About Easy Solar Calc

Easy Solar Calc is a set of free calculators for DIY solar, off-grid and backup power: appliance loads and electricity cost, battery banks and runtime, power stations, solar panels and their output, charge controllers and panel wiring, inverters, generators, generator fuel and propane. Each one shows the formula it uses and works the numbers through with your own inputs, so you can check them by hand.

The numbers are planning estimates. Appliances draw what their nameplates and meters say, not what a chart says; sun varies year to year; batteries lose capacity with cold, high current and age. Where a figure is typical rather than fixed — appliance wattages, peak sun hours, battery depth of discharge, fuel heat content — the page names its source, listed under “Sources” on each calculator. Wiring, fusing and connections to house wiring are governed by electrical codes and your equipment manuals; we leave those to qualified installers.

It is part of a small family of free project calculators that began with Easy Scale Calc (opens in a new tab). The site is supported by advertising and by Amazon affiliate links on product recommendations; neither changes what the calculators say. If you spot an error or want a calculator added, the contact page is the place to say so.

How we build and check the calculators

• Every calculator runs entirely in your browser — nothing you type is sent anywhere.
• Each page shows its formula, and every result shows the working with your own numbers, so you can check it by hand.
• Battery sizes use the usable share of rated capacity (80% for LiFePO4, 50% for lead-acid by default, editable), panel output includes the PVWatts default 14% system losses, and counts of panels and batteries are rounded up.
• Typical wattages, fuel heat content, sun hours and battery figures come from the government agencies, extension services and manufacturer data sheets linked on each page. Your appliance nameplate and your equipment manuals always take priority.
• We deliberately do not size wire, fuses or breakers: those come from NEC/ABYC ampacity tables and the equipment manual, and should be checked by a qualified installer.
• Every calculator is tested in both US and metric units, and every worked example is recomputed at build time so the numbers on the page always match the calculator.

How this content is made

The calculators, worked examples and guides on this site were drafted with the help of AI tools and then checked in several passes. Every number on a worked-example page is recomputed from the calculator at build time, so the text and the calculator can’t disagree. Formulas and published figures (manufacturer data sheets, NREL/NLR solar data, EIA fuel heat contents, extension-service wattages) are checked against the linked sources.

Automated checks run on every build: each calculator is tested in US and metric units and unit switching must round-trip exactly. If you find an error, tell us — corrections are fixed and logged below.

Sources we use

• National Laboratory of the Rockies (formerly NREL) — PVWatts® Calculator (opens in a new tab) — Look up the solar resource (peak sun hours) and yearly output for your own address; default system losses 14%.
• NLR (formerly NREL) — PVWatts® v8 API, NSRDB PSM V3 TMY data (opens in a new tab) — City figures on this site: fixed open-rack array facing due south, tilted at the site’s latitude, 14% system losses, queried October 2026.
• Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.
• Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
• U.S. Energy Information Administration — British thermal units (Btu) (opens in a new tab) — Heat content: gasoline 120,214 Btu/gal, diesel 137,381 Btu/gal, propane 91,452 Btu/gal; 1 kWh = 3,412 Btu.
• U.S. Energy Information Administration — Electric Sales, Revenue, and Average Price, Table 5A (2024 data, released October 2025) (opens in a new tab) — 2024 average residential customer: 863 kWh a month (about 10,359 kWh a year, 28.4 kWh a day); Louisiana 1,202 kWh a month, Hawaii 495.
• NIST Chemistry WebBook — Thermophysical properties of propane (saturated liquid) (opens in a new tab) — Liquid propane is about 31.6 lb/ft³ at 60 °F, which is 4.23 lb per US gallon (4.09 at 80 °F).
• Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
• Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
• Victron Energy — Which solar charge controller: PWM or MPPT? (2020) (opens in a new tab) — MPPT output current = panel watts ÷ battery voltage (100 W ÷ 13 V = 7.7 A); a PWM controller pulls the panel down to battery voltage (81 W from a 100 W panel at 25 °C, about the same as MPPT at 75 °C); temperature-coefficient method; example panel Voc 21.6 V, −0.35 %/°C.
• Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
• Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
• Crown Battery — Deep Cycle Battery Product Specifications (effective Oct 1, 2024) (opens in a new tab) — 20-hour capacities by BCI group for flooded, AGM and lithium 12 V batteries.
• U.S. Consumer Product Safety Commission — Portable Generator Hazards (Safety Alert 5123, 2021) (opens in a new tab) — Never run a generator indoors or in a garage, even with doors open; place it outside far from doors, windows and vents; CO alarms; never “backfeed” by plugging into a wall outlet.
• OSHA Fact Sheet — Using Portable Generators Safely (opens in a new tab) — Never attach a generator to a building’s wiring unless a qualified electrician has installed a transfer switch; 3–4 ft of clear space around the unit.
• U.S. DOE / EPA — fueleconomy.gov vehicle data (opens in a new tab) — EPA combined electricity use in kWh per 100 miles, measured at the wall outlet (charging losses included).
• Starlink — Standard kit specification sheet (opens in a new tab) — Power consumption: average 75–100 W.
• Starlink — Mini specification sheet (opens in a new tab) — Power consumption: average 25–40 W; input rating 12–48 V, 60 W.
• IRS — Residential Clean Energy Credit (opens in a new tab) — 30% credit for property installed 2022 through December 31, 2025; not available for property placed in service after December 31, 2025.

How the site is funded

Easy Solar Calc is free and has no sign-up. It is supported by display advertising and by affiliate links — when you buy something through a product link we may earn a small commission at no extra cost to you. Recommendations are chosen for fit with the job, not commission.

Corrections

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