What Size Generator Do I Need?

Choosing the correct generator size is one of the most important steps when buying a generator. A generator that is too small may overload, shut down or fail to start connected equipment. A generator that is unnecessarily large may cost more to buy, consume additional fuel and operate inefficiently under a very light load.

The correct generator size depends on the total running power required by your equipment, together with any additional starting power needed by motors, pumps, compressors, refrigerators and power tools.

Generator capacity may be shown in watts, kilowatts or kilovolt-amperes. Understanding these ratings and calculating the expected electrical demand will help you select a generator that provides safe, dependable power.

This guide explains how to calculate the generator size you need for home backup, construction tools, workshops, agricultural equipment, events and commercial applications.

Why Generator Size Matters

A generator must be capable of supplying the total electrical load without being operated continuously beyond its rated output.

An undersized generator may experience:

• Overload protection repeatedly tripping
• Engine speed dropping under load
• Low or unstable voltage
• Difficulty starting motors and compressors
• Generator overheating
• Alternator damage
• Damage to connected equipment
• Premature engine wear

An oversized generator can also create problems. A large generator operating continuously at a very low percentage of its capacity may consume unnecessary fuel and, in the case of diesel generators, may develop carbon build-up or wet stacking.

The aim is to select a generator with enough capacity to handle the expected running load and starting demand while retaining a sensible operating margin.

Understanding Watts, Kilowatts and kVA

Generator power is commonly expressed in watts, kilowatts or kilovolt-amperes.

Watts

Watts measure electrical power. Smaller appliances and portable generators are often rated in watts.

For example:

• 1,000 watts equals 1 kilowatt
• 2,500 watts equals 2.5 kilowatts
• 5,000 watts equals 5 kilowatts

Kilowatts

Kilowatts, shown as kW, are commonly used for portable, commercial and industrial generator ratings.

A 5kW generator can theoretically supply a combined electrical load of 5,000 watts, although it should not necessarily be operated continuously at its maximum output.

Kilovolt-Amperes

Kilovolt-amperes, shown as kVA, are commonly used for larger single-phase and three-phase generators.

kVA represents apparent power, while kW represents real usable power. The relationship depends on the generator’s power factor.

Many three-phase generators use a power factor of 0.8. As a general example:

• 10kVA at a 0.8 power factor equals approximately 8kW
• 20kVA at a 0.8 power factor equals approximately 16kW
• 50kVA at a 0.8 power factor equals approximately 40kW

Always check the generator manufacturer’s stated kW and kVA ratings rather than relying on an assumed conversion.

Running Watts vs Starting Watts

Generator sizing must account for both running watts and starting watts.

Running Watts

Running watts are the power an appliance or tool consumes during normal operation.

A heater rated at 2,000 watts will generally require approximately 2,000 watts while operating because it is mainly a resistive load.

Starting Watts

Starting watts are the additional power some equipment requires for a short period when switching on.

Equipment containing an electric motor may require significantly more power during start-up than during normal operation.

Examples include:

• Refrigerators
• Freezers
• Water pumps
• Air compressors
• Power tools
• Air-conditioning units
• Workshop machinery
• Ventilation fans

A motor that uses 1,000 watts while running may require 2,000, 3,000 or more watts momentarily when starting.

The precise starting demand should be obtained from the equipment manufacturer, data plate or technical documentation wherever possible.

Maximum Output vs Rated Output

Generator specifications often show both maximum output and rated or continuous output.

Maximum Output

Maximum output is the highest power the generator can produce for a short period.

This capacity may help the generator cope with a temporary starting surge, but it should not be treated as the normal continuous operating output.

Rated Output

Rated output is the power the generator is designed to supply during normal operation.

Generator selection should be based primarily on rated or continuous output, with the maximum output used only to assess short-term starting capability.

For example, a generator advertised as 5.5kW maximum may have a continuous rated output of only 5.0kW.

How to Calculate the Generator Size You Need

The basic generator-sizing process involves listing every item that may operate at the same time and identifying both its running and starting power.

Step 1: List the Equipment

Create a list of all appliances, tools or machines that the generator may need to power simultaneously.

For example:

• Refrigerator
• Freezer
• Lighting
• Television
• Internet router
• Water pump
• Power tools

Step 2: Find the Running Power

Check the data plate, operating manual or manufacturer’s technical information for each item.

Power may be shown in watts, kilowatts or amps.

Do not rely on broad online estimates where accurate equipment data is available.

Step 3: Identify Starting Loads

Identify which items contain motors, compressors, pumps or transformers.

Record the highest starting demand for each item. In many calculations, only the largest single starting load needs to be added to the combined running load, provided the equipment will not all start simultaneously.

Step 4: Add the Simultaneous Running Load

Add together the running watts of all equipment that may be used at the same time.

Step 5: Add the Largest Starting Requirement

Add the additional starting requirement of the largest motor-driven item.

The result gives a practical minimum generator requirement.

Step 6: Add a Safety Margin

Add a reasonable operating margin so the generator is not continuously working at its absolute limit.

A margin of approximately 15% to 25% is often appropriate for general sizing, although the correct allowance depends on the load type and application.

Large motors, electronic power supplies, welders and irregular loads may require a more detailed assessment.

Example Generator Size Calculation

Consider a simple household backup requirement:

The combined running load is:

200W + 250W + 300W + 150W + 50W = 950W

The largest starting requirement is the freezer at 1,000W. Because its normal running load of 250W is already included, the additional starting allowance is 750W.

The estimated requirement is therefore:

950W + 750W = 1,700W

After adding an operating margin, a generator with a continuous output of approximately 2.0kW to 2.5kW may be appropriate.

This example is illustrative only. Actual appliance ratings and starting currents vary.

How to Calculate Power from Amps

Some equipment data plates show current in amps rather than power in watts.

For a simple single-phase load, power can be estimated using:

Watts = Volts × Amps

For example, a 230V appliance drawing 10 amps would have an apparent demand of approximately:

230V × 10A = 2,300W

However, motors and other inductive equipment may have a power factor below 1.0, and starting current can be considerably higher than normal operating current.

For three-phase loads or complex equipment, obtain advice from a suitably competent electrician or generator specialist.

What Size Generator Do I Need for Home Backup?

The required generator size depends on whether you intend to power only essential items or most of the property.

Essential Household Backup

A smaller generator may be suitable for selected essentials such as:

• Refrigerator
• Freezer
• Lighting
• Television
• Internet router
• Phone chargers
• Heating controls

Depending on the actual appliance ratings and starting demand, a generator in the region of 2kW to 4kW may be sufficient for a carefully managed essential load.

Larger Household Backup

A larger generator may be required for:

• Multiple refrigerators and freezers
• Water pumps
• Electric ovens
• Kettles
• Microwaves
• Electric showers
• Heating systems
• Workshop equipment

High-consumption heating appliances can increase the required generator size considerably.

Rather than attempting to power every appliance, it is often more practical to identify essential circuits and manage the load.

A portable petrol generator may be suitable for short-term essential backup, while a larger diesel generator may be more appropriate for extended or frequent outages.

Connecting a Generator to a House

A portable generator should not be connected directly to a property using an improvised lead or by feeding power through a wall socket.

This practice, sometimes called backfeeding, can energise external wiring and create a potentially fatal risk to utility workers, electricians and members of the public.

Any connection to a property’s fixed electrical installation must use suitable changeover equipment designed and installed by a qualified electrician.

The installation may require:

• A manual or automatic changeover switch
• Correct earthing arrangements
• Suitable circuit protection
• Appropriate generator input connections
• Load separation or essential-circuit planning

What Size Generator Do I Need for Power Tools?

Power tools often have significant starting demands, particularly tools fitted with larger electric motors.

Equipment may include:

• Circular saws
• Cut-off saws
• Angle grinders
• Mixers
• Compressors
• Pumps
• Bench saws
• Welders

Check the tool’s rated input rather than its advertised mechanical output.

For example, a saw described as a 2kW machine may require considerably more than 2kW during start-up.

Conventional open-frame generators are commonly selected for site tools because they offer robust construction and strong starting capacity.

The generator should also provide the correct sockets and voltage for the equipment being used.

110V Site Generators

Many UK construction sites use 110V tools to reduce electrical risk.

A generator intended for site work may provide:

• 110V sockets only
• 230V sockets only
• Dual-voltage 110V and 230V outputs

On a dual-voltage generator, the full rated output may not be available through every individual socket.

Check:

• The output available at 110V
• The output available at 230V
• The rating of each socket
• The maximum current per outlet
• Whether both voltages can be used simultaneously

What Size Generator Do I Need for a Refrigerator or Freezer?

Refrigerators and freezers normally have relatively low running power but a much higher starting demand when the compressor starts.

A refrigerator may consume only a few hundred watts while running but require several times that amount momentarily during start-up.

When powering more than one refrigeration appliance, consider whether their compressors may start at the same time.

An inverter generator may be suitable for smaller household backup requirements because it provides clean electrical output and quieter operation.

Confirm that the inverter generator has enough surge capacity to start the compressor successfully.

What Size Generator Do I Need for a Pump?

Pumps can have very high starting currents and should be sized carefully.

Examples include:

• Water pumps
• Sump pumps
• Well pumps
• Drainage pumps
• Irrigation pumps
• Sewage pumps

The motor starting current may be several times the normal running current.

The pump manufacturer should be asked for:

• Rated power
• Full-load current
• Starting current
• Voltage
• Phase
• Starting method

Larger pumps may use star-delta, soft-start or variable-frequency drive systems, each of which affects generator sizing.

What Size Generator Do I Need for a Welder?

Welding equipment can place a demanding and rapidly changing load on a generator.

The generator must be compatible with the welder’s:

• Input voltage
• Input current
• Maximum power demand
• Power factor
• Duty cycle
• Electronic control system

Do not size a generator for a welder solely from the welding output current.

Refer to the welder manufacturer’s recommended generator capacity. Some electronically controlled welders require low-distortion power and may not operate correctly from an unsuitable conventional generator.

What Size Generator Do I Need for an Event?

Event power requirements may include:

• Lighting
• Sound systems
• Refrigeration
• Catering equipment
• Payment terminals
• Heating equipment
• Inflatables
• Display equipment

Add the running power of every item expected to operate simultaneously and identify any equipment with compressors, heating elements or motors.

Catering appliances such as kettles, coffee machines, fryers and water heaters can create a very high continuous load.

Where low noise is important, consider an enclosed silent generator rather than an open-frame machine.

Sensitive audio, lighting and payment equipment may also benefit from a stable, low-distortion electrical supply.

What Size Generator Do I Need for a Caravan or Motorhome?

Caravan and motorhome loads may include:

• Battery chargers
• Lighting
• Televisions
• Small microwaves
• Refrigerators
• Heating controls
• Air-conditioning systems

An inverter generator in the region of 1kW to 3kW may be sufficient for light leisure use, depending on the connected appliances.

Air-conditioning units and microwaves may require a larger generator because of their starting and running demand.

The generator must always be operated outdoors and away from the caravan, awning, windows and ventilation openings.

What Size Generator Do I Need for Commercial Backup?

Commercial generator sizing should begin with a detailed load assessment.

Loads may include:

• Lighting circuits
• Computers and servers
• Refrigeration
• Heating and ventilation systems
• Pumps
• Lifts
• Security systems
• Production equipment
• Electric motors

The assessment should consider:

• Normal operating load
• Peak demand
• Motor-starting currents
• Essential and non-essential circuits
• Future expansion
• Load sequencing
• Single-phase imbalance
• Harmonic loads

Professional load monitoring may be advisable before selecting a large commercial or standby generator.

Single-Phase vs Three-Phase Generator Sizing

Most domestic appliances and portable tools use a single-phase supply.

Industrial motors, machinery and commercial equipment may require a three-phase generator.

Three-phase generators require careful load distribution.

The generator’s total power rating is divided across three phases. A 30kVA three-phase generator does not necessarily provide 30kVA from one single-phase outlet.

Excessive imbalance can cause:

• Voltage instability
• Alternator overheating
• Reduced usable output
• Protection devices tripping
• Damage to connected equipment

Where both single-phase and three-phase loads are present, the installation should be assessed by a suitably competent electrical professional.

How Much Spare Capacity Should a Generator Have?

A generator should normally have some capacity above the calculated expected demand.

Benefits of retaining a margin include:

• Reduced risk of overload
• Improved ability to start motors
• Lower engine stress
• Better voltage stability
• Capacity for small future additions

For many straightforward applications, a margin of approximately 15% to 25% above the calculated requirement may be suitable.

However, the correct margin depends on the nature of the load. A system containing large motors or irregular electronic loads may require greater capacity.

Excessive oversizing should also be avoided, particularly with diesel generators expected to operate for long periods.

Should I Size a Generator for Everything at Once?

Not necessarily.

Load management can reduce the generator size required by preventing high-demand appliances from operating simultaneously.

For example, you may choose not to run:

• A kettle and microwave together
• Multiple power tools simultaneously
• Heating appliances at the same time as large motors
• Several pumps at once

Automatic load-management systems may also start and stop equipment in a controlled sequence.

Manual load management can work for small temporary systems, but critical commercial installations require a properly designed control strategy.

Why Motor Starting Method Matters

The way an electric motor starts can substantially affect generator size.

Direct-on-Line Starting

Direct-on-line motors draw a high inrush current when starting. This can create a substantial voltage dip and may require a larger generator.

Star-Delta Starting

Star-delta starting reduces the starting current compared with direct-on-line starting, although the starting torque is also reduced.

Soft Starters

Soft starters limit starting current electronically and may reduce the required generator capacity.

Variable-Frequency Drives

Variable-frequency drives can provide controlled motor starting but may introduce harmonic currents and require careful compatibility checks.

Large motor applications should be assessed by a generator specialist or electrical engineer.

Resistive, Inductive and Electronic Loads

Resistive Loads

Resistive loads convert electricity mainly into heat.

Examples include:

• Kettles
• Heaters
• Toasters
• Incandescent lighting

Their starting demand is normally similar to their running demand.

Inductive Loads

Inductive loads contain motors or transformers and often require additional starting power.

Examples include:

• Pumps
• Compressors
• Refrigerators
• Power tools
• Fans

Electronic Loads

Electronic loads may include switch-mode power supplies, variable-speed drives and digital control systems.

These loads may draw current in an irregular way and can introduce harmonics into the electrical system.

Sensitive equipment may require a generator with low total harmonic distortion and good voltage regulation.

Can a Generator Be Too Large?

Yes. Excessive oversizing can reduce operating efficiency.

Potential disadvantages include:

• Higher purchase cost
• Higher fuel consumption
• Greater weight and size
• Higher maintenance costs
• Poor diesel-engine loading
• Carbon build-up or wet stacking

A generator should normally operate within a healthy portion of its capacity rather than remaining almost unloaded for long periods.

Load banks may be used during maintenance and testing where standby diesel generators do not regularly experience sufficient load.

Can a Generator Be Too Small?

An undersized generator may appear to operate normally until a large appliance or motor attempts to start.

Signs that a generator is too small include:

• The engine slows suddenly
• Lights dim when equipment starts
• Voltage falls
• Overload protection trips
• Motors fail to start
• The generator overheats
• The output becomes unstable

Disconnect the load and investigate the cause rather than repeatedly resetting overload protection.

Petrol vs Diesel Generator Sizing

The calculation process is broadly the same for petrol and diesel generators.

Petrol generators are often chosen for:

• Lower-power portable use
• Occasional household backup
• Power tools
• Leisure applications
• Short-duration operation

Diesel generators are often chosen for:

• Extended daily operation
• Commercial backup
• Industrial loads
• Large motors
• Prime-power installations
• Three-phase applications

Fuel type should be selected after establishing the required output and duty cycle.

Generator Sizing and Noise

A generator operating close to full load may be louder than the same machine operating at a moderate load.

Selecting a generator with a sensible operating margin can reduce the amount of time it spends at maximum engine effort.

However, choosing a very large generator solely to reduce noise is inefficient. A correctly sized acoustic or inverter model is normally a better solution.

Generator Sizing and Run Time

A larger generator does not automatically provide a longer run time.

Run time depends on:

• Fuel-tank capacity
• Electrical load
• Engine efficiency
• Fuel type
• Eco mode or engine-speed control

Compare published run-time figures at the same percentage load.

A generator with a large fuel tank may run longer than a more efficient generator with a much smaller tank.

What Information Should I Gather Before Buying?

Before requesting a recommendation or quotation, gather:

• A list of all connected equipment
• The running watts or amps for each item
• The starting current of motors and compressors
• The required voltage
• Whether the load is single phase or three phase
• The operating hours per day
• The preferred fuel type
• The required noise level
• The required socket types
• Whether automatic startup is needed
• Whether the generator will connect to fixed wiring
• The installation location

Accurate information will result in a more reliable generator recommendation.

What to Check Before Buying

Before selecting a generator, check:

• The continuous or prime output
• The maximum or standby output
• The largest starting demand
• The total simultaneous running load
• The available safety margin
• The required voltage
• The required phase arrangement
• The socket types and current ratings
• The generator noise level
• The fuel-tank capacity
• The expected run time
• The starting method
• The emissions level
• The generator weight and dimensions
• The warranty and service requirements

Do not select a generator from its model number or maximum advertised power alone.

When Should You Get Professional Advice?

Professional generator sizing is recommended where the installation includes:

• Large electric motors
• Three-phase machinery
• Variable-frequency drives
• Welders
• Lifts
• Medical equipment
• Data centres or servers
• Automatic mains-failure systems
• Commercial refrigeration
• Whole-building backup
• Parallel generator systems

A qualified professional may carry out load monitoring, motor-starting calculations, voltage-drop assessments and phase-balancing checks.

Conclusion

The correct generator size is determined by more than simply adding together the wattage shown on a few appliances.

You must calculate the combined running load, identify the largest starting requirement and include a sensible operating margin.

Small inverter and petrol generators may be suitable for essential household equipment, leisure use and lighter tools. Larger diesel and three-phase generators are generally better suited to prolonged commercial, agricultural and industrial applications.

Always base your decision on the generator’s rated or continuous output rather than its short-term maximum figure.

For complex installations, large motors or fixed building connections, obtain advice from a suitably qualified electrician or generator specialist before purchasing.