Electrical Calculators
Free Electrical Calculators — Generators, Solar, UPS & Power Banks
Sizing electrical equipment is not guesswork — every wrong watt either costs you money or leaves you in the dark. We built four free calculators that use industry-standard formulas (NREL, Honda, APC, Anker) to give you the exact answer in 30 seconds. No signup, no email, no upsells. Pick the one you need below.
⚡ Generator Wattage Calculator
Find out what size portable inverter generator you need based on the appliances you want to run at the same time. The calculator distinguishes between running watts (steady draw) and starting watts (the brief surge when a motor kicks on) — get either wrong and your generator either trips or refuses to start the load.
Pick your use case (home backup, RV, camping, jobsite), check the appliances you'll run simultaneously, and the calculator returns total running watts, peak starting watts, and a recommended generator size with a 25 percent safety margin. Includes altitude derating for high-elevation users where gasoline engines lose ~3 percent of power per 1,000 feet.
Best for: anyone choosing a 2,000–10,000 W portable generator for home backup, RV travel, jobsite work, or off-grid use.
☀️ Solar Panel Sizing Calculator
Calculates how many solar panels and what total array wattage you need based on your daily kWh usage, local peak sun hours, and real-world system losses. Uses the NREL/Unbound Solar formula: (daily kWh × safety factor) ÷ (peak sun hours × (1 − derate)).
Handles three system types: grid-tied with net metering (sized for annual average), off-grid (sized for the worst-sun month with 2–3 days of battery autonomy), and hybrid (grid + battery backup with 70–80 percent winter coverage). Outputs panel count, recommended inverter size, battery storage in kWh, and required roof space in square feet.
Best for: homeowners pricing out residential solar, off-grid cabin or RV owners, and anyone trying to verify a contractor's quote.
🔋 UPS Runtime Calculator
Tells you exactly how many minutes your uninterruptible power supply will run your servers, PC, or network gear during an outage. Uses the standard formula: (Battery V × Ah × efficiency × age factor) ÷ load in watts, with adjustments for the Peukert effect on lead-acid batteries under heavy load.
Includes 9 preset load profiles (router, home PC, gaming PC, home server, NAS, light rack, full rack) and accounts for battery age — most UPS batteries lose 20 percent capacity after 2–3 years and 50 percent after 4. Also clarifies the VA-vs-Watts trap: a 1,500 VA UPS at power factor 0.8 delivers only 1,200 W of real power.
Best for: anyone planning home office backup, server room protection, or network gear ride-through during brief outages.
🔌 Power Bank Capacity Calculator
Find real charges per power bank →
Reveals how many full phone, tablet, or laptop charges your power bank actually delivers — not the marketing number on the box. The "10,000 mAh" you bought delivers about 6,600–7,400 mAh of usable charge after voltage conversion (3.7V battery → 5V USB output) and 15 percent efficiency loss.
Uses the Xtorm/Anker formula: Rated mAh × (3.7 ÷ 5) × charging efficiency, where efficiency is 0.85 for wired USB-C, 0.70 for wireless/MagSafe, or 0.65 for cheap cables. Includes 15 popular device presets (iPhone 16, Galaxy S25, Pixel 9, iPad, MacBook Air) and checks the FAA 100 Wh airline carry-on limit automatically.
Best for: anyone shopping for a travel power bank, sizing emergency backup for off-grid trips, or wondering why their 20,000 mAh bank doesn't last 6 charges.
Why These Calculators Use Real Formulas
Most online "calculators" are dressed-up multiplication that ignores voltage conversion, the Peukert effect, panel temperature derating, or starting-watt surge math. Ours don't. Each calculator was built from formulas published by the actual manufacturers — Honda, Generac, NREL, APC, Anker — and verified against their own sizing tools.
The result is that you get the same number a paid installer would quote you, without the upsell. Use them to verify a quote, plan a DIY project, or just understand what you're buying before you click "purchase."
Frequently Asked Questions
Are these calculators free? Yes. No signup, no email gate, no premium tier. Each calculator runs entirely in your browser — your inputs never leave your device.
How accurate are the results? Within 5–10 percent of professional sizing tools, which is closer than most consumer hardware spec sheets. We use the same industry formulas (NREL PVWatts, IEEE UPS guidelines, FAA aviation rules) that licensed installers and engineers rely on. For critical installs (grid-tied solar, medical UPS, data center), always verify with a licensed professional.
Do you sell equipment? The calculators are independent of any single brand. We don't push specific products — the recommendation text tells you what class of generator, panel, UPS, or power bank fits your need, and you choose where to buy.
Can I share these tools? Yes. All four calculators are free to link to from anywhere — blogs, forums, YouTube descriptions, social media. We appreciate the link back.
Free electrical calculators · No signup required · Estimates only, verify against equipment labels and local code before purchase.