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Solar Project Planner

All-in-one flagship workbench for sizing solar PV arrays, modeling shading losses, export tariffs, and 25-year ROI.

Instant Client-Side Math NREL & DOE Aligned 100% Free & Private

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Technical Overview

What is the Solar Project Planner?

The Solar Project Planner is EnerMath's comprehensive flagship engineering workbench that unifies array sizing, roof geometry, shading loss analysis, inverter DC/AC ratio sizing, and 25-year cash-flow modeling into a single interactive simulation.

Instead of jumping between disconnected calculators, enter your monthly electricity bill, local utility import and export tariffs, peak sun hours, and roof constraints once to evaluate recommended kW DC capacity, panel count, annual kWh yield, simple payback period, and lifetime CO₂ abatement.

Use this planner to benchmark residential or light-commercial solar quotes, test the impact of adding battery self-consumption, and verify whether net billing or time-of-use export credits justify a larger array.

Primary Model Inputs

monthly utility bill, retail tariff ($/kWh), usable roof area (sq ft), and shading loss (%)

Calculated Engineering Outputs

recommended system size (kW), annual generation (kWh), Year-1 bill savings, payback period, and system ROI

Mathematical Methodology

Core Engineering Formula & Industry Benchmarks

E_annual (kWh) = P_dc (kW) × PSH (hrs/day) × 365 × (1 - L_shade) × η_sys

System size is constrained by min(Load Target kW, Usable Roof Area ÷ 65 sq ft/kW) using NREL PVWatts derating.

Standard Industry Reference Values

US Residential Installed Cost
$2.65 – $3.15 / Watt DC
Source: NREL Solar Cost Benchmark
Standard System Derating Factor
78% – 84% DC-to-AC
Source: NREL PVWatts v8
LFP Battery Round-Trip Efficiency
90% – 94.5% AC/DC
Source: DOE Energy Storage Handbook
Annual Module Degradation Rate
0.40% – 0.55% / year
Source: NREL Photovoltaic Degradation Compendium
Step-by-Step Guide

How to Use the Solar Project Planner

1

Gather your baseline utility bills or project specs and enter your values for monthly utility bill, retail tariff ($/kWh), usable roof area (sq ft), and shading loss (%).

2

Adjust the interactive sliders or numeric inputs to match your local utility tariff, climate zone, and system size.

3

Review the real-time calculated metrics—specifically recommended system size (kW), annual generation (kWh), Year-1 bill savings, payback period, and system ROI.

4

Toggle available incentives (such as the 30% Federal Clean Energy Credit or local utility rebates) to compare gross vs. net payback.

5

Run a sensitivity check by testing conservative, expected, and high-inflation utility rate assumptions.

Real-World Case Study

Worked Example: Solar Project Planner

Real-World Engineering Scenario: Solar Project Planner

Baseline Assumptions

A homeowner with a $180/month electric bill at $0.18/kWh, 800 sq ft of unshaded south/west roof area, and 10% shading loss.

Modeled Outcome

An 8.3 kW rooftop array generating ~10,900 kWh/year, saving ~$1,960 in Year 1 with an estimated 7.2-year payback.

Engineering Takeaway

Sizing to 90–100% of annual consumption maximizes self-consumption before lower net-billing export rates apply.

Domain Intelligence

Key Engineering & Financial Insights

Self-Consumption Beats Export Credits

Under modern net-billing (NEM 3.0) tariffs, consuming your own solar power directly or storing it in an LFP battery is worth 3× to 4× more than exporting midday power to the grid.

DC-to-AC Oversizing Optimization

Sizing your solar array at 1.20× to 1.30× your inverter's continuous AC rating widens the shoulder production curve in morning and late afternoon with <1.5% annual clipping loss.

30% Federal Tax Credit Impact

Applying the 30% Federal Clean Energy Credit (Section 25D residential / Section 48E commercial) directly reduces net capital at risk and shortens simple payback by 2.5 to 3.5 years.

Utility Inflation Compounding

Because retail electricity tariffs historically rise 3.5%–5% per year, your nominal annual solar savings grow every year even after accounting for 0.5%/yr panel aging.

Why Use the Solar Project Planner?

Convert complex formulas into clear numbers you can act on quickly.

Measure whether your current plan is on track against your goals.

See how changing one variable affects outcomes before committing money.

Stress-test downside scenarios so surprises are less likely later.

Identify hidden cost drivers that reduce long-term financial efficiency.

Prioritize next actions based on measurable impact instead of guesswork.

Create a repeatable decision process you can use month after month.

Improve clarity when discussing options with family, lenders, or advisors.

Track progress over time and correct course early when assumptions change.

Use the results to improve structure and reduce long-term cost.

Technical Q&A

Frequently Asked Questions

The Solar Project Planner evaluates monthly utility bill, retail tariff ($/kWh), usable roof area (sq ft), and shading loss (%) using industry-standard thermodynamic and discounted cash-flow equations (E_annual (kWh) = P_dc (kW) × PSH (hrs/day) × 365 × (1 - L_shade) × η_sys). It computes recommended system size (kW), annual generation (kWh), Year-1 bill savings, payback period, and system ROI in real time directly in your browser so you can stress-test multiple scenarios instantly.
Your local utility tariff structure ($/kWh or fuel price), baseline usage volume, and equipment efficiency rating are the highest-sensitivity drivers. Even a $0.04/kWh shift in electricity rates or a 30% tax credit can shift payback timelines by 2 to 4 years.
Yes—where applicable, our models include toggles or adjustments for the 30% Federal Clean Energy Credit, Energy Efficient Home Improvement credits, and local utility rebates. Always verify your tax liability and local utility program rules before finalizing project financing.
Run three scenarios—conservative, expected, and optimistic—using the exact hardware wattage, SEER2/COP efficiency, and turnkey pricing from each installer's bid. Compare the resulting net payback period and 10-to-25-year cumulative savings rather than looking at upfront price alone.
No. All calculations inside the Solar Project Planner execute 100% client-side in your browser with <50ms latency. None of your utility bills, property specs, or financial inputs are stored or shared.