Answers to common electrical and solar calculation questions

Volts describe electrical potential, amps describe current, and watts describe instantaneous power. For DC and simple resistive loads, watts = volts × amps. Watt-hours measure energy over time: a steady 1,000-watt load running for one hour uses 1,000 Wh, or 1 kWh. Use the Ohm's Law Calculator or kWh Calculator for the corresponding calculation.

A kilowatt (kW) is a rate of power; a kilowatt-hour (kWh) is an amount of energy. A 2 kW heater running for three hours uses 6 kWh. Utility energy charges are normally based on kWh, not kW alone. The U.S. EIA explains these units with similar examples.

For DC or single-phase AC at power factor 1, use amps = watts ÷ volts. A 1,500 W load at 120 V is 12.5 A. For AC loads with a power factor below 1, divide by volts multiplied by power factor. Balanced three-phase calculations also include √3. Use the Watts to Amps Calculator and select the actual system type.

Multiply watts by operating hours, divide by 1,000 to get kWh, then multiply by the energy rate on the bill. Use measured average power for equipment that cycles. Add fixed fees, taxes, time-of-use rates, and demand charges separately because a simple energy calculation does not include them. Model the energy portion with the Electricity Cost Calculator.

Both checks matter, and they solve different problems. Ampacity and overcurrent protection limit conductor heating under the applicable rules. A voltage-drop calculation checks how much voltage is lost along the run. Calculate each requirement separately and use a conductor that satisfies all applicable constraints. PowerTotals' wire-size tool sizes only for resistive voltage drop; it does not certify ampacity or code compliance.

Follow the calculator label. PowerTotals Voltage Drop Calculator asks for one-way run length and accounts for the return path in single-phase and DC resistance calculations. Do not double the distance before entering it unless a tool explicitly requests total conductor length.

Do not treat 3% as a universal pass/fail rule. Project criteria depend on the circuit, equipment, jurisdiction, and governing standard. Use the target specified by the designer or authority having jurisdiction, and consider the combined feeder and branch-circuit path. PowerTotals keeps the target editable and tracks publication metadata on Standards Status.

Voltage drop occurs under load, so a no-load meter reading will not show the modeled drop. Real results can also differ because source voltage, actual current, conductor temperature, connections, route length, power factor, and reactance differ from the inputs or from a simplified resistance model. Compare the tool's assumptions with the measurement conditions on the methodology page.

The same number of volts lost is a larger percentage of a lower supply voltage. For equal power, a lower-voltage circuit also draws more current, which increases voltage drop and I²R loss. That is why long 12 V or 120 V runs can require more conductor area than a higher-voltage run delivering comparable power.

Start with nominal energy in watt-hours: battery volts × amp-hours. Multiply by the usable depth of discharge and system efficiency, then divide by the average load in watts. Real runtime can be shorter because of temperature, battery age, high discharge rate, inverter standby draw, and cycling loads. Try these variables in the Battery Runtime Calculator.

No. Amp-hours alone do not state energy unless voltage is known. A 12 V, 100 Ah battery has 1,200 nominal Wh, while a 24 V, 100 Ah battery has 2,400 nominal Wh. Usable delivered energy will be lower after discharge limits and conversion losses. The EIA also distinguishes a storage system's power capacity from its energy capacity.

Size energy capacity from the critical loads and desired backup duration, not from total solar array wattage alone. Separately verify that the battery and inverter can supply the highest simultaneous running load and short startup surges. Then check how quickly the available solar or grid charger can refill the battery. Start with the Home Backup Power Calculator.

Start with annual electricity use in kWh and the share you want solar to offset. Estimate annual production per installed kW for the location, roof direction, tilt, shade, and system losses. Divide the target annual energy by that production factor, then divide the required DC system size by panel wattage and round up to a whole panel. Use the Solar Panel Calculator for a location-adjusted estimate.

Nameplate watts are measured under standardized test conditions, not every roof condition. Actual energy yield changes with sunlight, module temperature, shade, dirt, orientation, wiring, inverter conversion, clipping, and downtime. Compare systems using modeled annual AC energy, not panel watts alone. NREL PVWatts models location, tilt, azimuth, losses, and inverter behavior.

Usually size storage around the energy that must be shifted or backed up: evening loads, time-of-use periods, or critical loads during an outage. Then verify that the array can recharge it under the intended seasonal conditions. An array-to-battery ratio alone does not describe the household load profile or backup goal. Compare scenarios with the Solar Battery Calculator.

Series connections add voltage while current stays near the string current; parallel connections add current while voltage stays near one string voltage. The valid arrangement must stay within the charge controller or inverter operating-voltage, maximum-voltage, and current limits across expected temperatures. Shading and mismatched modules also affect the design. Check a proposed layout with the Solar String & MPPT Calculator.