Free Solar Calculators That Actually Work

Every tool on this site is derived from a primary source: a published physics formula, a national electrical code clause, or a verified dataset from a research institution. We do not scrape competitor calculators or accept vendor-supplied numbers.

Sizing and yield estimates start from the NREL PVWatts derate model. We take the product of nameplate DC capacity, plane-of-array irradiance from a TMY3 dataset for the user's location, system losses (soiling, wiring, inverter efficiency, mismatch, age), and shading. The result is a conservative estimate of annual AC output expressed in kilowatt-hours. Peak sun hours, used in shorter calculations, are derived from the same irradiance data and divided by the standard 1 kW/m2 reference.

Voltage drop and conductor sizing follow NEC Article 690 for US residential PV. The formula is Vdrop = 2 x I x R x L for a one-way run, with R taken from NEC Chapter 9 Table 8 ampacities and adjusted for temperature and conduit fill per Table 310.16. We surface the calculation steps so installers can audit the result against the same code reference their inspector uses.

Tilt-angle optimisation uses the latitude rule for fixed-tilt arrays, modified for seasonal bias when a user selects winter or summer production weighting. For tracking arrays, we model single-axis backtrack geometry. Roof azimuth is treated as a continuous input rather than a cardinal-direction picker.

Financial calculations use the federal residential clean energy credit (currently 30 percent through 2032 under the Inflation Reduction Act), MACRS five-year accelerated depreciation for commercial systems, and state-level incentive structures pulled from the DSIRE database. Payback period is computed against the user's actual utility tariff, not a national average.

Solar economics and code references differ substantially between countries. We cite primary authorities for each market we publish in, rather than relying on a single English-language source and converting currencies.

• United States (en-us): National Renewable Energy Laboratory (NREL) for irradiance and PVWatts, Solar Energy Industries Association (SEIA) for installation cost benchmarks, EnergySage for live installer quote data, the Energy Information Administration (EIA) for retail electricity rates, and IRS Form 5695 instructions for the federal Residential Clean Energy Credit.
• United Kingdom (en-gb): Microgeneration Certification Scheme (MCS) standards, Energy Saving Trust technical guidance, Ofgem for tariff caps and Smart Export Guarantee rates, the Department for Energy Security and Net Zero (DESNZ) for policy and feed-in legacy data.
• Australia (en-au): Clean Energy Council for installer accreditation and product lists, Clean Energy Regulator (CER) STC zone maps, the Australian Energy Regulator (AER) Default Market Offer for baseline tariffs, and SunWiz quarterly reports for installation volumes.
• Canada (en-ca): Natural Resources Canada (NRCan), CanmetENERGY laboratory publications for module performance under Canadian conditions, and the Canada Greener Homes Loan programme for financing parameters.
• Germany (de): Bundesnetzagentur for grid-connection rules and feed-in tariff (Einspeisevergutung) registers, Verbraucherzentrale for consumer cost guidance, BSW Solar (Bundesverband Solarwirtschaft) for industry data, and the EEG paragraph 48 schedule for monthly degression of tariffs.
• France (fr): Agence de la transition ecologique (ADEME) for technical and economic guidance, the non-profit Hespul for installer training references, EDF Obligation d'Achat (OA) tariff schedules, and the Commission de regulation de l'energie (CRE).
• Spain (es): Instituto para la Diversificacion y Ahorro de la Energia (IDAE), Union Espanola Fotovoltaica (UNEF), the Comision Nacional de los Mercados y la Competencia (CNMC), and Real Decreto 244/2019 for self-consumption rules.
• Brazil (pt-br): Agencia Nacional de Energia Eletrica (ANEEL), Associacao Brasileira de Energia Solar Fotovoltaica (ABSOLAR), and Lei 14.300/2022 for distributed-generation framework including Fio B compensation rules.
• Italy (it): Gestore dei Servizi Energetici (GSE) for incentive administration, Anie Rinnovabili for trade data, Autorita di Regolazione per Energia Reti e Ambiente (ARERA), and standard CEI 64-8 for installation rules.
• Netherlands (nl): Holland Solar trade association, TKI Urban Energy research programme, and the Autoriteit Consument en Markt (ACM) for tariff and net-metering policy.

Most solar calculators on the open web exist to capture leads for installer networks. They ask for a postcode and an email address before returning a number, and the number is typically a marketing estimate tuned to encourage a sales call rather than a derivation a buyer can defend. We disagree with that pattern and decided to publish a free alternative.

SolarCalculatorHQ is independent. We have no affiliation with installers, manufacturers, or financing providers. There are no referral fees, no lead resale, and no paid placements anywhere on the site. We do not require a signup, an email address, or any personal information to use any tool. Every result is computed in the browser from inputs the user controls, with the formulas and source references shown alongside.

The site is locale-aware because solar economics is fundamentally local. The Inflation Reduction Act tax credit applies only in the United States. EEG-paragraph-48 monthly degression governs feed-in rates in Germany. The Lei 14.300/2022 Fio B schedule changes Brazilian compensation each year. Spain operates self-consumption under Real Decreto 244/2019 with rules that differ from neighbouring France's EDF OA contracts. We localise currency, units, terminology, code references, and tariff structures rather than translating a US calculator into ten languages.

The site is funded by display advertising. We are transparent about that because it determines what we will and will not do. We will not sell user data. We will not insert affiliate links into recommendations. We will not write toward a particular installer or product line.

We re-derive every calculator when an authority changes a rate or code clause. Ofgem reviews the energy price cap in January, April, July, and October. EEG paragraph 48 degression occurs on the first of each month. The AER Default Market Offer takes effect on 1 July annually. ANEEL adjusts Fio B percentages annually under Lei 14.300. We update the relevant pages within seven days of each official change and log the revision on the page.