About TheGreenWatt
Solar education built by a physicist who installed his own panels and decided to do the math properly. No fluff, no lead generation — just NREL data and peer-reviewed formulas.
Who Runs This Site
I'm Marko Visic, a physicist based in Ljubljana, Slovenia. I got into solar energy the way most people do — by installing panels on my own house and then falling down a rabbit hole of datasheets, inverter specs, and NREL weather models.
TheGreenWatt started in 2021 as a single calculator I wrote because every other "solar calculator" on the web either required my phone number, returned obviously wrong numbers, or both. I wanted a tool that used the same NREL data that professional installers use, but without the sales pitch attached to the results.
Five years later, the site has grown to 157 articles, 16 interactive calculators, peak sun hours data for all 50 US states and DC, a 30-term solar glossary, and sizing guides for every major home appliance. The physics background helps — when I write about temperature coefficients or system derate factors, I'm working from the underlying equations, not paraphrasing someone else's blog post.
I run this site solo. There is no editorial team, no content farm, no venture capital asking for growth metrics. The site grows by being accurate and useful, which is the only growth strategy I know how to execute.
What You Can Do On This Site
TheGreenWatt covers the full solar journey — from "how does this work?" to "how many panels do I need?" to "is it worth the money?" Everything is organized into seven categories:
Photovoltaic fundamentals, panel specs, and a 30-term glossary
System sizing, output estimation, and production by kWh target
Pricing, tax credits, ROI, financing, net metering, and payback
Battery sizing, chemistry comparison, charge controllers, and wiring
Series vs parallel, inverter types, and the installation process
Cleaning, monitoring, degradation, snow, clouds, and troubleshooting
Sizing guides for specific loads: AC, EV, RV, well pump, and 30+ appliances
All 16 Calculators
Every calculator is open-formula — the equation is documented in the article, the source for each constant is cited, and the results are verified against NREL PVWatts output. No signups required. Your inputs stay in your browser.
Methodology
The peak sun hours data that powers the state pages and calculators comes from NREL PVWatts v8, fetched in April 2026. For each of the 51 locations (50 states + DC), we simulated a reference system with these parameters:
- System capacity
- 1 kW DC
- Tilt
- 20° (typical residential)
- Azimuth
- 180° (south-facing)
- DC-to-AC derate
- 0.83 (14% DC losses × 96% inverter)
- Array type
- Fixed roof mount
- Weather data
- NSRDB TMY (typical meteorological year)
- Location per state
- Largest city (by population)
- Last fetched
- April 2026
The Core Solar Output Formula
Cost Data
Solar system pricing uses the NREL Q1 2024 U.S. Solar Photovoltaic System Cost Benchmark as the baseline: $2.50–$3.20 per watt installed for residential systems. State-level pricing is adjusted based on NREL regional data and verified against installer quotes from EnergySage marketplace reports. The average US electricity rate ($0.17/kWh) comes from EIA Electric Power Monthly Table 5.6.A.
Battery Data
Battery calculations use chemistry-specific efficiency factors: lithium iron phosphate (LiFePO4) at 95% round-trip efficiency, AGM at 85%, and flooded lead-acid at 80%. These values are from DOE battery technology references and validated against manufacturer-published specs from Battle Born, Victron, and Renogy.
Appliance Data
Appliance wattage figures come from the EPA ENERGY STAR Product Finder database and DOE Appliance Energy Use charts. When ENERGY STAR data is unavailable, we use nameplate ratings from manufacturer documentation with a realistic duty cycle applied (e.g., a refrigerator runs its compressor about 33% of the time, not continuously).
Editorial Standards
Every article on TheGreenWatt follows these editorial standards:
Sources Required
Every article cites at least 5 sources. Sources must be government databases (NREL, EIA, DOE, EPA), peer-reviewed standards (IEC, NEC), or manufacturer-published datasheets. We do not cite blogs, forums, AI-generated content, or "solar quotes" sites as primary sources.
Numbers Verified
Every numerical claim (cost per watt, kWh output, payback period) can be traced back to its source data and reproduced using the formula shown in the article. Calculator outputs are cross-checked against NREL PVWatts for known reference systems.
Regular Updates
Articles display their "last updated" date. Cost data is reviewed when NREL publishes new benchmarks (quarterly). State data is refreshed annually from NREL PVWatts. Outdated articles are flagged and updated within 30 days of new data availability.
Corrections Policy
If we get something wrong, we fix it and update the article's "last updated" date. For significant corrections (wrong formula, incorrect data), we add a note explaining what changed. Readers can report errors via the contact page.
Independence
TheGreenWatt is independently operated. No solar installer, manufacturer, or financing company has editorial influence over our content. Product recommendations are based on published specifications and independent testing data, not sponsorship relationships.
Data Sources
Every numerical claim on this site comes from one of these whitelisted sources:
Peak sun hours, solar irradiance by location, system production modeling. PVWatts is the industry-standard tool used by professional solar installers.
pvwatts.nrel.govElectricity prices by state ($0.17/kWh national average), household consumption data (10,791 kWh/yr average), utility rate trends.
www.eia.gov/electricity/monthlyInstalled solar cost per watt by system size and market segment. Updated quarterly.
www.nrel.gov/solar/market-research-analysis/solar-installed-system-cost.htmlState and federal solar incentives, net metering policies, tax credit details for all 50 states.
www.dsireusa.orgSolar testing standards (IEC 61215, IEC 61730, IEC 61853) that define how panels are rated and measured.
webstore.iec.chAppliance wattage ratings, energy consumption data, and efficiency standards used in our appliance sizing guides.
www.energystar.govBattery technology references, system loss factors, solar technology overviews.
www.energy.gov/eere/solar/solar-energy-technologies-officeElectrical safety requirements for solar installations: wire sizing (Article 310), overcurrent protection, and solar-specific rules (Article 690).
www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70Specific panel and battery specs from published product documentation — we only cite specs from manufacturer-published PDFs, never third-party listings.
What This Site Doesn't Do
Trust is easier to build by being explicit about what we won't do.
No installer leads. We don't connect you to local installers, don't sell your contact info, don't earn referral fees from solar companies.
No AI-generated content published without human review. Claude Code helps with the build process, but every formula, data point, and editorial decision is verified by a human.
No tracking beyond basic analytics for aggregate traffic patterns. No user profiling, no ad retargeting, no data broker partnerships.
No affiliate manipulation. When we recommend products, the recommendation comes from the data, not from who pays the highest commission.
No paywalls or signups. Every calculator, every data table, every state page is free and accessible without creating an account.
What We Do Instead
Cite every data source. Every number links back to NREL, EIA, EPA, or a manufacturer datasheet.
Show the formula. If a calculator gives you a number, you can see exactly how it was computed.
Publish the date. Every page shows when it was last updated and when the underlying data was fetched.
Correct errors publicly. If we get something wrong, we fix it and note the correction.
Keep everything free. No paywalls, no signups, no gated content.
Contact
For data corrections, methodology questions, or just to say hi:
info@thegreenwatt.comSee also: Contact page
Data Freshness
The peak sun hours dataset was last refreshed in April 2026 from NREL PVWatts v8 (NSRDB).
Each article shows its own "last updated" date. Cost data is benchmarked against NREL quarterly reports. Calculator formulas are versioned and tested against known reference outputs.