Electrical & Off-Grid Power Calculators
Free, no-signup calculators for wire sizing, voltage drop, ampacity, conduit fill, Ohm's law and breaker size — plus solar arrays, battery banks, inverters, generators, RV 12V wiring and energy cost. Each tool ships with the formula, a worked example, a reference table and a source. Results are estimates for planning and education.
Topics
Tables and guides
- Reference tables — AWG wire chart, NEC ampacity, conduit fill, voltage drop, resistivity, peak sun hours and appliance wattage.
- Guides — the why behind each calculation, step by step.
All calculators
Wire & Circuit Sizing
- Voltage Drop Calculator
- Wire Size Calculator
- Wire Ampacity Calculator (NEC 310.16)
- Ohm's Law Calculator
- Conduit Fill Calculator (EMT)
- Breaker / OCPD Size Calculator
Off-Grid & Solar Sizing
- Off-Grid Solar System Size Calculator
- Solar Panel Size Calculator: How Many Panels Do You Need?
- Battery Bank Size Calculator (Amp-Hours & kWh)
- Inverter Size Calculator (Continuous & Surge Watts)
- Solar Charge Controller Size Calculator (MPPT / PWM)
- Watt-Hours Calculator: Convert Ah and Volts to Wh
- Series and Parallel Battery Calculator
- Solar Panel Output Calculator (kWh per Day)
Generators & Backup Power
RV, Van & Marine Power
- RV & Van Solar Power Calculator
- 12V & 24V DC Wire Size Calculator
- Battery Runtime Calculator
- DC-DC & Alternator Charger Sizing Calculator
- Appliance Amp Draw Calculator (Watts to Amps)
Energy Use & Cost
Motors, Lighting & Conversions
Reference tables
- AWG Wire Size Chart: Circular Mils, Resistance & Ampacity
- NEC 310.16 Ampacity Chart (60 / 75 / 90°C, Cu & Al)
- EMT Conduit Fill Chart: Areas & 40% Fill
- Voltage Drop Chart per AWG (20 A, 100 ft, 120 V)
- Wire Resistance Table: Ohms per 1,000 ft by AWG
- Peak Sun Hours by US State (NREL Average)
- Appliance Wattage Chart: Running vs Starting Watts
Guides
- How to Size Electrical Wire for Amps and Distance
- Voltage Drop Explained: the 3% Rule and Formula
- NEC Conduit Fill Explained: How Many Wires Fit
- AWG Wire Gauge Chart: Ampacity and Resistance
- Ohm's Law Made Simple: V, I, R and P
- How to Size an Off-Grid Solar System
- Battery Bank Sizing: Amp-Hours, Autonomy and DoD
- MPPT vs PWM Charge Controllers and How to Size Them
- What Size Generator Do I Need? Running vs Starting Watts
- 12V Wiring for Vans and RVs: DC Wire Size
- How Much Does It Cost to Charge an EV at Home?
- Breaker Sizing and the Continuous-Load 125% Rule
What you can work out here
WattCalcs is a free, no-signup hub of calculators for electrical and off-grid power work, built for DIYers, electricians, and solar, RV and van builders. Every tool is deterministic: it runs an explicit formula on the values you enter and shows that formula, a worked example, a reference table and a cited source right on the page, so you can check the reasoning rather than trust a black box. Nothing here depends on a price feed or a login — the math is timeless physics and the constants are stable engineering reference data.
The site is organised into six areas. Wire and circuit sizing — voltage drop, wire size, ampacity, conduit fill, Ohm's law and breaker size — is the electrical core, where the answer is whichever of the ampacity and voltage-drop checks demands the larger conductor. Off-grid and solar sizing works from your daily watt-hours out to panels, batteries, inverters and charge controllers. Generators and backup power add up running and starting watts to a generator rating. RV, van and marine power handles the heavy low-voltage DC wiring those builds need. Energy use and cost turns wattage and your own electricity rate into money. And motors, lighting and conversions covers the everyday W/A/V/kVA and three-phase conversions that sit under everything else.
Not sure where to start? Browse the guides for the why behind each calculation, or the reference tables for the AWG chart, NEC ampacity, conduit fill and peak sun hours. Because electrical work is safety-critical, every result is an estimate for planning and education: verify all sizing with a licensed electrician and your authority having jurisdiction against the NEC edition adopted where you live.