6 Best Free Solar Calculators for Installers, by Task

Free solar calculators for installers are web tools that help with jobs on site. They size strings (how many panels wire in a row). They match MPPT (maximum power point tracking) charge controllers, the units that charge a battery from panels. They also estimate yield (how much power the panels will make), check loads, plan racking and size batteries.

Victron MPPT Sizing Calculator ranks first at 76.5/100. It is a thin category. Six calculators rank, and five are tied to one hardware brand. Each has a free tool. Sunny Design has a paid PRO tier, and Sol-Ark does not label its tool free. We left out homeowner savings calculators.

For full design platforms with a free tier, see our free solar design software guide. For free tools across every installer job, see best free solar software. More design tools sit in our design and simulation hub.

Quick picks

✓ Verified on the company’s own pricing page. ◐ Published by the company outside its pricing page (help centre, store or press release). ? We could not read the company’s pricing page, so the figure is not confirmed. Prices checked October 2026.

How did the 6 tools score?

• Use-case fit 35%
• User reviews 20%
• Pricing 15%
• Ease of use 10%
• Support 10%
• Momentum 10%

Each bar shows the weighted score out of 100.

Sub-scores at a glance (0-10)

A 5.0 on reviews, ease, support or momentum is our neutral score. We use it when public evidence is thin. It is not a judgement.

How did we rank these free solar calculators?

How the score is weighted

• Use-case fit 35%
• User reviews 20%
• Pricing 15%
• Ease of use 10%
• Support 10%
• Momentum 10%

We looked at 14 calculators from 22 to 25 October 2026, using company websites, release notes, forums and public reviews. We did not run projects through the tools.

We dropped three because we could not confirm they still exist or are on offer. That left 11 scored and 6 ranked. Five scored below our minimum match score (4 out of 10). They sit under Also considered with the three we dropped. We score each calculator on six criteria:

• Use-case fit (35%): How well it does this page’s job. We score its best single task, such as string sizing, MPPT matching, yield, structural load, racking or battery sizing. A calculator that does one job well can score 10. We did not reward breadth across tasks.
• User reviews (20%): G2, Capterra and app-store ratings. A few reviews count for less. Most free brand calculators have no review listings. They get a neutral 5.0 instead of losing points.
• Pricing (15%): Free and brand-neutral tools score highest. Free but brand-tied tools score lower. Paid solar features score lowest.
• Ease of use, support and momentum (10% each): Review sub-scores, forum posts and dated updates. Each is neutral without evidence.

Scores are specific to each page. We never use third-party ratings in rating markup. Full method: how we score software.

1. Victron MPPT Sizing Calculator: best for matching PV arrays to off-grid and hybrid charge controllers

Best for: Installers who specify Victron charge controllers. Score: 76.5/100.

What it is: A free web calculator from Victron Energy, based in the Netherlands. It matches a PV (solar panel) array to a SmartSolar or BlueSolar MPPT charge controller. It also matches arrays to EasySolar or RS units. These combine an MPPT and an inverter (the unit that turns solar power into mains power). Off-grid, marine, RV and hybrid installers use it.

Why we picked it: It does one job very well. It costs nothing, and real installers use it every day. That earned the top score, even though it only covers Victron kit.

Key features:

• A wizard that matches modules to Victron MPPT controllers.
• A database of more than 164,000 PV modules, plus custom panels behind an "advanced" switch (restored September 2024).
• Voltage checks based on local temperatures. The cheapest suitable controller shows first, with others listed.
• Session memory, setup export and 13 languages.
• A new web version since 25 August 2024. An Excel sheet is still on offer for offline or custom use.

Pricing: Free. Output is limited to Victron controllers. Checked October 2026 on Victron Professional’s release note.

Pros (from user posts on the Victron Community and our research):

• You can specify the right Victron controller in minutes.
• It fits a daily routine. One installer and dealer uses it on most jobs and files the printout with the customer record (August 2024).
• When the 2024 redesign dropped custom panels, Victron staff answered users and brought them back within a month.

Cons (from user posts and our research):

• It only recommends Victron controllers.
• The database misses small flexible marine and RV panels and some older modules. You may have to enter specs by hand.
• Users asked for stricter oversizing limits (100, 115 or 130%) after some controllers failed at 125%.

Reviews: No user reviews found as of October 2026. Forum users like it for daily use. They note the web and Excel versions can disagree.

What we could not confirm: whether Victron has added the stricter oversizing limits.

mppt.victronenergy.com

2. Sunny Design: best for string sizing and PV-storage design on SMA inverters

Best for: Residential and C&I (commercial and industrial) installers who fit SMA inverters. Score: 69.0/100.

What it is: Inverter maker SMA gives away Sunny Design as its planning tool. It runs on the web, iPad and Android tablets. Installers and planners use it to size strings. They also design yield, shading, storage and hybrid systems around SMA inverters.

Why we picked it: It is the broadest free tool here. It does string sizing, yield and storage design in one place. For SMA installers, that means one tool instead of three.

Key features:

• PV string sizing with automatic string alignment against SMA inverters.
• Yield and self-consumption forecasts with shading losses.
• Visual roof planning with shading analysis.
• Battery storage, off-grid and PV-hybrid system design.
• A schematic circuit diagram and SMA device list (CSV) to download.

Pricing: Sunny Design is free, but some features need an account. Sunny Design PRO costs EUR 600 per year net (excluding VAT), with a one-month free trial. PRO adds sector coupling with heat pumps, CHP (combined heat and power) and EV charging. It also adds battery peak-shaving, tariff management and detailed costing. Checked October 2026 on the SMA Sunny Design page.

Pros (from our research):

• String sizing, yield and storage design sit in one free tool, so you do not re-enter the array.
• The CSV device list drops straight into your ordering, and the schematic gives you a start on paperwork.
• You can size a job on a tablet at the customer’s home, not just at a desk.

Cons (from our research):

• It is built around SMA kit only.
• Sector coupling and detailed costing need PRO at EUR 600 per year.
• You get a schematic, not a full single-line diagram, and some features need an account.

Reviews: No user reviews found as of October 2026, and no installer forum threads.

What we could not confirm: whether it models non-SMA inverters, and how often it is updated. Release notes up to version 6.10 carry no dates.

Read our full Sunny Design review or our Sunny Design alternatives.

sunnydesignweb.com/sdweb

3. PVGIS: best for a free yield estimate anywhere, with no login

Best for: Quick yield checks and radiation data for any brand, anywhere except the poles. Score: 65.8/100.

What it is: The European Commission Joint Research Centre (JRC) runs PVGIS. It is a free web tool with an API (a way for other software to pull its data). It gives radiation data and yield estimates for grid-tied, tracking and off-grid battery systems. Installers and consultants use it for quick first checks.

Why we picked it: It is the only ranked calculator with no brand lock-in and no login. The European Commission runs it, so the data source is public and stable.

Key features:

• Grid-connected yield estimates for fixed and tracking systems.
• Off-grid PV with battery estimates.
• Hourly, daily and monthly radiation time series.
• TMY (typical meteorological year) files in CSV, JSON or EPW (a weather file for simulation tools).
• Horizon profile from terrain data, plus a free API used by pvlib-python.
• PVGIS 5.3 has run since September 2024. PVGIS 6, with a new backend and 2014-2024 solar data, is in public beta.

Pricing: Free, with no sign-up. Data is free for public use if credited "PVGIS © European Communities, 2001-2026". Some commercial sites use "PVGIS" in their names and sell premium reports. The official tool is on the JRC site. Checked October 2026 on the JRC PVGIS page.

Pros (from a Buildhub user and our research):

• Estimates can track real output closely. One self-builder saw 4.5 MWh predicted against 4.26 MWh actual in year one (April 2023).
• You can export hourly output to a spreadsheet to size inverters across several arrays.
• No login and a single-page map, so new staff can run it without training.

Cons (from a Buildhub user and our research):

• Hourly data is in UTC, so you must shift it for local and summer time.
• No layout, 3D near-shading or detailed electrical modelling.
• No commercial support or SLA (service-level agreement). It is a public research service with a manual.

Reviews: No user reviews found as of October 2026. Forum users rely on it for a first estimate before detailed design.

Read our full PVGIS review.

re.jrc.ec.europa.eu/pvg_tools

4. Unirac U-Builder: best for racking layout, BOM and engineering report

Best for: US installers of any size who use Unirac racking. Score: 64.0/100.

What it is: US racking maker Unirac offers U-Builder as a free online racking tool. It comes with U-Builder Go for fast budget BOMs (bills of materials) and U-Builder PRO. US installers who mount on Unirac roof and ground racking use it.

Why we picked it: It is the only ranked tool for racking and code loads. It turns a free layout into a BOM, an engineering report and state letters for AHJs (your local permit office).

Key features:

• Layout on Google Maps imagery with several subarrays and roof zones.
• Wind, snow and seismic criteria by ZIP code, with uplift, point loads and maximum rail span.
• A live BOM that shows the lowest-cost rail option.
• An engineering report with ballast map and design loads, plus state certification letters for AHJs.
• CAD export, an install plan, and links with HelioScope, Aurora and OpenSolar.

Pricing: Free. Unirac says it is "Free for everyone, no startup costs or hidden fees". Its separate Plan Set Services are paid and quoted per project. They cover AHJ and utility plan sets, electrical and battery design, and PE (professional engineer) stamps in 50 states. Checked October 2026 on the Unirac U-Builder page.

Pros (from user and trade reports and our research):

• You get a live BOM and code loads filled in by ZIP. You skip manual wind, snow and seismic lookups (Greentech Renewables, 2016).
• Once you learn the workflow, it can carry the design through to the end (Mike Holt forum, December 2025).
• Aurora, HelioScope and OpenSolar links mean you do not redraw the array.

Cons (from one installer on the Mike Holt forum):

• It feels "really clunky, and unprecise", and adding a second roof area is hard.
• There is no user manual, and the tutorials did not help.
• No email or phone support contact is visible.

Reviews: No user reviews found as of October 2026. Older trade coverage praised it, while a December 2025 installer thread criticises its usability and support.

unirac.com/u-builder

5. KACO String Sizing Tool: best for quick string checks against KACO inverters

Best for: Installers of any size who fit KACO inverters. Score: 57.3/100.

What it is: This free web tool comes from German inverter maker KACO new energy. It checks your string layout for a chosen module against a KACO inverter’s limits. KACO installers use it next to KACO’s free blueplanet PV-designer for fuller planning.

Why we picked it: It is the most direct string check on this page. You enter a layout and get a pass or a warning against KACO’s limits.

Key features:

• Pick a KACO inverter and cos-phi (power factor).
• Module database by maker and type.
• Set module temperature, number of strings and modules per string.
• Checks DC-to-AC (Pdc/Pac) ratio, MPP (maximum power point) voltage, open-circuit voltage, array current and system voltage against limits. It shows warnings.
• Print or save results locally.

Pricing: Free. The tool page says it is "offered to the user free of charge". The blueplanet PV-designer, built on the PV*SOL engine, is "always free of charge". Checked October 2026 on the KACO String Sizing Tool page.

Pros (from our research):

• One pass checks five electrical limits and warns you before a string goes wrong.
• You can open it in a browser and run a check in the field, with no login mentioned.
• When a job needs more, the free PV-designer takes over and exports PDF or PV*SOL files.

Cons (from our research):

• It works with KACO inverters only.
• It checks a layout you enter. It does not suggest every valid layout.
• No update date is shown, so you cannot tell how current the module database is.

Reviews: No user reviews or forum posts found as of October 2026.

What we could not confirm: which KACO inverter families are in the dropdown.

6. Sol-Ark PV Sizing Tool: best for string checks and battery sizing around Sol-Ark hybrid inverters

Best for: Small US installers who work with Sol-Ark hybrid inverters. Score: 54.8/100.

What it is: Sol-Ark, a US hybrid-inverter maker, publishes web calculators for its own inverters. They check PV strings. They also estimate battery bank and hybrid inverter size from daily energy use and US peak sun hours. Installers use them to scope backup and off-grid systems. Note: we could not run a calculation in the tool. The features below come from its page and Sol-Ark’s resources pages.

Why we picked it: It is the only ranked tool that sizes a battery bank from backed-up loads. It also uses local US sun hours. That suits US backup jobs on Sol-Ark kit.

Key features:

• PV sizing: pick a Sol-Ark inverter (USA/Canada 5K-1P-N to 60K-3P-480V-N, Latin America 5K-1P-L, 8K-1P-L, 8K-2P-L) and a module from the library or your own specs.
• Set modules per string, strings per MPPT, wire run and temperatures. It checks Voc (open-circuit voltage) and Isc (short-circuit current) per MPPT, voltage drop and DC- or AC-coupled limits.
• Battery storage and hybrid inverter sizing from backed-up loads.
• ZIP-code lookup of worst-case monthly peak sun hours (US only).
• An estimate of backup and off-grid energy needs.

Pricing: No charge shown. The tool page has no login, sign-up, email gate or payment step. But Sol-Ark does not label it free. Its Terms of Use say using it lets Sol-Ark contact you. Checked October 2026 on Sol-Ark’s PV sizing tool page.

Pros (from our research):

• You get a quick "does this string work?" answer for each Sol-Ark MPPT, including voltage drop.
• It sizes on worst-case monthly sun hours, so off-grid customers are less likely to run short in winter.
• It runs in a browser, with no login or app to install.

Cons (from our research):

• It covers Sol-Ark inverters only, and the sun-hours ZIP lookup is US-only.
• Sol-Ark does not label it free, and using it lets Sol-Ark contact you.
• There are no dated releases or changelog, so you cannot see when it last changed.

Reviews: No user reviews or forum posts found as of October 2026.

sol-ark.com/solar-panel-sizing/SolarPanelSizing.html

How do the 6 free solar calculators compare?

Also considered

The first five scored below the minimum. Most are too narrow or not free for this job. We dropped the last three before scoring.

• SkyCiv Structural 3D + Load Generator (general structural software, scored 53.3/100): its Load Generator covers solar panel wind and snow loads. It uses ASCE 7-16 (the US structural loads standard) and AS/NZS 1170.2 (the Australian and New Zealand wind code). But the free Basic account cannot generate solar panel loads. You need a paid Professional plan or the standalone Load Generator module. We could not check their prices.
• Victron Toolkit app (scored 48.8/100): a free, open-source Victron app with a generic DC and AC cable voltage-drop calculator. It is useful but too narrow to rank. App Store 3.2/5 (11 ratings, checked October 2026).
• Enphase IQ Microinverter Compatibility Calculator (scored 45.7/100): checks whether a module is electrically compatible with Enphase IQ microinverters for warranty purposes. It is a pass/fail check, not sizing.
• FIMER String Sizer (scored 45.7/100): FIMER describes a free tool that lists every allowed string layout for a FIMER inverter. But the tool URL redirects from https to http. It would not load for us in October 2026, so we could not confirm it works.
• Tigo Solar Optimizer Benefits Calculator (scored 34.0/100): a sales aid that estimates the extra yield and savings from adding Tigo optimisers. It is not a string-sizing or layout tool.
• Fortress PV String Size Calculator (excluded): we could not confirm the calculator still exists (no company page found in October 2026).
• Luxpower Load Calculator and AI Panel Calculator (excluded): we could not confirm the calculator exists (no company page found in October 2026).
• HeatSpring free solar design tool (Megawatt String Sizing Tool) (excluded): the free download now leads to a paid course page. The tool is no longer available.

How do you choose the best free solar calculators for installers?

Start from the task, then check the brand. The right free tool usually comes from the maker you already stock.

Which free tool sizes strings?

On SMA inverters, use Sunny Design (SMA lock-in). On KACO, use the KACO String Sizing Tool (KACO lock-in).

On Sol-Ark hybrids, use Sol-Ark’s tool. For Enphase microinverters, use the compatibility calculator in Also considered.

We found no free, brand-neutral string sizing calculator we could check. Mixed fleets still need a hand calculation or a design platform. For cable voltage drop, the Victron Toolkit app’s calculator is generic despite the Victron name.

Which free tool sizes an MPPT charge controller?

For off-grid, marine, RV and hybrid systems on Victron, use the Victron MPPT Sizing Calculator (Victron lock-in). Check the oversizing ratio yourself, as stricter limits are not confirmed.

Which free tool estimates yield?

Use PVGIS (no lock-in) for a brand-neutral estimate or TMY file. Inside an SMA design, Sunny Design (SMA lock-in) does the job. Neither makes a report that banks accept for loans.

Which free tool checks wind and structural load?

On Unirac racking, U-Builder fills in wind, snow and seismic loads by ZIP for free (Unirac lock-in). SkyCiv makes solar panel loads only on a paid plan.

Which free tool lays out racking?

Unirac U-Builder (US only) gives you layout, BOM, engineering report and AHJ letters at no cost. For racking-neutral work, see our mounting structure design software guide.

Which free tool sizes a battery?

For a first-pass US backup or off-grid size, use the Sol-Ark tools (Sol-Ark lock-in). Sunny Design (SMA lock-in) designs storage and hybrid systems. PVGIS (no lock-in) models off-grid PV with a battery.

See also our battery storage design software guide, battery design features and off-grid solar design software. The full list of free tools is on our free solar software page.

Want the wider field? Browse every tool we have checked in our design and simulation category, or see our list of free solar software.

Free solar calculator FAQs

Sources

All checked 22 to 25 October 2026.

• Victron MPPT Sizing Calculator: 2024 release note, earlier release note, community thread, calculator vs spreadsheet thread
• SkyCiv: free account, ASCE 7 solar panel wind loads, solar industry page, solar design API, G2, Capterra
• Sunny Design: SMA product page
• PVGIS: JRC PVGIS page, PVGIS 5.3 release, Buildhub thread
• Unirac U-Builder: product page, Aurora help article, Mike Holt forum, Greentech Renewables
• KACO: String Sizing Tool, PV system design page
• Sol-Ark: PV sizing tool, sizing tools, battery and hybrid inverter tool
• Also considered: Victron Toolkit, Victron Toolkit App Store, Enphase calculator, FIMER String Sizer, Tigo calculator, HeatSpring post

Last updated 28 October 2026. Next scheduled review: 23 January 2027.