Solar PV System Design Calculator (Excel) for Homeowners and Small Installers

A practical, step-by-step tool to size your rooftop solar, estimate energy, and understand payback (usable worldwide)

 

If you’re a homeowner planning rooftop solar (or a small installer doing quick feasibility checks), the hardest part is usually not the panels. It’s the math. System size, string sizing, shading spacing, cable sizing, expected yearly energy, bill savings, and then the big question: is it worth it?

To make this easier, I built a Solar PV System Design Calculator (Excel workbook) that lets you do the core design calculations in a structured, guided way. It’s designed for USA homeowners first, but it also works well for India, Canada, and other regions by using region presets and editable assumptions.

This post also doubles as a user manual so you can start using the workbook confidently.

Who this Solar PV Excel tool is for

• Homeowners who want to size a rooftop solar system and estimate savings before talking to installers
• Small solar installers who want a fast, consistent pre-design calculator for proposals
• DIY solar learners who want to understand how solar PV sizing works in real life

What this Excel Solar PV design calculator can do

Depending on how deep you want to go, the workbook supports both quick estimates and detailed checks:

• Recommend a DC system size (kWdc) based on your yearly usage
• Estimate 1st-year solar energy generation (kWh/year) and monthly energy
• Calculate annual bill savings and simple payback
• Support module + inverter selection, including a small equipment library and custom entry
• Run string sizing checks (series modules min/max based on voltage and temperature)
• Help with row spacing / shading clearance planning
• Do basic cable sizing and voltage drop checks (DC and AC)
• Generate a clean Quote / Summary page you can print or share as a proposal 

 User Manual: How to use the Solar PV System Design Calculator (Excel) 

Before you start: what you need to collect

You’ll get the best results if you have:

1. Your annual electricity consumption (kWh/year)
• You can get this from your utility portal or by adding the last 12 bills.
2. Your electricity tariff / rate
• USA: $/kWh
• India: ₹/kWh
• Canada: CAD/kWh
3. Your location solar potential (Peak Sun Hours or equivalent)
• If you don’t know, start with the workbook’s suggested values and refine later.
4. Optional but helpful:
• Roof usable area
• Any shading obstacles (water tank, parapet wall, nearby building)

Step 1: Start on the “Start Here” sheet

Open the workbook and go to Start Here.

• Read the short notes
• Use the navigation links to go to Region Presets and Inputs
• If Excel shows a security bar, click Enable Editing (no macros needed)

Step 2: Choose your region (Region Presets)

Go to Region Presets and select:

• USA (primary)
• Canada
• India
• UK / EU
• Other / Custom

This will populate suggested defaults (you can override later). The idea is simple: start with sensible assumptions, then customize based on your site and preferences.

Tip for installers: If you work in multiple regions, this sheet helps you stay consistent across proposals.

Step 3: Fill the Inputs sheet (this is the main control panel)

Now go to Inputs. Fill the cells in the “User Inputs” area. Focus on these first:

 

A) Consumption and target sizing

• Annual consumption (kWh/year)
• Optional: if you want to offset only part of your usage (example: 80%), enter your target offset

The tool uses this to recommend a DC size (kWdc).

B) Tariff and savings

• Tariff / electricity rate (per kWh)

This drives your estimated savings.

C) Solar radiation / PSH

• Peak sun hours (PSH) or equivalent solar resource
• If you don’t know your PSH, use the suggested values and adjust after you check a reliable solar map for your city.

D) System assumptions (easy to keep default)

• Performance Ratio (PR)
• Temperature derate assumptions (if provided)
• Loss assumptions (soiling, wiring, inverter, etc.)

If you’re a homeowner: you can keep these at defaults for a first estimate.

Step 4: Decide how you want to use the tool (Quick vs Detailed)

This workbook supports two modes:

Mode 1: Quick estimate (best for homeowners)

Just complete Region Presets + Inputs.

• You’ll instantly get:
• Recommended kWdc
• Estimated 1st-year energy (kWh/year)
• Annual savings
• Simple payback

This is perfect for “Should I go solar?” and “How big should my system be?”

Mode 2: Detailed design checks (best for installers)

After Inputs, also complete:

• Module & Inverter
• String Sizing
• Shading & Spacing
• Cable Sizing
• Protection

This is perfect when you want a technically sound pre-design before final engineering.

Step 5: Select module and inverter (Module & Inverter sheet)

Go to Module & Inverter and choose:

• A module from the library (or enter your own datasheet values)
• An inverter (or enter your own)

 

Why this matters:

• Module voltage varies with temperature
• Inverter MPPT voltage window limits how many modules can be in series
• This sheet sets up accurate string sizing

If you’re a homeowner and you don’t know what to choose: pick a common modern module (400–550W range) and a typical residential inverter size.

Step 6: Check string sizing (String Sizing sheet)

This sheet answers questions like:

• How many modules can I connect in series safely in winter (cold Voc limit)?
• Will the inverter MPPT still work in summer heat (hot Vmp minimum)?
• How many strings per MPPT are reasonable?
• Total modules and string configuration for your target kWdc 

 

If the sheet flags warnings:

• Reduce modules in series if cold voltage is too high
• Increase modules if hot voltage is too low for MPPT
• Adjust inverter selection if it’s mismatched

This is one of the most valuable pages for installers.

Step 7: Plan spacing and shading clearance (Shading & Spacing)

This section helps you estimate:

• Row-to-row spacing to reduce inter-row shading
• Clearance around obstacles (parapet wall, water tank, nearby building)

 

Homeowners can use this to sanity-check roof fit.

Installers can use it for an early layout plan.

Step 8: Cable sizing and voltage drop (Cable Sizing)

This helps check:

• DC cable sizing for acceptable voltage drop
• AC cable sizing from inverter to ACDB / main panel
• A practical “round up to standard cable size” approach

Important note: This is a calculator aid, not a replacement for local electrical code compliance. Always verify with a licensed electrician and applicable codes (NEC in the USA, CEC in Canada, etc.).

 

Step 9: Economics (payback, NPV, IRR)

Go to Economics if you want more than simple payback.

You can enable:

• Electricity price escalation
• Solar degradation per year
• Optional financing / loan inputs
• NPV and IRR style evaluation (useful for serious buyers and proposal quality)

Homeowners: even if you don’t use NPV/IRR, it’s a great learning tool to understand long-term value.

Step 10: Quote / Report (shareable summary)

Finally, visit:

• Report: technical summary
• Quote: a clean proposal page you can print or share

Installers can paste their branding and use this as a lightweight proposal template.

 

Best practices to get accurate results

• Use 12-month consumption, not one month
• Update tariff for slab / time-of-use if applicable (or use an average)
• Pick a realistic PR (don’t assume perfect conditions)
• If your roof has shading, reduce expected energy or improve assumptions cautiously
• Treat this tool as a decision helper, then validate with a site survey

 

FAQ (common homeowner questions)

1. Is this tool only for USA or any specific region?
   No, it’s built to be usable across regions. Choose a region preset and adjust tariff, PSH, and assumptions. 
2. Do I need to know electrical engineering?
   No. For basic sizing and savings, you only need consumption, tariff, and location assumptions. 
3. Can I use it for proposals as a small installer?
   Yes. Use the detailed sheets (module, inverter, string sizing, spacing, cable, protection) and then generate the Quote sheet.

 

Want the workbook?

Grab it here on my page or request using this form. If you try it, tell me your city/region and your monthly bill range, and I’ll help you set the assumptions correctly so the estimate is realistic.

 

Important note:

This is a calculator tool meant to help you plan and understand numbers better. For final installation, always validate with local codes and a licensed professional.