---
title: "Solar proposal sizing checklist: verify the number before you sign"
description: "Use bills, planned efficiency work, roof life, shade and future electric loads to test how a solar proposal arrived at its system size."
canonical: "https://betterpowered.com/articles/solar-proposal-sizing-checklist/"
locale: "en-US"
contentKind: "article"
articleFormat: "checklist"
articleFormatLabel: "Checklist"
topic: "roof-solar-resilience"
topicLabel: "Roof, solar & resilience"
datePublished: "2026-08-25T14:51:44.785Z"
dateModified: "2026-08-25T14:51:44.807Z"
author: "BetterPowered Editorial Team"
---

# Solar proposal sizing checklist: verify the number before you sign

A panel count is not a sizing explanation. Trace the proposal from measured consumption and roof constraints to modeled production and disclosed assumptions.

A solar proposal can contain a precise panel count and still leave the central question unanswered: **what load and what roof was that size designed for?** Before accepting the number, trace it from the home's measured electricity use through planned efficiency work, roof constraints, shade and known future loads. The goal is not to find a universal “right” size. It is to make this proposal's assumptions visible enough to compare, challenge and preserve.

Use this solar proposal sizing checklist before signing:

- confirm the consumption history and billing period used;
- separate fixed utility charges from metered electricity;
- document efficiency work that will change the baseline;
- resolve roof condition, remaining life and usable area;
- review the shade, orientation and production-model inputs;
- add future electric loads as explicit scenarios;
- reconcile proposed capacity with estimated annual production; and
- test savings claims with current utility rules and disclosed assumptions.

## 1. Start with a complete electricity record

The Federal Trade Commission advises reviewing the last year of metered electricity in kilowatt-hours and also identifying fixed delivery or administrative charges. The U.S. Department of Energy likewise says prior bills help establish annual and seasonal electricity needs. Read the [FTC's solar consumer guidance](https://consumer.ftc.gov/articles/solar-power-your-home) and DOE's [step-by-step guide for consumers going solar](https://www.energy.gov/cmei/systems/articles/walk-me-through-it-step-step-guide-consumers-going-solar).

Ask the proposer for a table showing each service period, billing days and metered kWh used in the sizing calculation. Mark estimated reads, missing months and unusual periods. If the proposal uses a monthly average, require the underlying months as well; an average alone hides seasonality.

Do not treat the total amount due as electricity consumption. Fixed charges may remain even when grid purchases fall, and a dollar total can change because of tariff components rather than a change in kWh. For a practical way to organize the record, see [How to read twelve months of electric bills before upgrading](/articles/how-to-read-twelve-months-of-electric-bills/).

## 2. Decide which load baseline the system is meant to serve

DOE recommends considering a home energy assessment and any recent or future changes that affect consumption before sizing solar. Its homeowner guide also notes that efficiency improvements reduce the amount of solar energy the home needs. That makes sequence part of sizing, not a footnote.

Ask for three separate cases when they apply:

1. **Historical case:** the unadjusted billed kWh for the stated period.
2. **Efficiency case:** the baseline after committed work, with each adjustment and its evidence listed.
3. **Future-load case:** the baseline after defined new electric loads, kept separate from efficiency reductions.

Do not let an unverified efficiency estimate silently reduce the load, and do not count a completed improvement twice. If insulation, air sealing, HVAC or other work is still only being considered, the proposal can show a sensitivity case instead of presenting the reduction as settled. The sequence is explored further in [Solar and battery after efficiency: what should you size?](/articles/solar-and-battery-after-efficiency-what-to-size/).

## 3. Resolve the roof before accepting panel count

DOE's [Homeowner's Guide to Solar](https://www.energy.gov/cmei/systems/homeowners-guide-solar) identifies roof age, tree cover, size, shape and slope as suitability factors. The FTC also tells consumers to consider the roof's angle, age, condition and orientation, and to compare what happens if roof repair is needed after installation.

The proposal should therefore identify:

- the roof planes included in the design;
- the condition information relied on and who supplied it;
- any roof work assumed before installation;
- the usable layout after required access and obstructions are accounted for; and
- who bears the scope and cost of removing and reinstalling the array if later roof work is needed.

A roof drawing does not establish remaining roof life. If the proposal depends on postponing roof work, obtain the appropriate roof evaluation and make that decision separately. Use [Check the roof before you size solar](/articles/check-the-roof-before-you-size-solar/) to organize that review.

## 4. Make shade and production-model inputs reviewable

Solar potential depends on site location, roof orientation, shade and the selected system. DOE says online mapping tools can start the evaluation, while an installer can perform a more detailed assessment. The current [PVWatts calculator](https://pvwatts.nlr.gov/) estimates grid-connected PV production, but its result depends on user inputs and model assumptions; it is an estimate, not a production promise.

Ask the proposal to state the model and version, location or weather data, array tilt, azimuth, DC capacity, inverter assumptions, system-loss assumptions and how nearby trees or structures were represented. If shade changes by roof plane or season, a single unexplained loss value is not enough to audit the design. Ask for the site observations or shade analysis behind it.

Then compare the model output with the proposed layout. The roof planes, module count and stated equipment capacity should correspond to the capacity entered in the production model. Any difference should be explained in writing.

## 5. Add future loads as named scenarios

DOE specifically advises accounting for recent or future changes in household electricity use and gives an electric vehicle as one example. A future load should not appear as an unsupported percentage added to the bill history.

For each planned load, record what is changing, the evidence used for its annual kWh estimate, the expected start date and whether it replaces an existing energy use or adds a new one. Keep uncertain projects in a separate scenario. That lets you compare a system sized for today's load with one designed around a documented future plan without pretending both forecasts are facts.

If backup power is also a goal, do not use the solar array's annual energy estimate as a substitute for a battery-load plan. Define the circuits, power needs and desired duration separately; see [How to plan home battery backup loads before choosing capacity](/articles/home-battery-backup-load-planning/).

## 6. Reconcile capacity, production and the target load

Require one worksheet that connects these fields:

| Field | What to verify |
| --- | --- |
| Historical consumption | The exact bill periods and total metered kWh used |
| Efficiency adjustments | Each committed change, direction of adjustment and supporting basis |
| Future-load adjustments | Each named load, annual kWh basis and timing |
| Target annual load | The scenario the proposal is actually designed around |
| Proposed array capacity | DC system capacity and the equipment configuration that produces it |
| Modeled production | Annual and, when available, monthly kWh with model inputs disclosed |
| Utility treatment | Current fixed charges, export-credit rules and any interconnection sizing constraints confirmed with the responsible utility |

Capacity and estimated energy production should not be used interchangeably. Ask the proposer to show how the stated array capacity becomes the annual and seasonal production estimate for this site, then compare that estimate with the selected load scenario. Do not assume that matching annual totals means the home will never buy electricity from the grid; the FTC notes that production, consumption and utility arrangements all affect the bill.

## 7. Separate the sizing case from the savings case

DOE says solar savings depend on consumption, system size, generation, roof direction and sunlight, as well as utility rates and compensation for exported energy. The FTC cautions that future utility rates are difficult to predict and recommends comparing costs, production commitments, warranties and roof-repair terms across proposals.

If a proposal includes bill savings or payback, ask for the current tariff, fixed charges, export-credit treatment, rate-escalation assumption, system-degradation assumption, financing terms and maintenance responsibilities. These are scenario inputs, not guaranteed outcomes. Recalculate the comparison when any of them changes, and do not allow a projected dollar result to substitute for the kWh sizing worksheet.

## 8. Compare proposals on the same facts

The FTC recommends obtaining detailed bids from multiple companies with system size, expected performance, full installation cost, production guarantees if offered, and equipment and workmanship warranties in writing. To compare sizing rather than sales presentation, give each proposer the same bill set, efficiency plan, future-load list and roof information. Ask each one to identify any assumption it changed.

Pause the decision if a proposal provides only panel count, omits the bill periods, treats a future load as a guess, ignores planned efficiency work, cannot connect the roof layout to the production model, or presents savings without current utility assumptions. A useful proposal should let another qualified reviewer reproduce the logic even if that reviewer would choose different assumptions.

## Keep a one-page decision record

Before signing, preserve the chosen load scenario, proposal revision, roof decision, shade evidence, model inputs, estimated production, utility information, warranties and unresolved conditions. Date every source. This record will not guarantee production or savings, but it will show what the system was intended to do and which facts require rechecking before installation.

## Primary and official sources

- [Walk Me Through It: A Step-By-Step Guide for Consumers Going Solar](https://www.energy.gov/cmei/systems/articles/walk-me-through-it-step-step-guide-consumers-going-solar) — U.S. Department of Energy
- [Homeowner's Guide to Solar](https://www.energy.gov/cmei/systems/homeowners-guide-solar) — U.S. Department of Energy
- [PVWatts Calculator](https://pvwatts.nlr.gov/) — National Laboratory of the Rockies, a U.S. Department of Energy laboratory
- [Solar Power for Your Home](https://consumer.ftc.gov/articles/solar-power-your-home) — Federal Trade Commission
- [Energía solar para su hogar](https://consumidor.ftc.gov/articulos/energia-solar-para-su-hogar) — Federal Trade Commission

> This checklist is educational and preliminary. System design, roof scope, production, savings, permits, interconnection and contract terms depend on the property, responsible professionals, current utility requirements and the signed agreement.

## Use this answer in a whole-property plan

- [Whole-home upgrade planning](https://betterpowered.com/whole-home-upgrade-planning/)
- [Energy advisory services](https://betterpowered.com/energy-advisory-services/)
- [How BetterPowered uses illustrative scenarios](https://betterpowered.com/methodology/illustrative-upgrade-scenarios/)
- [BetterPowered Contractor Standard](https://betterpowered.com/contractor-network/standard/)

## Primary sources

- [Walk Me Through It: A Step-By-Step Guide for Consumers Going Solar](https://www.energy.gov/cmei/systems/articles/walk-me-through-it-step-step-guide-consumers-going-solar) — U.S. Department of Energy
- [Homeowner's Guide to Solar](https://www.energy.gov/cmei/systems/homeowners-guide-solar) — U.S. Department of Energy
- [PVWatts Calculator](https://pvwatts.nlr.gov/) — National Laboratory of the Rockies
- [Solar Power for Your Home](https://consumer.ftc.gov/articles/solar-power-your-home) — Federal Trade Commission
- [Energía solar para su hogar](https://consumidor.ftc.gov/articulos/energia-solar-para-su-hogar) — Federal Trade Commission

> This article is educational and preliminary. Project scope, savings, eligibility, permits and tax treatment depend on the property and current program rules.