Overview
This tool helps you compare two core design paths for residential and small-community solar systems: a net-zero grid-tied system and a fully off-grid system. Both can power the same load but differ in cost structure, reliability, and policy sensitivity.
The calculator estimates CAPEX (upfront cost) and LCOE (levelized cost of electricity — the average $/kWh over the system’s life). Using your local irradiance, load, and component data, it automatically sizes panels, inverters, and batteries, then computes energy balance, grid bills, and overall economics.
Grid-Tied (Net-Zero)
Designed to offset ~100% of annual energy use through grid exports and credits. Uses the grid for backup — no batteries required. Lowest cost per kWh, but depends on policy (buy/sell rate and net-metering terms).
Off-Grid
Independent of the utility grid, with solar and batteries sized for multi-day autonomy. Provides resilience and energy security but requires higher CAPEX, periodic battery replacements, and careful load management.
Decision Hook
If grid extension is expensive or unavailable, off-grid may be cheaper overall despite a higher LCOE. Otherwise, a grid-tied system provides faster payback and lower complexity.
When to Choose Each Path
- Choose Grid-Tied if the grid is available, export credit policies are fair, and reliability is high.
- Choose Off-Grid if grid connection costs exceed ~$20–30k or reliability/outage risk is unacceptable.
- Choose Hybrid (PV + small battery) if export credits are low and you want partial backup power.
Key Metrics Explained
- CAPEX: Total upfront investment (panels, batteries, inverter, installation, and grid costs).
- LCOE: Present-value cost of energy per kWh over system lifetime.
- Autonomy: Number of days the system can operate without solar input (relevant for off-grid).
- Payback: Years required for savings to equal initial investment (for grid-connected systems).
Content adapted for educational comparison; use time-series modeling (e.g., PVWatts, SAM) and actual tariffs for design accuracy.
How Net Metering Works
Net metering allows solar owners to stay connected to the utility grid while offsetting their electricity bills. Your system’s meter tracks both imports (electricity you use from the grid) and exports (excess energy you send back). The utility then applies credits to your bill based on these energy flows.
Step-by-Step Energy Flow
- Generation: Your PV array produces DC power, converted to AC by the inverter.
- Self-Use: The home consumes this solar power instantly when available.
- Export: Surplus solar energy is exported to the grid and tracked by a bi-directional meter.
- Credit: Each exported kWh earns a credit at the sell rate (¢/kWh).
- Import: When production is low (night or cloudy days), power is imported from the grid at the buy rate.
- True-Up: At month or year end, your total imports and exports are reconciled by the utility.
Bill or Credit Calculation
The calculator uses these rates to determine your bill outcome:
Annual Bill ($) = (Import kWh × Buy Rate) − (Export kWh × Sell Rate)
If the result is positive → you owe the utility. If negative → you receive a credit. When Annual Production ≈ Annual Load, the system achieves “Net Zero.”
Example Scenario
Suppose a home uses 10,000 kWh/yr and the PV system produces 11,000 kWh/yr. At a Buy Rate = $0.20/kWh and Sell Rate = $0.10/kWh:
- Export = 1,000 kWh → credit of $100
- Import = 0 kWh → no grid usage
- Annual Bill = −$100 (credit carried forward)
In this example, the household achieved net-positive generation; excess credits may roll over or expire depending on the utility’s true-up policy.
Common Policy Variations
- Full Retail Credit: Exports valued at the same rate as imports (traditional NEM 1.0).
- Avoided Cost Credit: Lower sell rate (reflects utility’s wholesale value, e.g. NEM 3.0).
- Monthly True-Up: Credits reset each month, favoring self-consumption.
- Annual True-Up: Credits roll for 12 months, good for seasonal solar regions.
Tips for Optimization
- Size PV slightly below total annual load if export credits are low.
- Use smart appliances or small batteries to shift solar use into evenings.
- Track local tariff updates—small changes in sell rate can change ROI.
- Compare hybrid (PV + battery) setups if you face low export rates.
Content adapted for educational comparison; always check your utility’s current net-metering and interconnection rules.
Case Studies (Quick Read)
1) Off-Grid Cabin (8 kW + 48 kWh)
A remote cabin located 0.5 miles from the nearest grid line — avoiding costly grid extension (~$50,000+). The system includes an 8 kW PV array and a 48 kWh LiFePO₄ battery bank providing ~2 days of autonomy.
- Load: 9,000 kWh / year (≈ 25 kWh/day)
- Irradiance: 4.5 kWh/m²/day → annual yield ≈ 11,000 kWh
- Storage: 48 kWh usable at 80% DoD; battery life ≈ 12 years
- System Cost: ~$45,000 (PV + batteries + inverters + install)
- LCOE: ~$0.38 / kWh (higher due to storage replacement)
Includes a 3 kW generator for winter reserve and backup. No grid bill, full autonomy, but higher upfront cost.
2) Net-Zero Suburban Roof (10 kW)
A typical suburban home in a region with 5.0 kWh/m²/day solar resource, annual load ≈ 10,500 kWh. The array is sized to generate ~ 110% of annual use, exporting surplus energy through net metering.
- Buy Rate: $0.20 / kWh
- Sell Credit: $0.10 / kWh (NEM 3.0 policy)
- System Cost: ~$23,000 (panels + inverter + install + permits)
- LCOE: ~$0.23 / kWh
- Payback: ~8 years with 30% ITC and current tariffs
Export credits reduce bills significantly; homeowner reaches net-zero on annual basis, with grid backup maintained.
3) Hybrid Microgrid (6 kW + 15 kWh Battery)
A semi-rural home uses a hybrid system that remains grid-connected but includes a small 15 kWh battery for evening use and backup during outages. This setup balances economics and resilience.
- Load: 8,000 kWh / year
- Irradiance: 5.5 kWh/m²/day → annual generation ≈ 8,800 kWh
- Battery: 15 kWh usable; 1 day autonomy
- System Cost: ~$28,000 (PV + battery + inverter + install)
- LCOE: ~$0.27 / kWh
Content adapted for educational comparison; use time-series simulation (e.g., PVWatts + tariff engine) for design validation.
Policy Landscape & Practical Tips
Policy and tariff structures play a major role in determining whether a grid-tied, hybrid, or off-grid system makes the most sense financially. Export credit values, interconnection fees, and incentives can shift payback periods by years. This section outlines the key factors to monitor before committing to a system design.
How Policy Impacts System Choice
- Export Credits: Define how much you earn for each excess kWh sent to the grid. Lower credits favor self-consumption and batteries.
- Time-of-Use (TOU) Tariffs: Electricity prices vary by time of day; exporting midday solar at low value and importing in the evening at high value can erode savings.
- Net Billing vs Net Metering: Under net billing, exports are valued lower than imports; under net metering, they are typically equal. Know your local policy.
- Incentives & Tax Credits: Federal or state incentives can reduce upfront CAPEX by 20–40%, improving LCOE for both grid-tied and off-grid systems.
Check Tariffs
Review your utility’s time-of-use and export credit rates. A favorable net-metering policy (1:1 buy/sell) makes grid-tie more cost-effective; under reduced export credits (e.g., 30–50% of buy rate), battery self-use becomes more valuable.
Backup Value
Adding even a small 10–15 kWh battery can shift self-consumption into high-value hours, reduce exposure to TOU rates, and provide outage resilience — often improving payback under NEM 3.0 or net billing frameworks.
Extension Cost
For remote sites, compare the total cost of extending grid service (distance × $/ft) with the CAPEX of an off-grid solar + battery system. If extension cost exceeds $25k–$40k, off-grid autonomy often becomes economically favorable despite higher LCOE.
Practical Steps Before Installation
- Contact your utility or regulator to confirm the latest net-metering and interconnection policy.
- Model your expected export/import balance using average daily load and irradiance.
- Evaluate hybrid (PV + battery) setups if your export credit is below 50% of your buy rate.
- Account for permit, interconnect, and maintenance fees in the CAPEX estimate.
- Lock in your net-metering terms before policy revisions — grandfathered rates can preserve ROI.
Example Policy Sensitivity
A 10 kW rooftop system producing 15,000 kWh/year in a region with a $0.20/kWh buy rate:
- At 1:1 Net Metering → near net-zero bill; payback ≈ 7 years.
- At 50% Export Credit → ~20% higher payback time; hybrid system recommended.
- At 25% Export Credit → battery storage nearly essential for economic parity.
Content adapted for educational comparison; use time-series simulators and verified utility tariffs for final financial modeling.
Solar Sizing & Cost Tool (Net-Zero vs Off-Grid)
This interactive calculator compares the levelized cost of electricity (LCOE) for on-grid and off-grid systems and estimates net-metering outcomes. Enter your array size, production, load, and cost assumptions — then test buy/sell rates to see how policy changes affect your annual bill.
How to Use the Calculator Tool
This section describes what each variable means and where to find the values you’ll need for the calculator. It follows the spreadsheet tabs: Inputs, Definitions, Calculations, NetMetering, and Examples. View Tool Working →
- Array Size (kW): Total PV capacity — use your installer’s quote or system design target.
- Annual Load (kWh): Yearly household or facility electricity use from your utility bills.
- Irradiance (kWh/m²/day): Local solar potential from NREL PVWatts or Global Solar Atlas.
- Performance Ratio (PR): Accounts for inverter and wiring losses; typically 0.75–0.85.
- Battery Capacity (kWh): Total usable storage; multiply nominal by depth of discharge (DoD).
- Buy Rate ($/kWh): Grid electricity price from your tariff.
- Sell Credit ($/kWh): Rate paid for exports under your net-metering policy.
- CAPEX(Capital Expenditure): Total installed cost including modules, inverter, battery, and BOS (Balance of System).
- O&M: Annual operating and maintenance cost, usually 1–2% of CAPEX.
- CRF (Capital Recovery Factor): Converts upfront CAPEX to an annualized cost based on discount rate and lifetime.
- LCOE (Levelized Cost of Electricity): Average lifetime cost per kWh produced.
Tip: You can estimate irradiance and performance ratio using free tools like PVWatts, and obtain buy/sell rates directly from your utility tariff sheet.
Outputs include:
- On-grid and off-grid LCOE
- CAPEX estimates and cost ratios
- Import/export energy balance
- Net-metering bill or credit outcome
📄 View the underlying assumptions sheet:
Open Google Sheet →
🧮 See the live calculator logic:
View Tool Working →
💻 Access the GitHub repository:
GitHub →
Once inside GitHub, click the “Code” green button → select
Download ZIP to get all the website HTML files.
🎬 Watch the demo video:
Demo Video →
🌐 Visit the full educational page:
One Community – Solar Energy Microgrid Setup and Maintenance →
These external links provide open-source documentation, spreadsheets, and visual references for the Solar Cost Analysis Tool. Use them to trace how assumptions, costs, and energy data flow through the underlying model.
How to find each input:
- Load (kWh): Use last 12 months of utility bills; sum kWh (or compute average daily = annual/365). Off-grid cabins: estimate from appliances × hours/day.
- Solar irradiance (kWh/m²/day): Use PVWatts (NREL) or Global Solar Atlas for your location. The tool multiplies by a Performance Ratio for losses.
- Distance from grid & $/ft: Ask the utility/contractor for line extension quote; rural overhead vs underground differs widely.
- Panel details: Module nameplate watts and price per module (or per W). Enter both; we’ll compute module count and installed $/W.
- Battery: Enter nominal voltage, Ah per battery (or kWh per unit), days of autonomy, DoD, and $/kWh (or pack price/size). Tool sizes bank kWh and estimates cost.
- Inverter: Peak kW needed (sum of coincident loads + surge factor) and $/kW. Off-grid inverters often cost more.
- Buy/Sell rates: Your tariff’s import rate and export credit (or tariff-specific $/kWh).
- Installation: Lump-sum labor/permits/inspection; add BOS if not in panel price.
Values last updated: —