Energy calculators and engineering reference
The arithmetic behind a permit or interconnection application: power and current, AC and DC, energy, storage, array sizing and voltage drop, each showing its formula. Below the calculators: every formula an application asks for, every way to connect, how load differs from generation, and typical sizes and demand figures with their sources.
Calculators
Volts × amps → watts
Watts → amps (feeder and breaker sizing)
AC ⇄ DC conversion
Utility inverters convert DC to AC at about 98–99% efficiency at their rated point (see the inverter table below); use your datasheet's CEC or European weighted efficiency for a real design.
kW ⇄ kVA ⇄ kVAR
Annual energy and capacity factor
Battery sizing
Solar array: modules, inverters, land
Land default 8.3 acres per MWac is NREL's total-area figure for single-axis tracking plants over 20 MW (7.5 for fixed tilt); DC/AC 1.3 is Berkeley Lab's utility average. Module and inverter defaults are illustrative: use your equipment datasheets.
Voltage drop
Circular-mil areas from NEC Chapter 9, Table 8. The 3% figure is the NEC informational-note guideline for a branch circuit (5% feeder plus branch), not a code requirement.
Average demand and load factor
NEC dwelling load (standard method, general loads)
A planning estimate following the NEC Article 220 standard method (values per EC&M's code articles, linked in the Load reference below); the electrician's calculation, on the code edition your city adopted, governs the permit.
Peak demand of a home, item by item
Set how many of each item the home has. The total assumes everything runs together, which is the worst case; the NEC dwelling calculation above applies the code's demand factors instead.
Running and starting watts come from two utilities' published generator-sizing charts (DTE Energy and the Eugene Water & Electric Board) and DOE for EV chargers. Where a chart gives a range, the upper value is used. Starting watts are for motors; check the nameplate for your own equipment.
Formulas an application asks for
| Calculation | Formula | Where an application uses it |
|---|---|---|
| Real power | P (W) = V × I (DC) · V × I × PF (single-phase) · √3 × VLL × I × PF (three-phase) | Service and feeder sizing on utility and county electrical permits; inverter AC output. |
| Apparent and reactive power | kVA = kW ÷ PF · kVAR = √(kVA² − kW²) | Transformer and inverter ratings (kVA); reactive capability in ERCOT and utility interconnection studies. |
| Current | I = P ÷ (V × PF) single-phase · I = P ÷ (√3 × V × PF) three-phase | Breaker, conductor and service sizes on the electrical permit and the utility service request. |
| PV continuous current | Idesign = 1.25 × Isc (and conductors at 1.25 × that, NEC 690.8) | PV source and output circuit sizing on the electrical permit. |
| Maximum PV voltage | Vmax = modules per string × Voc × temperature correction (NEC 690.7) | String design: must stay below the inverter and module maximum system voltage. |
| DC/AC ratio (inverter loading ratio) | ILR = MWdc ÷ MWac | The MW on an interconnection request is the AC limit at the point of interconnection; the DC array is larger. |
| Annual energy | MWh/yr = MWac × 8,760 h × capacity factor | Offtake, revenue and emissions figures; some utility applications ask for expected annual kWh. |
| Capacity factor | CF = actual MWh ÷ (MW × hours in the period) | Comparing technologies and checking a production estimate. |
| Battery energy | MWh = MW × duration (h) · energy charged = MWh delivered ÷ round-trip efficiency | Storage MW and MWh on the ERCOT request and the PUCT registration; county fire-code reviews. |
| Load factor | LF = average kW ÷ peak kW · average kW = kWh ÷ hours | Large-load requests and utility service applications ask for peak demand and load factor. |
| Demand from a bill | peak kW ≈ kWh ÷ (hours × LF) | Estimating peak when only energy (kWh) is known. |
| Voltage drop | VD = 2 × K × I × L ÷ CM (single-phase / DC) · √3 × K × I × L ÷ CM (three-phase) | Conductor sizing on long runs; K = 12.9 copper, 21.2 aluminum; CM from NEC Chapter 9, Table 8. |
| Transformer current | Isecondary = kVA × 1,000 ÷ (√3 × VLL) | Padmount and substation transformer sizing in utility service and interconnection packages. |
| Modules and inverters | modules = MWdc × 10⁶ ÷ module W · inverters = MWac × 1,000 ÷ inverter kW (round up) | Equipment lists for the electrical permit, the PUCT registration and lender packages. |
| Land | acres ≈ MWac × acres per MWac | Site control, the lease exhibit, stormwater disturbed-acres on the TCEQ notice. |
| Disturbed area for stormwater | one acre or more disturbed → TCEQ TXR150000 coverage | The TCEQ notice of intent asks for disturbed and total acres. |
Every way to connect
| Way to connect | What it is | Typical projects | What you file | More |
|---|---|---|---|---|
| Behind the meter (self-consumption) | Generation or storage on the customer's side of the utility meter, serving the building first; any export is governed by the utility's distributed-generation rules. | Homes, businesses, farms; usually under 1 MW. | The utility's DG interconnection application; city or county electrical permit. | Texas utility application links · Solar for homes and businesses |
| Distribution interconnection (front of the meter, small) | A project connects to the utility's distribution lines (in Texas generally under 10 MW and below 60 kV) and sells power or serves the grid directly. | Community solar, small solar and storage, typically 1–10 MW. | The utility (TDSP) distributed-generation process; in ERCOT the resource still registers. | Interconnection by market · PUCT 25.211 / 25.212 |
| Transmission interconnection (generation) | A generator connects to the high-voltage grid at a substation or a new tap on a line (the point of interconnection, POI) and sells into the wholesale market. | Utility-scale solar, wind, storage, gas; 10 MW and up in ERCOT. | ERCOT generation interconnection request (GINR) in RIOO-IS, studies by the transmission provider, then the SGIA. Outside ERCOT: the RTO or utility queue under FERC rules. | How to file an ERCOT request · Find a point of interconnection |
| Large load (consumer) interconnection | A big consumer (data center, industrial plant, electrolyzer) connects to the transmission system to take power; it is studied for whether the grid can serve it. | Data centers, crypto mining, industry; in Texas, 75 MW and up falls under SB 6 (2025). | The utility or transmission provider's large-load process; ERCOT large-load interconnection studies. | Data centers and large loads |
| Co-located load and generation | A load sits at the same site as a generator and takes power from it, with or without drawing from the grid; the grid still studies what crosses the POI. | Data center next to a solar, storage or gas plant. | Both the generation and large-load processes; ERCOT asks for a load information form when a load under 75 MW is co-located with generation. | ERCOT Resource Integration |
| Storage charging from the grid | A battery is both load (charging) and generation (discharging); the request covers both directions at the POI. | Standalone storage. | ERCOT GINR for the resource; charging studied as load. | How to file an ERCOT request |
| Islanded or microgrid | Generation and storage serve a load with no connection, or with a connection that can open and run alone. | Remote sites, critical facilities, oil and gas pads. | No grid interconnection if fully isolated; local electrical permits still apply. | Permits by county |
Load versus generation
Generation
Puts power onto the grid: solar, wind, gas, nuclear, and batteries when they discharge. The question an interconnection study asks is whether the grid can carry the power away from the point of interconnection without overloading lines or upsetting stability. The request is sized in MW at the point of interconnection (the AC limit), and the developer pays for upgrades the studies assign.
Load
Takes power from the grid: homes, buildings, data centers, factories, and batteries when they charge. The question is whether the grid can deliver enough power to the site, at the peak, with the reliability the customer needs. A load request is sized by peak demand (MW) and load factor, often with a ramp schedule; large loads in Texas follow the SB 6 rules.
Both
A battery, or a load co-located with generation, is studied in both directions. The meter at the point of interconnection decides what counts as each: power flowing out is generation, power flowing in is load. Co-location can shrink what crosses the meter but not what the grid must be ready for if one side trips.
Typical sizes and demand, with sources
Load: what takes power from the grid
Demand and energy use, from homes to buildings to large loads.
Residential
| Item | Value | Source |
|---|---|---|
| Average home, United States2024; about 10,360 kWh a year | 863 kWh/month | U.S. EIA, Electric Sales, Revenue and Price, Table 5a (2024) |
| Average home, Texas2024; average bill $163.72 at 14.94¢/kWh | 1,096 kWh/month | U.S. EIA, Electric Sales, Revenue and Price, Table 5a (2024) |
| Home peak demand, best guess: what one home battery backs upBest guess, not a measured peak. No public source gives a typical per-home peak: it depends on size, climate and HVAC, and utilities keep that data in their load research. A Powerwall 3 delivers 11.5 kW continuous and starts motors up to 185 locked-rotor amps, which installers size to cover a home's essential loads. For a real home, use the NEC calculator above. | 11.5 kW | Tesla Powerwall 3 datasheet, North America (manufacturer) |
| Home backup energy, typicalMedian home battery in 2025; the same as one Powerwall 3 (13.5 kWh) | 13.5 kWh | Berkeley Lab, Distributed Solar & Storage 2026 Data Update |
| Average home demandImplied by EPRI: 100–1,000 MW ≈ 80,000–800,000 average homes, so about 1.25 kW each; EIA's 863 kWh/month works out to 1.18 kW | 1.2 kW | EPRI, Powering Intelligence 2026 FAQs |
| Average home, United States (EIA FAQ)Earlier-year figure on the EIA FAQ page | 10,791 kWh/year | U.S. EIA, How much electricity does an American home use? |
| NEC general lighting and receptacles, dwellingNEC 220.12 (220.14(J) since 2020) | 3 VA/ft² | EC&M (Mike Holt), One-Family Dwelling Unit Load Calculations |
| NEC small-appliance circuitsNEC 220.52(A) | 1,500 VA each, at least 2 | EC&M (Mike Holt), One-Family Dwelling Unit Load Calculations |
| NEC laundry circuitNEC 220.52(B) | 1,500 VA | EC&M (Mike Holt), One-Family Dwelling Unit Load Calculations |
| NEC lighting demand factor, first 3,000 VANEC Table 220.42 (220.45 in 2023) | 100 % | EC&M (Mike Holt), One-Family Dwelling Unit Load Calculations |
| NEC lighting demand factor, 3,001–120,000 VAAbove 120,000 VA the table uses 25% | 35 % | EC&M (Mike Holt), Load Calculations Part 2 (2020 NEC) |
| NEC electric dryer, minimumOr nameplate if larger (220.54) | 5,000 W | EC&M (Mike Holt), Load Calculations Part 2 (2020 NEC) |
| EV charger, Level 1120 V × 16 A | 1.9 kW | U.S. DOE, Federal Energy Management Program: EV charging |
| EV charger, Level 2240 V × 32 A; 6.7 kW at 208 V | 7.7 kW | U.S. DOE, Federal Energy Management Program: EV charging |
Commercial
Utility scale
| Item | Value | Source |
|---|---|---|
| Texas large-load threshold (SB 6, 2025)PUCT sets interconnection standards for loads of 75 MW or more and may lower the threshold | 75 MW | Texas Legislature, SB 6 (89th Leg.) enrolled bill analysis |
| New data center, typical sizeEPRI: roughly the demand of 80,000 to 800,000 average homes; planned sizes are rising | 100–1,000 MW | EPRI, Powering Intelligence 2026 FAQs |
| U.S. data centers, share of all electricity (2023)Projected 6.7–12% by 2028 | 4.4 % | U.S. DOE announcement of the LBNL 2024 U.S. Data Center Energy Usage Report |
| U.S. data centers, share of electricity by 2030 (EPRI)About 4–5% today; range depends on how many planned projects are built | 9–17 % | EPRI, Powering Intelligence 2026 FAQs |
Generation: what puts power onto the grid
System sizes, equipment ratings, ratios and land, from rooftop to utility scale.
Residential
| Item | Value | Source |
|---|---|---|
| Median home solar system (2025) | 7.7 kWdc | Berkeley Lab, Distributed Solar & Storage 2026 Data Update |
| Median home battery (with solar)2025; 37% of new home solar systems added a battery | 13.5 kWh | Berkeley Lab, Distributed Solar & Storage 2026 Data Update |
| Home battery: Tesla Powerwall 311.5 kW continuous; built-in solar inverter takes up to 20 kW of panels | 13.5 kWh | Tesla Powerwall 3 datasheet, North America (manufacturer) |
| Enphase IQ8 microinverters, continuous outputIQ8 240, IQ8+ 290, IQ8M 325, IQ8A 349, IQ8H 380; CEC 97% | 240–380 VA each | Enphase IQ8 Series datasheet (manufacturer document) |
| SolarEdge Home Wave string inverters3.0, 3.8, 5.0, 6.0, 7.6, 10.0, 11.4 kVA at 240 V; CEC 99% | 3,000–11,400 VA | SolarEdge Home Wave single-phase datasheet, North America (manufacturer document) |
| NEC PV maximum circuit current690.8(A); conductors sized at another 125%, so 1.56 × Isc | 1.25 × Isc | EC&M (Mike Holt), NEC Requirements for Solar, Part 2 |
| NEC PV maximum system voltage, one- and two-family homes690.7; 1,000 V for other buildings. Voc corrected for the coldest expected temperature | 600 V | EC&M (Mike Holt), NEC Requirements for Solar, Part 2 |
Commercial
| Item | Value | Source |
|---|---|---|
| Median non-residential solar system (2025)Businesses, schools, nonprofits and government buildings | 41 kWdc | Berkeley Lab, Distributed Solar & Storage 2026 Data Update |
| NEC PV maximum system voltage, other buildings690.7 | 1,000 V | EC&M (Mike Holt), NEC Requirements for Solar, Part 2 |
Utility scale
| Item | Value | Source |
|---|---|---|
| Solar DC/AC (inverter loading) ratio, averageSteady near 1.3 since 2017 | 1.3 | Berkeley Lab, Utility-Scale Solar 2026 Edition |
| Solar capacity factor, AC, fleet averageRegional averages 16% (least sunny) to 33% (sunniest) | 23 % | Berkeley Lab, Utility-Scale Solar 2026 Edition |
| Solar land, fixed-tilt over 20 MWTotal site area; directly occupied area 5.8 | 7.5 acres/MWac | NREL, Land-Use Requirements for Solar Power Plants (2013), Table ES-1 |
| Solar land, single-axis tracking over 20 MWTotal site area | 8.3 acres/MWac | NREL, Land-Use Requirements for Solar Power Plants (2013), Table ES-1 |
| Solar land, 1–20 MW plantsTotal site area, fixed to tracking | 7.6–8.7 acres/MWac | NREL, Land-Use Requirements for Solar Power Plants (2013), Table ES-1 |
| Utility module: First Solar Series 7 TR1 (thin film)2,300 × 1,216 mm (2.80 m²), 38.4 kg, 18.8–19.7% efficient; Voc about 231–233 V. About 1,820 modules per MWdc at 550 W | 525–550 W | First Solar datasheet, filed with the Kentucky PSC (Crab Run Solar, case 2025-00276) |
| Utility module: JinkoSolar Tiger Neo 72HL4-BDV (bifacial TOPCon)2,278 × 1,134 mm, 32.0 kg, 21.3–22.1% efficient, 1,500 V DC; Voc 50.3–51.1 V, Isc 14.0–14.3 A. About 1,750–1,820 modules per MWdc | 550–570 W | JinkoSolar datasheet (manufacturer) |
| Solar + storage hybrids, battery duration (2025)ERCOT hybrids are the shortest | 3.4 hours | Berkeley Lab, Utility-Scale Solar 2026 Edition |
| Sungrow SG350HX-US string inverter1,500 V DC; 320 kVA at 40 °C; 99.02% maximum efficiency | 352 kVA | Sungrow SG350HX-US product page (manufacturer) |
| SMA Sunny Central UP central inverterAt 35 °C; 1,500 V DC; CEC 98.5% | 4,000–4,600 kVA | SMA Sunny Central UP datasheet (manufacturer) |
| Tesla Megapack 2 XL, energy per unit2-hour unit 1.93 MW at 92.0% round trip; 4-hour 0.98 MW at 93.7% | 3.85–3.92 MWh | Tesla Megapack 2 XL datasheet, filed with the Connecticut Siting Council |
| Utility battery round-trip efficiency2019 average; pumped storage 79% | 82 % | U.S. EIA, Today in Energy, Feb 12 2021: batteries and pumped storage return about 80% |
| Utility battery duration, built through 2020Older EIA figure | 3.0 hours | U.S. EIA, Today in Energy (battery storage durations) |
| New U.S. wind turbine, average size (2023)Hub height 103.4 m; rotor 133.8 m | 3.4 MW | U.S. DOE, Land-Based Wind Market Report: 2024 Edition |
Typical values for planning. Equipment ratings come from the maker's datasheet, code values from the edition your jurisdiction adopted, and utility requirements from the utility. Each row links the source it came from.