, see the net-metering guide.

The one-page audit table

• Twelve-month billed kWh total and monthly range — source: utility bills.
• Current tariff name, effective date, and import prices — source: utility tariff.
• Fixed and non-bypassable annual charges — source: bills and tariff.
• Proposed annual and monthly production — source: seller's production report.
• Self-consumed and exported kWh — source: interval load model or clearly labeled assumption.
• Export credit, netting interval, and true-up — source: distributed-generation rider or commission order.
• Cash price and financing total — source: contract and lender disclosure.
• Federal, state, utility, and certificate incentives — one row and one primary administrator document each.
• Simple payback and scenario range — run in the site calculator from the rows above; the CSV covers year-one energy value only.

The decision rule

Do not accept a proposal whose annual-savings figure cannot be rebuilt from the twelve-month bills, the serving utility's tariff, a roof-specific production report, and an explicit self-consumption/export split. The goal is not a more complicated calculator. It is a smaller set of numbers with a document behind every one.