Watts Math
Free home electrical and HVAC calculators with the code references shown: voltage drop and wire size by NEC tables, breaker sizing, Ohm's law, air-conditioner and heater BTUs, mini split sizing, generator wattage, electricity cost, and solar panel sizing.
Most sizing calculators give you a number and no reason to believe it. Every tool here cites the code table or sizing chart it uses, shows the arithmetic, and says what the number does not cover -- the conduit-fill derating, the Manual J that a rule of thumb stands in for, the local amendment.
Wiring and circuits covers wire gauge, breakers, voltage drop and Ohm's law. Heating and cooling sizes air conditioners, heaters and mini splits. Power and energy covers generators, what things cost to run, and solar.
Wiring & circuits
Voltage drop in volts and percent for a copper or aluminum run at any length, current and wire gauge, single-phase, three-phase or DC, using NEC Chapter 9 resistance values and the 3% and 5% recommendations.
Wire gauge & breakerThe wire gauge and breaker size for a load, from NEC ampacity tables at 75°C, the 125% continuous-load rule, the small-conductor breaker limits, and the voltage drop over your run length.
Ohm's law: V, I, R, PEnter any two of volts, amps, ohms and watts and get the other two, plus AC conversions between watts and amps with power factor for single-phase and three-phase.
Heating & cooling
How many BTUs of cooling or heating a room or house needs, from the ENERGY STAR sizing chart with adjustments for ceiling height, sun, occupants, kitchen, insulation and climate, in BTU/h and tons.
Mini split sizingThe mini split size (9K, 12K, 18K, 24K BTU and up) for each zone, with a multi-zone total, from room area, sun exposure, ceiling height and climate, and the power it will draw at your SEER2.
Power & energy
What size generator you need for the appliances you want to run: running watts summed, the largest motor's starting surge added, and the standard generator size that covers it with headroom.
Cost to run an applianceWhat any appliance costs to run per day, month and year from its wattage, hours of use and your electricity rate, with kWh shown and a table of common household loads.
Solar panels neededHow many solar panels you need to offset your annual usage, from peak sun hours for your region and the PVWatts derate, with system size, roof area, installed cost and payback at your electricity rate.
Frequently Asked Questions
Where do the numbers come from?
Each calculator names its source on the page: NEC Chapter 9 Table 8 for conductor resistance, NEC 310.16 for ampacity, 240.4(D) for small-conductor breaker limits, the ENERGY STAR sizing chart for cooling BTUs, manufacturer tables for generator watts, EIA for electricity rates, and NREL PVWatts for solar losses. Nothing here is invented; the arithmetic is shown so you can check it.
Can I use these instead of an electrician?
No. They tell you what the general case in the national code says so you can plan a job and understand a quote. Local amendments, conduit fill, temperature, terminal ratings and the inspector's judgement all apply, and mistakes in electrical work are dangerous. Use the numbers to have a better conversation with the person doing the work.
Do I need an account, and is anything saved?
No account, nothing saved. Every calculation runs in your browser and nothing you type is sent anywhere.