Understand the method.
Then check the inputs.

Worked examples explain a decision; they do not predict your result

The examples in this library use explicitly chosen inputs to show how a calculation changes when assumptions change. They are not measurements of a customer’s home, equipment ratings, lender offers, or verified utility tariffs. A project-specific review must replace those inputs with documented information. Publication means an article passed Origin’s editorial workflow; it does not mean a search engine indexed it or an independent agency approved it.

Monthly payment and remaining utility cost

Combined monthly budget equals solar payment plus remaining utility amount plus any reserve the household chooses to set aside. The difference equals the starting bill minus that combined amount. A $250 starting bill, $160 solar payment, $35 remaining utility amount, and $15 reserve produce a $210 combined budget and a $40 difference. Multiplying by twelve gives $480 only if those inputs remain unchanged. This is arithmetic, not a seasonal bill forecast.

The reserve is retained household money, not a quoted Origin fee. A negative difference is displayed as additional outlay. No tax benefit, home-value change, future rate increase, or future sale is assumed.

Fixed-payment ownership loans

Monthly payment equals P × r ÷ [1 − (1 + r)^−n]. P is principal, r is annual interest rate divided by 100 and twelve, and n is the number of monthly installments. At zero interest, payment is P ÷ n. Total scheduled payments equal the unrounded monthly payment multiplied by n; modeled interest equals that total minus principal.

The model separates cash installation scope from financed principal. It assumes equal end-of-month payments, no down payment, no deferral, no balloon, no recast, and no prepayment. Its annual interest rate is not an APR disclosure incorporating additional finance charges. Actual schedules can differ because of dates, fees, features, and rounding. Obtain the lender’s written disclosures and payment table.

Lease escalation and prepaid timing

For a lease with an annual escalator, year y’s monthly payment equals the starting monthly amount multiplied by (1 + escalator ÷ 100)^(y − 1). Multiply by twelve and add all years to find the nominal scheduled total. A starting payment of $100 and a 2% escalator produce a $102 monthly amount in year two, not an additional $2 increase every month.

A prepaid amount is compared at time zero. A separate timing illustration discounts each year’s scheduled lease payments using an end-of-year convention. This is an approximation because actual payments occur monthly. Assumed discount rates are sensitivity inputs, not promised returns. If a prepaid amount is borrowed, model the loan separately. Ownership, service, transfers, roof work, and end-of-term terms must be checked for equivalence before interpreting the price difference.

Battery energy and average-load runtime

Available energy equals starting stored energy × (1 − reserve ÷ 100) × modeled conversion efficiency. Runtime equals available energy in kWh divided by average load in kW. Draft cases use a 90% conversion assumption and no solar recharge. For example, 10 kWh with a 20% reserve and 90% conversion yields 7.2 kWh for loads. A continuous 500 W average load is 0.5 kW, giving 14.4 hours of modeled runtime.

Starting energy is not interchangeable with every manufacturer’s usable-capacity rating. Avoid deducting the same operating limit or loss twice. Energy arithmetic does not establish inverter output, motor-starting capability, circuit suitability, or outage operation of existing solar. Those are design questions for qualified professionals. These examples are not emergency plans or assurances for critical medical equipment.

Annual solar generation, direct use, imports, and exports

Direct use is the smaller of household consumption and generation multiplied by the target direct-use share. Imports equal consumption minus direct use. Exports equal generation minus direct use. Both annual balances must reconcile. Draft cases use an artificial $0.16/kWh import rate, a stated export rate, and $35 monthly fixed charges. These do not represent a particular utility.

Export value is capped at modeled imported-energy charges. It cannot erase fixed charges or create a cash payout; unused modeled credit is reported separately. This is a teaching convention, not net-metering law or a utility tariff. Monthly settlement, minimum charges, time-of-use rates, demand charges, taxes, fuel adjustments, and carryovers are not modeled. Annual generation matching annual use does not prove zero imports or a zero bill.

Rounding, evidence, and review

Calculations use unrounded values internally and round displayed amounts for readability. Small differences can arise when recomputing from rounded intermediate figures. Each public article should identify a reader’s decision, explain assumptions, show calculations or evidence, and identify what could change its conclusion. Similar cases usually belong in one guide rather than many near-identical articles.

Origin’s review controls check completeness and compare text and reader intent with other articles in this library. They cannot verify a tariff, confirm a borrower’s eligibility, inspect a roof, or establish an equipment warranty. Consult actual agreements, current utility documents, and qualified advisers where needed. Request a correction through info@solarroofgen.com if a calculation needs clarification.