This home solar sizing and ROI workbook helps you turn electricity bills, roof conditions, equipment specifications, and financing assumptions into a repeatable estimate. Use the workflow to size solar panels, compare battery options, check inverter compatibility, estimate annual savings, and identify when your figures need to be updated.
Overview
A solar system should be sized around your actual energy use and installation conditions—not a generic system size. A useful estimate answers four separate questions:
- How much electricity does the property use?
- How much solar energy can the available roof or ground area produce?
- What equipment is needed to convert, store, and use that energy?
- Does the expected saving justify the total cost over the period you plan to own the system?
This workbook is designed for early planning and comparison. It is not a substitute for a structural inspection, electrical design, shading study, permit review, or installer proposal. Treat the result as a decision-making range, then ask a qualified professional to verify the design before purchasing or connecting equipment.
For a spreadsheet, create five tabs: Inputs, System Design, Cash Flow, Scenarios, and Actual Results. Keeping assumptions separate from formulas makes the workbook easier to audit when prices, utility rates, or production readings change.
How to estimate
1. Start with twelve months of electricity use
Collect your monthly bills and record kilowatt-hours (kWh), not only the amount paid. Add the monthly values to calculate annual consumption. If you have less than a full year of records, label the result as provisional because heating, cooling, occupancy, and seasonal equipment can materially change demand.
Also record your current electricity rate and any fixed charges separately. A simple energy-cost estimate is:
Annual energy cost = annual kWh × energy rate + annual fixed charges
Use the rate that applies to the electricity your solar system could offset. If your tariff has time-of-use periods, demand charges, export credits, or other billing rules, model those elements separately rather than applying one average rate to every kWh.
2. Estimate usable solar area
Measure the roof sections or ground area that could physically hold panels. Exclude access paths, skylights, vents, chimneys, required setbacks, fragile surfaces, and areas that remain shaded for a significant part of the day. Note the direction, tilt, and shade exposure of each usable section.
Panel count can be estimated with:
Panel count = usable area ÷ panel footprint
Round down, then check whether the resulting layout can be wired within the inverter’s voltage and current limits. A roof may have enough total area but still be unsuitable for the desired system because of obstructions or electrical constraints.
3. Convert panel count into system capacity
Multiply the planned number of panels by the panel’s rated wattage and divide by 1,000:
Array size in kW = panel count × panel wattage ÷ 1,000
For a production estimate, use a local or installer-provided annual yield assumption in kWh per kW per year. Then apply a conservative system factor for temperature, wiring, inverter conversion, soiling, mismatch, and shade:
Annual solar production = array size × local yield × system factor
Do not treat this as a guaranteed output. Compare the estimate with an installer’s production model and revisit it after the first complete year of monitoring.
4. Match the inverter
Record the inverter’s maximum DC input, operating voltage range, maximum current, AC output rating, and supported battery configuration. The panel string design must remain within those limits at both expected cold and hot conditions. Microinverters, string inverters, and hybrid inverters can produce different design and monitoring outcomes, so compare the complete system rather than the inverter price alone.
5. Size storage from loads and runtime
A battery is sized by the loads it must serve, the desired backup duration, and the portion of its capacity that can be used without exceeding the manufacturer’s limits. List essential loads such as refrigeration, internet equipment, lighting, medical equipment, pumps, or selected outlets. For each load, record watts and expected daily operating hours:
Daily energy = watts × hours ÷ 1,000
For backup, add the essential loads and multiply by the required hours or days. Then account for battery usable capacity and inverter losses. A battery that reduces evening grid purchases is not automatically the best battery for outage backup; the operating objective must be stated first.
For more detailed battery decisions, see the LiFePO4 solar battery buying guide and the home appliance backup runtime checklist.
Inputs and assumptions
Use one clearly labeled input cell for each assumption. Avoid burying numbers inside formulas. Your workbook should include the following fields:
- Monthly and annual electricity consumption in kWh
- Energy rate, fixed charges, export credit, and tariff structure
- Usable roof area, panel dimensions, orientation, tilt, and shade adjustment
- Panel wattage, efficiency, expected degradation, and warranty term
- Local production yield and overall system factor
- Inverter type, efficiency assumption, output rating, and replacement allowance
- Battery chemistry, nameplate capacity, usable capacity, round-trip efficiency, and warranty terms
- Equipment, installation, electrical work, permitting, and maintenance costs
- Financing cost, incentive or tax treatment, and expected electricity-rate change
- Analysis period and discount rate for a net present value calculation
For an equipment comparison, keep the distinctions practical:
| Component | What to compare | Why it matters |
|---|---|---|
| Solar panels | Efficiency, rated power, degradation assumption, warranty, dimensions | Determines roof use, expected output, and long-term production |
| LiFePO4 battery | Usable kWh, power rating, temperature limits, warranty, compatible inverter | Separates energy capacity from the ability to run loads |
| Lead-acid battery | Usable depth of discharge, ventilation, maintenance, cycle expectations | Nameplate capacity may overstate practical storage |
| Inverter | DC input limits, AC output, efficiency, backup functions, monitoring | Controls system compatibility and how loads are served |
For simple payback, use:
Simple payback period = net installed cost ÷ first-year net savings
For a more complete view, create one cash-flow row per year. Include energy savings, export revenue or credits, maintenance, battery replacement allowances, financing costs, and degradation. Net present value (NPV) discounts future cash flows to today’s value, but its result depends heavily on the discount rate and analysis period. Show those assumptions beside the result instead of presenting NPV as a universal answer.
Worked examples
Suppose a household uses 8,400 kWh per year. Its preliminary design uses 20 panels rated at 400 watts:
20 × 400 ÷ 1,000 = 8 kW of panel capacity
If the chosen local production assumption is 1,200 kWh per kW per year and the system factor is 0.80, the estimated annual production is:
8 × 1,200 × 0.80 = 7,680 kWh per year
This is an estimate, not a promise. It suggests the array may offset a substantial portion of annual use, but monthly production will not match monthly consumption. The household should compare winter output, summer surplus, export rules, and the timing of its loads before choosing a battery.
For a backup example, assume essential loads total 600 watts and must run for 10 hours:
600 × 10 ÷ 1,000 = 6 kWh of load energy
If the system experiences conversion losses and the battery reserve is limited, the selected nameplate capacity must exceed 6 kWh. Confirm the battery’s continuous power rating as well: a battery may have enough energy for a long runtime but still fail to start a high-surge pump or appliance.
Use three scenarios in the workbook: conservative, central, and optimistic. Change only defensible inputs such as production factor, electricity-rate growth, installed cost, export value, or battery use. A decision that works only under the optimistic scenario deserves additional review.
When to recalculate
Re-run the workbook whenever a major input changes. Update electricity rates, export credits, incentives, financing terms, equipment quotes, and installation costs before making a purchase. Policy and utility programs can change, so verify any current treatment with the relevant provider or a qualified local professional rather than relying on an old estimate.
After the first twelve months, compare the system’s actual production with the estimate and record the difference. Check whether shade, soiling, outages, curtailment, or monitoring gaps explain the result. The solar system performance review checklist can help organize that annual review.
Recalculate before adding panels, purchasing a battery, replacing an inverter, changing major appliances, or preparing to sell or refinance the property. Keep copies of bills, quotes, warranties, production reports, and the workbook version used for each decision. Supporting documents also make warranty and maintenance follow-up easier; see the solar panel warranty guide for a record-keeping checklist.
Next step: create the five workbook tabs, enter twelve months of bills, measure usable solar area, and save a conservative, central, and optimistic scenario. Treat the output as a living estimate—one that becomes more useful each time you replace assumptions with verified prices or measured production.
