Should You Get Solar Panels, an EV, or Both?

A financial model showing why solar panels and electric vehicles are worth more together than apart — especially now that electricity prices are rising and the cost of going green is declining.

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20-year net benefit from solar alone
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20-year net benefit from an EV alone
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Extra value from doing both together

Why this calculator exists

Electricity prices have risen sharply in recent years and projections suggest more of the same. At the same time, the cost of rooftop solar continues to fall every year. Moreover, electric vehicle prices continue to fall, while gas prices remain high. The result is that the financial case for going green is stronger than it has been at any point in history, and the landscape is shifting fast enough that an evaluation from even two or three years ago may already be out of date. This necessitates a tool that can consider the changing price landscape and help you make an informed decision about whether to buy now or wait for prices to fall further. Importantly, the savings shown here are not just inflation-adjusted — they represent returns above and beyond what your money would earn in a financial portfolio. A net benefit of $0 means the solar or EV investment ties that return, and anything positive means it beats it.

Additionally, the tool must capture the fact that solar and EVs are complements: an EV charged with rooftop solar has zero fuel cost, and the EV's battery can replace expensive standalone home battery storage for solar. The tool below quantifies that synergy, so you can see whether the combined package makes financial sense for your situation, even if each investment alone looks marginal.

Your Inputs

Adjust these parameters to match your situation. The defaults are for Spring, TX.

☀️ Solar & Home

hrs ?Find yours at Google Project Sunroof (sunroof.withgoogle.com)
kWh ?Check your utility bill. Average US home is ~900 kWh/mo.
¢/kWh ?Find on your electricity bill. US avg ~16¢. TX avg ~14¢.
$ ?Leave blank to use formula estimate ($3K fixed + $2.20/W). Enter your actual quote if you have one.
$ ?Federal 25D ITC expired Dec 2025. Enter any state/local incentives. Check dsireusa.org.

🔋 Battery Storage

?If Yes, sizes a standalone home battery for Scenario A using the ratio below. Default No: solar alone has no battery storage.
kWh ?kWh of standalone battery to add on top of the EV's V2H in Scenario C. 0 = the EV is the only storage. Costs kWh × $/kWh from the "Battery cost per kWh installed" rate below.
$/kWh ?Tesla Powerwall 3: ~$1,000/kWh. Market avg ~$1,100/kWh.
kWh/kW ?Rule of thumb: 1-1.5 kWh of battery per kW of solar capacity for grid-tied systems.

🚗 EV & Driving

mi/yr ?Average US driver: 12,000-15,000 mi/yr.
MPG ?Your current ICE vehicle's fuel economy.
$/gal ?Check GasBuddy for local prices.
$ ?Price difference between comparable EV and ICE, net of any EV credits and lower maintenance.
kWh ?Usable EV battery capacity for vehicle-to-home. Most EVs have 60-80 kWh total; 40 is conservative after driving reserve.

📈 Price Projections

%/yr ?US electricity prices have risen ~7% annually in recent years. EIA projects continued increases.
%/yr ?Long-run average nominal gas price growth.
%/yr ?Expected average inflation over the next 20 years.

⚙️ System Details

%?Solar inverter efficiency. Typical: 80-90%.
mi/kWh?Typical EV: 3-4 miles per kWh.

💰 Financial Preferences

%?What your money would earn otherwise (e.g., stock market ~9% nominal).
?0 = counterfactual is illiquid. 1 = fully liquid. Adjusts discount rate for illiquidity of solar investment.
%?How much you'd sacrifice future $ for present $. Enters discount rate via liquidity factor.

🌱 Non-Financial Preferences

%?What % extra would you pay on electricity to remove carbon footprint?
%?What % extra would you pay on fuel to remove carbon footprint?
%?What % extra would you pay for guaranteed electricity access? Applied when battery storage is present.

📉 Price Decline Rates

%/yr?Annual real price decline for residential solar (SEIA 5-year avg: ~4%).
%/yr?Annual real price decline for home batteries.
%/yr?Annual real price decline of EV premium as battery costs fall.

Your Results

These figures represent the value of each investment — Solar Only —, EV Only —, Solar + EV — in upfront cash — over and above a portfolio earning 9% annually. A result of $0 means parity with the portfolio — anything positive means solar/EV beats it.

Scenario A
Solar Only
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Net benefit over 20 years
Solar capacity—
Installation cost—
Tax credits—
Battery cost—
Independence benefit—
Net cost—
Total NPV of savings—
Buy now or wait?—
Scenario B
EV Only (Grid Charging)
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Net benefit over 20 years
EV price premium—
Total NPV of fuel savings—
Year 1 fuel savings—
Year 20 fuel savings—
Buy now or wait?—
Scenario C
Solar + EV (Solar Charging)
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Net benefit over 20 years
Combined solar capacity—
Solar cost (net of credits)—
EV price premium—
Extra battery installed—
Battery cost—
Net cost—
Total NPV of savings—
Buy now or wait?—

Complementarity: Why Both > Each Alone

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Synergy value = Scenario C − Scenario A − Scenario B
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Avoided EV electricity costs (solar charges EV for free)
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Less: incremental solar cost for EV capacity
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Avoided battery hardware cost (EV V2H replaces standalone battery)
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Independence benefit unlocked by Solar+EV (EV V2H counts as storage)
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Non-financial premium on solar-charged EV

How It Works

Click to expand the methodology behind each part of the model.

Capacity is computed from your electricity usage: Capacity (kW) = monthly_kWh × 12 ÷ (sunlight_hours × inverter_efficiency%)

Panel production each year accounts for degradation — panels start at 100% efficiency, drop to 97% in year 2 (initial burn-in), then decline 1% per year thereafter.

Real annual savings = monthly production × electricity price × 12, deflated by CPI growth. Each year's electricity price compounds at the growth rate you set.

Discounted savings = real savings ÷ (1 + discount_rate)^(year−1). The discount rate reflects your opportunity cost of capital, adjusted for liquidity and impatience preferences.
ICE fuel cost each year: annual_miles ÷ MPG × gas_price, with gas prices compounding at the gas growth rate, deflated by CPI.

EV electricity cost (grid charging): monthly_EV_kWh × electricity_price × 12, also deflated by CPI. Monthly EV kWh = annual_miles ÷ 12 ÷ EV_efficiency.

Grid-charging savings = ICE cost − EV electricity cost. Notice these decrease over time when electricity rises faster than gas.

Solar-charging savings = full ICE cost (EV electricity is free). These increase over time with gas prices.
Solar installation cost uses a concave formula: $3,000 + $2.20/W × capacity. The $3K fixed component (permits, design, mobilization) means larger systems cost less per watt — a 15 kW system is ~$2.40/W vs ~$2.95/W for a 4 kW system. Source: EnergySage and a1solarstore 2026 pricing data.

Tax credits default to $0. The federal 30% Residential Clean Energy Credit (Section 25D) expired December 31, 2025 under H.R. 1. Check DSIRE for state/local incentives.

Battery cost is sized at a configurable ratio per kW of solar (default 1.25 kWh/kW). Installed cost averages ~$1,000/kWh in 2026 (EnergySage/Tesla Powerwall 3 data).
Environmental premium: A willingness-to-pay multiplier on financial savings. If you'd pay 10% extra for carbon-free electricity, solar savings are worth 10% more to you. Applied separately for solar and EV.

Independence benefit: If battery storage exists (standalone or EV V2H), the model credits an independence value equal to the independence preference % × the financial NPV. This reflects the value of guaranteed electricity access during grid outages.
The synergy equals Net Benefit(C) − Net Benefit(A) − Net Benefit(B) and comes from three sources:

1. Avoided EV electricity costs: When charging from solar instead of the grid, the EV's electricity cost drops to zero. The NPV of those avoided grid charges is the largest financial component of the synergy.

2. Avoided battery cost (V2H): A typical EV has 60-80 kWh of battery. Vehicle-to-Home (V2H) technology lets the EV replace a standalone home battery (typically 13-17 kWh). This saves $13,000-17,000 in battery costs.

3. Concave solar costs: Adding EV capacity to the solar array increases total kW but the incremental cost per watt is lower than the average (fixed costs already paid). This partially offsets the extra panels needed.
Real discount rate = (counterfactual_growth + liquidity × impatience) − CPI. With defaults (9% growth, 0 liquidity, 2% CPI), the discount rate is 7%. This represents the real return your money would earn otherwise.

"Buy now or wait" answers: if equipment prices decline at X%/year, how many years until the cost drops enough that buying later is better? The formula is log(NPV / Cost) ÷ log(1 − decline_rate). A negative number means the net benefit is already positive — buy now. A positive number means wait that many years.The practical implication: the net benefit figures in this tool are not savings relative to doing nothing. They are savings relative to investing the same money in a portfolio earning your counterfactual rate. A net benefit of $0 means solar or EV exactly matches your portfolio return — making it a remarkably high bar to clear.

Limitations

Independence is the only modeled battery benefit. We currently only capture the independence benefit from having a battery. There may be other benefits we don't model — for example, protection from dynamic price increases during times of no solar production (cloudy days, evenings, winter), or arbitrage between time-of-use rate periods.
Solar capacity sizing is a simple heuristic. There may be more sophisticated ways to calculate optimal capacity, since costs per watt decrease with total watts due to fixed cost. In other words, one would have to calculate the extra benefit from adding an extra kW (savings or payment from selling to grid) against the extra cost. Instead, here we simply pick capacity that covers your average electric bill.
Sources: EnergySage, a1solarstore, NREL ATB 2024, SEIA, IEA Global EV Outlook 2025, BloombergNEF Battery Price Survey 2025.
Federal ITC expiration: H.R. 1 signed July 4, 2025.
This tool is for informational purposes only and does not constitute financial advice. Consult a financial advisor for personalized recommendations.