WattGauge

RV · Off-Grid · Home Solar

Solar calculators
that show the math.

Size your RV solar system, off-grid cabin, or home setup with our free tools. No sign-up. No login. Every formula shown.

Featured Tools

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RV Solar Sizing

Size your panel array, battery bank, and charge controller for van life or RV travel.

↳ 500 Wh/day, 5 sun hrs → 125W panels

Off-Grid Cabin System

Full off-grid system design: panels, batteries, inverter size, and estimated cost range.

↳ 3 kWh/day, 3 days backup → 750W array, ~11 kWh bank

Home Solar Savings

Annual savings, payback period, and 25-year ROI using your state's sun hours and utility rates.

↳ $180/mo bill in CA → ~5 yr payback

Battery Bank Sizing

Size a battery bank for backup or off-grid use: DoD, voltage, chemistry, and string layout.

↳ 5 kWh/day, 3 days, 48V → 20 × 100Ah

Solar Payback & ROI

Simple payback, IRR, NPV, and a 25-year savings chart, with rebates and cash, loan, or lease financing.

↳ 6 kW, $16.5K in CA → ~5 yr payback

Solar + EV Planner

Extra panels needed to cover EV charging, fuel savings vs. gas, and annual charging cost.

↳ 200 mi/wk → +6 × 400W panels

Why WattGauge

01

All math shown

Every calculator discloses the exact formula it uses. Understand your numbers, not just trust them.

02

Built for every setup

RV, van, off-grid cabin, or home rooftop: each calculator is tailored to how that system actually works.

03

Runs in your browser

All calculations happen locally. Your inputs never leave your device. Works offline once loaded.

FAQ

Do I need to sign up or enter my email?
No. Every calculator runs for free with no account, no email, and no sign-up. You can also print or save any result page as a PDF straight from your browser.
How accurate are the results?
These are engineering estimates that match the standard formulas used by solar installers. Real-world results vary based on panel orientation, shading, temperature derating, and wiring losses. Add 20–25% margin to your panel and battery sizing.
Where does the data come from, and how current is it?
Peak sun hours come from NREL (National Renewable Energy Laboratory) public datasets. Utility rates are EIA 2026 residential state averages and are reviewed annually. The calculators apply no federal tax credit by default, because the residential credit (Section 25D) ended December 31, 2025. The payback tool has an editable credit field for state incentives, and you can override rates and sun hours on every calculator page.
Can I share or bookmark my results?
Yes. All calculators encode your inputs into the URL (?params=...), so you can bookmark or share a link and get the same results back instantly.
Does it work on mobile and offline?
Yes. All tools are fully responsive and mobile-friendly. Once the page loads, all math runs locally in your browser. Disconnect from the internet and calculations keep working.
I found an error in a formula. What do I do?
Please report it via the Contact page with the calculator name, your inputs, and expected vs. actual output. All formulas are verified against NREL and standard solar engineering references, and errors are fixed quickly.