How do I check a rooftop solar savings estimate?

Check a rooftop solar savings estimate by reconstructing the electricity bill before and after installation. Separate electricity you use directly, electricity exported to the grid, electricity still bought from the utility and charges that remain regardless. Then add the complete installation cost, ongoing service, financing if applicable and money needed for future obligations. A production estimate is not a savings estimate by itself.

The number I would circle first is not the biggest one on the proposal. It is the price assigned to electricity the household will not use immediately. Valuing every exported kilowatt-hour at the retail purchase rate can make a modest arrangement look spectacular.

Start with the house you actually have

Gather actual electricity bills and their rate schedules. Identify consumption, fixed charges, seasonal changes and any time-dependent prices. Note planned changes such as an electric vehicle or different heating, but keep those forecasts separate from the historical record.

Ask for the proposed system’s production assumptions: location, roof orientation, shading, equipment, losses and expected changes over time. Kilowatts describe power; kilowatt-hours describe energy over a period. A system’s quoted capacity is not the number of units it will generate every hour or year.

The Department of Energy’s homeowner guide explains that savings depend on consumption, generation, utility prices and compensation for exported power. Its roof discussion also makes age and shade relevant. An attractive annual output number does not settle whether the physical installation suits the property.

Have appropriately qualified people assess the roof and installation. Ask who pays for electrical changes, permits, removal for roof work and any restoration. A missing necessary job belongs in the bid comparison before you agree, not in a hopeful footnote afterward.

Get the utility rule that applies to your system

Recover the actual tariff and connection requirements from the utility or other responsible authority. Check eligibility, system limits, import prices, export compensation, fixed charges and when credits are applied or expire.

The term net metering does not supply all those answers. A one-for-one annual credit, a lower export rate and a credit usable only against certain charges can produce different results from the same panels. A proposal should identify the actual arrangement it assumes.

The FTC’s solar purchasing guide points out that utility fixed charges can remain and recommends comparing detailed bids, including installation costs and maintenance responsibilities. Keep those items visible instead of treating the old electricity bill as entirely removable.

Where rates vary by time, reconstruct the relevant intervals. Annual generation and annual consumption alone cannot show whether the household exports at a low price and buys at a higher one later. The simple annual example below deliberately supplies flat prices and unrestricted usable credits; it is not a model of every utility.

Rebuild an original annual bill

Consider a wholly fictional home using 12,000 kilowatt-hours in one year. Its flat import price is $0.20 per kilowatt-hour, with $240 of fixed annual utility charges. Those prices include all utility charges applicable in this limited model.

Without solar, the energy costs $2,400. Adding the fixed charges produces a $2,640 annual utility bill. Consumption stays the same in every comparison below; lower bills do not come from secretly reducing the household’s energy use.

The proposed system generates 10,000 kilowatt-hours. The household uses 6,000 directly and exports 4,000. It still imports 6,000 to meet the rest of its 12,000-unit use. Generation is therefore smaller than annual household consumption, yet exports still occur because production and use do not always coincide.

The supplied export credit is $0.05 per kilowatt-hour. All $200 of credits are usable against that year’s bill, with no cap, expiration loss or additional fee. Imports cost $1,200. Add $240 of fixed charges and subtract $200 of export credits: the annual utility bill becomes $1,240.

Compared with $2,640 without solar, that is $1,400 of utility-bill savings. It is not a $2,000 saving obtained by multiplying all 10,000 generated units by the $0.20 retail price.

Separate avoided purchases from export income

Here is another way to check the same $1,400. Using 6,000 generated units directly avoids $1,200 of purchases. Exporting the remaining 4,000 supplies $200 of credits. The two benefits sum to $1,400.

Scroll across to reconcile the original annual energy flows and utility charges. These are invented inputs, not an actual roof assessment or rate quote.

The inflated $2,000 calculation overstates the supplied benefit by $600. It gives the 4,000 exported units a $0.20 value instead of their actual fictional $0.05 credit, a difference of $0.15 each.

No battery is included. Adding one would require a different price, energy-flow model, losses, operating arrangement and replacement assumptions. Do not insert storage benefits into a quote that has not paid for storage.

Include the cost of owning the system

The original fictional cash-purchase bid is $18,000, including the complete equipment, installation, approvals and necessary connection work. The roof is suitable under the supplied scenario and needs no preparatory replacement. There is no financing, incentive receipt or resale receipt in this model.

A separate $200 annual service arrangement covers all routine professional work required during the modeled year, including labor and travel. It supplies no unpaid owner labor. No repair or replacement occurs that year. These are explicit teaching assumptions, not claims that a real system will cost only $200 to maintain.

After service, the year’s operating cash saving is $1,200: $1,400 of utility savings less $200. The household also chooses to set aside $300 of that saving for future system obligations, leaving $900 available for other uses.

The $300 remains household money in a dedicated reserve. It is not an incurred repair expense or a second service bill. If a replacement later occurs, compare the actual payment with the reserve available then; do not deduct both the earlier saving transfer and the later payment as two separate economic costs.

The complete year’s external operating payments are $1,440: $1,240 to the utility and $200 for service. Another $300 is allocated internally. The household’s $2,640 baseline allocation therefore leaves $900 uncommitted after those payments and the reserve contribution.

Stress the self-use and export assumptions

Keep generation at 10,000 units but change direct use to 4,000. Exports rise to 6,000 and grid imports rise to 8,000. The solar system produces exactly as much as before; the household simply uses less of its production immediately.

Imports now cost $1,600 and export credits are $300. With the same $240 fixed charge, the bill is $1,540. Utility savings fall to $1,100. After the same $200 service and $300 reserve allocation, only $600 remains available.

For a separate sensitivity test, restore the original 6,000 units of direct use but set export compensation to zero. The bill becomes $1,440; utility savings are $1,200, and the amount left after service and reserve is $700.

These are separate branches, not simultaneous forecast changes. Neither has an assigned probability. Their purpose is to show which assumptions deserve evidence before a sales illustration becomes a household commitment.

Keep the upfront money and the horizon visible

Suppose the household has $32,000 allocated to this decision, including a protected $12,000 cash floor. Paying the complete $18,000 installation leaves $14,000, only $2,000 above that floor. Other household obligations are funded separately in this bounded example.

Annual savings do not arrive as a $900 check on installation day. Real utility bills, service payments and production vary through the year. Build the actual payment schedule before deciding that an annual surplus can cover a near-term bill.

Nor does this one-year example establish lifetime profit, a return percentage or a system’s value when selling the home. Future production, rates, repairs, replacement, removal and any transfer terms need their own dated assumptions. The simple-payback guide explains why recovering the initial cost does not finish the analysis.

If borrowing is proposed, obtain the cash price and the complete finance terms separately. Recover all payments, fees, changing obligations and any assumption that an incentive will fund a later payment. Verify actual current incentive eligibility and timing independently; none has been credited in this calculation.

Keep the estimate’s version and the inputs used to produce it. If a revised proposal changes both the system size and the assumed utility price, ask for a bridge showing each change separately. Otherwise a larger advertised saving can conceal a more optimistic tariff assumption rather than better equipment. Check whether projected future rate increases also affect fixed charges or export prices; they need not move together. A flat-rate model like this one cannot settle that question for a real household.

Leave the sales meeting with a reproducible record

Compare matched written bids rather than unrelated headline savings. Record the system, production assumptions, applicable utility arrangement, all upfront work, ongoing responsibilities and what happens if the installation underperforms.

A production warranty and a bill-savings promise answer different questions. Examine the actual remedy, responsible company, exclusions and time limits. A technically functioning panel does not guarantee a particular utility tariff or household consumption pattern.

The decision record should identify what is verified, what is estimated and what remains unresolved. A missing roof-work price or unconfirmed export tariff is a reason to complete the evidence, not to replace the gap with a cheerful average.

1. Reconstruct the existing bill from actual use and charges.
2. Separate direct solar use, exports and remaining imports.
3. Apply the actual utility prices and credit rules.
4. Include the complete installation and operating responsibilities.
5. Test unfavorable generation, self-use and compensation assumptions.
6. Check payment dates, protected cash and longer-term obligations.