Electrical Load Calculator for Generator, Breaker, Inverter and Solar Sizing
An electrical load calculator adds up every appliance and motor you plan to power, then turns the list into numbers you can buy against: maximum demand in kW and kVA, design current in amps, and the size of the breaker, generator, inverter, battery bank and solar array. Use the free tool below for single-phase or three-phase supplies, at home or on a commercial site. Under it you will find the formulas, two worked examples and the NEC 220.82 method for home service panels.
Quick answers
- Demand: quantity × watts × peak share, then divide each load by its power factor to get kVA.
- Current: kVA × 1000 ÷ volts for single-phase, or kVA × 1000 ÷ (√3 × volts) for three-phase.
- Generator: running kVA × 1.25, then check that it can carry the largest motor start.
- Running load: aim for roughly 50–80% of the generator kW rating once it is running.
Table of Contents
Table of Contents
Electrical Load Calculator
Electrical Load Calculator
List your loads once and get connected load, maximum demand, design current, daily energy and cost, plus ready-to-use sizing for the main breaker, generator, inverter, battery bank and solar array.
How to Use
- Pick a quick start preset, or clear the list and build your own.
- In System settings, choose single-phase or three-phase, enter the voltage, and set the design margin and your electricity tariff.
- Add loads with the quick add menu or the Add blank load button. For each load enter the quantity, power in watts, hours of use per day, power factor, the share of units running at the peak moment, and the starting surge multiple for motors.
- Open Backup and solar settings if you want battery and solar sizing, and adjust autonomy, sun hours and losses.
- Press Calculate. The summary panel and the tables below update at once, and they also refresh as you type.
- Check the sizing cards for the main breaker, generator, inverter, battery bank and solar array, then review which loads use the most energy.
- Use Copy summary to paste the result into a note or quotation, or Print to keep a record.
System settings
Load list
For cycling loads such as refrigerators, enter the equivalent hours the load actually runs each day rather than 24.
Backup and solar settings
Result summary
Share of daily energy by category
Equipment sizing
Main breaker
0
Generator
0
Inverter
0
Battery bank
0
Solar array
0
Load by load results
| Load | Category | Connected (W) | Demand (W) | Energy (kWh per day) | Share of energy |
|---|
Results by category
| Category | Connected (kW) | Demand (kW) | Energy (kWh per day) | Share of energy |
|---|
What the numbers tell you
Technical notes
Connected load, demand and energy
Connected load is the sum of every nameplate you list, as if everything ran at once. Maximum demand is the realistic peak, because not every unit runs at the same moment. You control this with the peak share on each load and with the overall diversity setting. Energy is a different question: it depends on how many hours each load really runs in a day, so a large load that runs briefly can use less energy than a small load that runs all day.
Power factor and kVA
Motors, compressors and some electronics draw more apparent power than the real power they convert into work. The calculator uses each load power factor to convert real power into apparent power, which is what cables, breakers and generators must carry. A low overall power factor means equipment must be larger for the same useful output.
Starting surge
Motor loads draw a short burst of current when they start, several times higher than their running current. The calculator assumes the largest starting load starts while everything else is already running, which is the usual design case for generators and inverters. If your largest motors never start together, this approach is sound; if they can start at the same moment, add the surge of each one by hand or use soft starters.
Breaker, generator and inverter sizing
The design margin is added on top of the calculated demand and the result is rounded up to the next standard size. Generator sizing also checks that the machine can ride through the largest motor start using its short-term overload capability. Inverter sizing checks both the continuous rating and the surge rating. Always compare the selected size with the manufacturer datasheet and derate for altitude, high ambient temperature and harmonic loads where they apply.
Battery and solar sizing
The battery bank is sized to carry the full daily energy for the number of autonomy days you set, allowing for the usable depth of discharge and for round-trip losses. The solar array is sized to replace one day of energy using the peak sun hours and a combined loss allowance. These are planning estimates. A detailed design should also consider charge controller limits, temperature effects, panel orientation and seasonal variation.
What is not covered
This tool does not size cables, calculate voltage drop, check short-circuit levels, or apply the load calculation rules of any specific wiring code. For dwelling service calculations under a national code, use a code-based residential calculator and have the final design checked by a qualified engineer.
How to use the electrical load calculator
- Pick a preset (home with air conditioning, small retail shop or three-phase workshop), or clear the list and build your own.
- Choose single-phase or three-phase, enter the voltage (line-to-line for three-phase, for example 400 V), then set the design margin and your tariff.
- Add each load with its quantity, watts, hours per day, power factor, peak share and motor starting multiple.
- Open Backup and solar settings if you want battery and solar sizing.
- Press Calculate, read the sizing cards, then copy or print the summary for your notes or quotation.
Measure real running watts with a plug-in watt meter or a clamp meter when you can. Measured values beat nameplates, which are often higher than what the appliance actually draws.
How the electrical load calculator works
The tool answers three questions: how big is the peak (kW and kVA), how much current does it draw (amps), and how much energy does it use (kWh).
Connected load, maximum demand and diversity
Connected load is the sum of every nameplate, as if everything ran at once. Maximum demand is the realistic peak, because not every unit runs at the same moment. You control it with the peak share on each load and the overall diversity setting. The idea is the same as the demand factor and diversity factor used in building design.
Energy is a separate question. It depends on hours of use, so a 3 kW heater running for 1 hour uses 3 kWh, while a 150 W refrigerator with an 8-hour equivalent run time uses 1.2 kWh.
Power factor, kW and kVA
Cables, breakers and generators must carry apparent power in kVA, not only the real power in kW. The calculator converts with kVA = kW ÷ power factor. A group of loads with a power factor of 0.85 needs about 18% more kVA than kW. Generator nameplates normally assume a power factor of 0.8, so a 100 kVA set delivers about 80 kW.
For the full explanation read kW vs kVA in cable sizing. If your overall power factor is low, the power factor correction capacitor calculator shows how much correction you need.
Starting surge
Motors draw a short burst of current when they start, several times their running current. The tool assumes the largest starting load starts while everything else is already running, which is the usual design case for generators and inverters. Typical starting multiples are 3 for small compressors and air conditioners, and 5 to 7 for direct-on-line induction motors. Soft starters, star-delta starters and drives cut that figure.
If two big motors can start together, add both surges by hand. Read diesel generator sizing for motor starting and allowable voltage dip for generator sizing before you finalise a set, and use the motor current calculator to get running amps from kW or HP.
Load factor
Load factor is average demand divided by maximum demand. Over a day it equals daily kWh ÷ (maximum demand in kW × 24). A low load factor means a big peak for little energy, such as a single large pump. The tool reports it in the result summary. See plant factor, plant capacity factor and load factor for the definitions.
Electrical load calculation formulas
| Quantity | Single-phase | Three-phase |
|---|---|---|
| Apparent power (kVA) from kW | kVA = kW ÷ PF | kVA = kW ÷ PF |
| Current (A) from kW | I = kW × 1000 ÷ (V × PF) | I = kW × 1000 ÷ (√3 × V × PF) |
| Current (A) from kVA | I = kVA × 1000 ÷ V | I = kVA × 1000 ÷ (√3 × V) |
| Real power (kW) from kVA | kW = kVA × PF | kW = kVA × PF |
V is the line-to-line voltage for three-phase. Example: a 25 kW three-phase load at 400 V and power factor 0.85 draws 25,000 ÷ (1.732 × 400 × 0.85) = 42.5 A.
How to calculate generator amps
Use the generator's rated kVA and voltage. A 100 kVA three-phase set at 400 V has a full-load current of 100,000 ÷ (1.732 × 400) = 144 A. A 10 kVA single-phase set at 240 V has 10,000 ÷ 240 = 41.7 A.
To get the real power it can deliver, multiply kVA by the power factor: 100 kVA × 0.8 = 80 kW. Use the generator circuit breaker sizing calculation to size the breaker on its output, and the full load amperage calculator for motor amps. For the feeder, use the AC cable size calculator.
Typical appliance watts and starting multiples
These are planning figures. The nameplate and the manufacturer's datasheet always win.
| Appliance | Running watts | Typical power factor | Starting multiple |
|---|---|---|---|
| LED lighting, whole house | 100–300 | 0.9–0.95 | 1 |
| Refrigerator | 150–300 | 0.8 | 3–4 |
| Central air conditioner, 2.5–3 ton | 2,500–3,500 | 0.85 | 3 (up to 6 without a soft start) |
| Sump or well pump, ½ hp | 750–1,000 | 0.75 | 3–4 |
| Washing machine | 400–800 | 0.8 | 2–3 |
| Microwave oven | 1,000–1,500 | 1.0 | 1 |
| Electric water heater element | 3,000–4,500 | 1.0 | 1 |
| Air compressor, 3.7 kW | 3,700 | 0.8 | 4–6 |
| Level 2 EV charger | 7,200–11,500 | 1.0 | 1 (continuous load) |
| Induction motor, direct-on-line | rated input | 0.8–0.88 | 5–7 |
| Induction motor, soft starter / star-delta / drive | rated input | 0.8–0.88 | 3 / 2 / 1–1.5 |
An EV charger is a continuous load, so size its circuit at 125% of the charger current. Use the EV charger load calculation tool to check whether your panel has room.
Worked example 1: home backup generator (single-phase, 240 V)
A house wants to run the essentials during an outage. The electric water heater stays off the backup circuit. Run the same numbers in the tool to compare.
| Load | Qty | Watts each | Peak share | Demand (W) | PF | Demand (VA) |
|---|---|---|---|---|---|---|
| Central A/C, 2.5 ton | 1 | 2,500 | 100% | 2,500 | 0.85 | 2,941 |
| Refrigerator | 1 | 200 | 100% | 200 | 0.80 | 250 |
| Sump pump, ½ hp | 1 | 750 | 100% | 750 | 0.75 | 1,000 |
| LED lighting | 1 | 150 | 100% | 150 | 0.95 | 158 |
| Ceiling fans | 4 | 70 | 100% | 280 | 0.90 | 311 |
| Microwave | 1 | 1,200 | 50% | 600 | 1.00 | 600 |
| Washing machine | 1 | 500 | 50% | 250 | 0.80 | 312 |
| TV, router, laptops | 1 | 350 | 100% | 350 | 0.95 | 368 |
| Water heater, 4,500 W | 1 | 4,500 | off | 0 | – | 0 |
| Total | 10,430 connected | 5,080 | 0.86 | 5,941 |
- Design current: 5,941 ÷ 240 = 24.8 A. With a 25% margin that is 31 A, so a 35 A breaker in NEC sizes (32 A in IEC sizes).
- Running requirement: 5.94 kVA × 1.25 = 7.4 kVA.
- Starting check: the A/C draws 2.94 kVA running. At a start multiple of 3 it adds 2 × 2.94 = 5.9 kVA, so the peak is 11.8 kVA. If the set can hold a 25% short-term overload, it needs 11.8 ÷ 1.25 = 9.5 kVA.
- Result: the start decides. Choose a 10 kVA (8 kW) set. Running demand is 5.08 ÷ 8 = 64% of its kW rating, inside the healthy range.
A soft-start kit that cuts the A/C multiple from 3 to 1.5 drops the peak from 11.8 to 7.4 kVA, so a smaller set becomes possible. For rules of thumb by house size, read what size generator for a house and how many kW generator to run a house. To list your own appliances first, use the generator wattage estimator.
Worked example 2: three-phase workshop (400 V)
| Load | Input kW | PF | Peak share | Demand kW | Demand kVA | Start multiple |
|---|---|---|---|---|---|---|
| Lathe motor | 7.5 | 0.85 | 100% | 7.50 | 8.82 | 6 |
| Air compressor | 3.7 | 0.80 | 100% | 3.70 | 4.63 | 5 |
| LED lighting | 2.0 | 0.90 | 100% | 2.00 | 2.22 | 1 |
| Office split A/C | 3.5 | 0.85 | 100% | 3.50 | 4.12 | 3 |
| Sockets and small tools | 3.0 | 0.95 | 50% | 1.50 | 1.58 | 1 |
| Total | 19.7 connected | 0.85 | 18.2 | 21.37 |
Design current is 21,370 ÷ (1.732 × 400) = 30.8 A. Running kVA with a 25% margin is 26.7 kVA. The largest motor, the lathe, is 8.82 kVA running, so the way you start it decides the generator size:
| Lathe starting method | Multiple | Peak kVA | Set needed (kVA) | Standard set | Running load of kW rating |
|---|---|---|---|---|---|
| Direct-on-line | 6 | 65.5 | 52.4 | 60 kVA | 38% |
| Soft starter | 3 | 39.0 | 31.2 | 40 kVA | 57% |
| Star-delta | 2 | 30.2 | 26.7 | 30 kVA | 76% |
"Set needed" is the larger of running kVA × 1.25 and peak kVA ÷ 1.25, using a 25% short-term overload allowance. Real sets are limited by voltage dip and by the manufacturer's motor-starting curve, so confirm with the datasheet. One cheap starter on one motor halves the generator in this case. Read generator sizing for VFD motors and generator sizing for pumps for drive and pump duty, and the motor protection calculator for the overload settings. In 480 V systems, use the 480 volt generator sizing chart.
Generator load calculation: formula and load percentage
Generator load percentage shows how hard the set is working: load % = running load (kW) ÷ generator kW rating × 100. This applies to any genset, which is simply the engine and alternator together.
How to calculate generator load percentage
- Take the maximum demand in kW from the calculator, for example 18.2 kW.
- Convert the generator rating to kW: kVA × 0.8. A 30 kVA set is 24 kW.
- Divide and multiply by 100: 18.2 ÷ 24 × 100 = 76%.
- Check the kVA side too: 21.4 kVA ÷ 30 kVA = 71%. The higher of the two percentages is the one that limits you, which here is the kW figure.
Which load percentage to aim for
Plan for a typical running load of 50–80% of the kW rating. Load-bank maker Avtron puts the efficient range for diesel engines at 70–80% of rated kW. Long periods below about 30% risk wet stacking, where unburned fuel and soot build up in the exhaust. An engineering explainer in Consulting-Specifying Engineer notes that NFPA 110 requires monthly tests at 30% of nameplate or the manufacturer's minimum exhaust temperature.
So avoid both extremes. A set that always runs at 95% has no headroom, and a set that idles at 20% wastes fuel and wears the engine. Once you know the load percentage, estimate fuel burn and running cost with the generator power consumption calculator.
Generator sizing calculator guide: home, commercial and three-phase
The method is the same at every size. List the loads, take running kVA with a 20–25% margin, check the largest motor start, then round up to the next standard size.
Home generator sizing
Size for the loads you will really run during an outage, not for the whole house. Refrigerator, lights, fans, router and a sump pump usually fit a small set. Central air conditioning, an electric range or an EV charger pushes you into the 10 kW range and above, mostly because of motor starting. For brand-specific sizing tools, try the Generac generator size calculator and the backup generator sizing calculator.
Commercial generator sizing
Commercial sites add motors, UPS units, drives and pumps. Non-linear loads such as UPS systems and variable frequency drives create extra heating in the alternator, so manufacturers often ask for more capacity than the kVA total suggests. Read generator sizing for UPS systems, generator sizing for VFD motors and generator sizing for pumps. For a final selection, run your load list through the maker's own tool, such as Caterpillar's genset sizing resources.
Three-phase generator sizing
Switch the tool to three-phase and enter the line-to-line voltage, such as 400 V or 480 V. It then works as a 3 phase generator load calculator. Spread single-phase loads evenly across the three phases so no phase carries much more current than the others. A three-phase set can feed single-phase loads, but one phase can deliver only a fraction of the three-phase rating, so check the datasheet. The 480 volt generator sizing chart lists common US sizes, and the types of generators guide explains the options. Buying in Pakistan? See the best generator companies in Pakistan.
Panel and service load: the NEC 220.82 method
The tool above gives a planning demand. The National Electrical Code has its own method for a US home service. Article 220 allows a standard method and an optional method. The optional method in 220.82 applies to a dwelling served by a single 120/240 V or 208Y/120 V three-wire service or feeder rated 100 A or more.
- General load: 3 VA per square foot for lighting and receptacles, plus 1,500 VA for each of two small-appliance circuits, plus 1,500 VA for laundry, plus the nameplate of fixed appliances.
- Demand: take the first 10 kVA at 100% and the rest at 40%.
- Heating or cooling: add the larger of air conditioning at 100% or central electric heat at 65%.
- Service size: divide the total by 240 V and round up to the next standard rating.
Example for a 2,000 sq ft house with a 12 kW range, 5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher and a 20 A, 240 V air conditioner:
| Step | Calculation | VA |
|---|---|---|
| General lighting | 2,000 × 3 | 6,000 |
| Small-appliance circuits | 2 × 1,500 | 3,000 |
| Laundry circuit | 1 × 1,500 | 1,500 |
| Fixed appliances | 12,000 + 5,000 + 4,500 + 1,200 | 22,700 |
| Total general load | 33,200 | |
| After demand factors | 10,000 + (23,200 × 0.40) | 19,280 |
| Air conditioner at 100% | 20 A × 240 V | 4,800 |
| Calculated load | 24,080 |
24,080 ÷ 240 = 100.3 A, so the minimum standard service is 125 A. Most installers would still specify 200 A to leave room for an EV charger or heat pump. For an existing home that is adding a load, 220.83 uses the first 8 kVA at 100% instead. Editions and local amendments differ, so the authority having jurisdiction has the last word.
For the wider code picture, read the home electrical wiring and NEC code guide. Outside the US, use the guides for BS 7671 in the UK, AS/NZS 3000 in Australia and CEC / CSA C22.1 in Canada. Adding an EV charger or heat pump? Check the EV charger load calculation tool and the heat pump electricity cost calculator.
Breaker, inverter, battery and solar sizing
The same load list feeds the other sizing cards.
- Main breaker: design current × (1 + margin), rounded up to the next standard size. Size the feeder with the AC cable size calculator.
- Inverter: needs a continuous rating above running kVA and a surge rating above the largest motor start. See the solar inverter sizing calculator.
- Battery bank: daily kWh × autonomy days ÷ (depth of discharge × round-trip efficiency). For 10 kWh a day, one day of autonomy, 80% depth of discharge and 90% efficiency: 10 ÷ (0.8 × 0.9) = 13.9 kWh.
- Solar array: daily kWh ÷ (peak sun hours × (1 − losses)). For 10 kWh, 5 sun hours and 25% losses: 10 ÷ (5 × 0.75) = 2.7 kW, or 7 panels of 400 W.
Continue with the solar panel size calculator by load, the DC cable size calculator, the solar charge controller guide and the solar panel kits comparison. To turn kWh into money, use your utility's calculator from the US electricity bill calculators or the Pakistan DISCO calculators.
What this calculator does not cover
The tool does not size cables, calculate voltage drop, check short-circuit levels or apply the load rules of a specific wiring code. Derate for altitude, high ambient temperature and harmonic loads where they apply. Use the earth cable size calculator for the earthing conductor. Typical wattages, power factors and starting multiples vary by product and region, so check nameplates, datasheets, local wiring regulations and your tariff. Have the final design reviewed by a licensed electrician or electrical engineer.
Frequently asked questions
What is an electrical load calculator?
It is a tool that totals the power of the appliances and motors on a supply and reports demand, current and equipment size. This one covers single-phase and three-phase, plus breaker, generator, inverter, battery and solar sizing.
How do you calculate electrical load?
Multiply each load's quantity by its watts and its peak share, then add them for demand in watts. Divide each by its power factor for VA. Divide total VA by the voltage, or by √3 × voltage for three-phase, to get amps.
How do you calculate generator load?
Divide the running demand in kW by the generator kW rating and multiply by 100. The kW rating is kVA × 0.8. Check the kVA percentage as well, and size the set so the starting peak also fits.
What load percentage should a generator run at?
Plan for 50–80% of rated kW. Efficient running is usually quoted at 70–80%, and long periods below about 30% can cause wet stacking in diesel sets.
What is the difference between kW and kVA on a generator?
kW is the real power the engine can deliver. kVA is the apparent power the alternator can carry. kW = kVA × power factor, and generator ratings assume 0.8.
Should I size a generator for starting or running watts?
Both. Size for running kVA plus a margin, then confirm the set can carry the largest motor start. In the home example above, the start decided the size.
Can a three-phase generator run single-phase loads?
Yes, but a single phase can supply only a fraction of the three-phase rating unless the set is reconnected. Keep loads balanced and follow the datasheet.
Does this calculator follow the NEC?
No. It is a planning tool that uses diversity and power factor. For a US dwelling service, use the NEC 220.82 method shown above and have the result checked locally.
How often should I recalculate my load?
Whenever you add or remove a large appliance, a motor or an EV charger. Review it each year if your usage changes with the seasons.
Method and sources
- Dwelling service method: EC&M, Dwelling Unit Calculations and One-Family Dwelling Unit Load Calculations, covering NFPA 70 (NEC) Article 220.
- Generator load range: Avtron Power Solutions and Consulting-Specifying Engineer, which cites NFPA 110 test requirements.
- Manufacturer sizing practice: Caterpillar genset sizing.
- Written and reviewed by Abdul Wassay, electrical engineer. Author profile and credentials.
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Electrical Load Calculator: Generator & Breaker Sizing
Free electrical load calculator for home and commercial loads. Get demand, amps, kVA, plus generator, breaker, inverter and solar sizing.
Price Currency: USD
Operating System: Web Browser
Application Category: UtilitiesApplication
