You cannot determine the kVA rating of a Firman ECO4990ES because the manufacturer does not publish a power factor for this model, and kVA is not a standard specification for portable generators sold in the UK. The generator is rated at 4,990 watts (W) maximum output, which equates to roughly 4.99 kVA at a power factor of 1.0, but UK generators typically list kW, not kVA (GOV.UK, 2026). The answer depends on whether you are calculating for resistive or inductive loads.
- Firman ECO4990ES maximum output is 4,990 watts or 4.99 kVA at PF 1.0.
- For inductive loads, divide kW by power factor (0.6–0.8) to get kVA.
- UK portable generators list kW, not kVA, per GOV.UK standards.
The variable that decides the kVA is the power factor of the connected equipment. For purely resistive loads, like heaters or incandescent lights, the power factor is effectively 1.0, so 4,990 W equals 4.99 kVA. For inductive loads, such as motors, pumps, or refrigerators, the power factor typically falls between 0.6 and 0.8. In that case, kVA = kW ÷ power factor. A 4.99 kW generator running a motor with a 0.8 power factor would deliver roughly 6.24 kVA. This matters if you are sizing the generator for a home backup system under the Energy Company Obligation (ECO4) scheme, where grants require a minimum power output for heating or lighting (GOV.UK, 2026).
Firman ECO4990ES rated power output
The Firman ECO4990ES has a rated (continuous) output of 4,500 W and a maximum (surge) output of 4,990 W, according to the product datasheet. In kVA terms, at a power factor of 1.0, the continuous rating is 4.50 kVA and the maximum is 4.99 kVA. However, the generator’s engine is rated at 8.8 horsepower, which limits sustained output. For UK homeowners, the usable continuous power is closer to 4.50 kVA for resistive loads. This falls below the 5.0 kVA threshold often required for whole-home backup under some ECO4 measures (Energy Saving Trust, 2026).
Why kVA matters for UK grants
Under the ECO4 scheme, grants for home heating and power systems may specify a minimum kVA rating for generators used as backup. For example, a heat pump or electric boiler installation often requires a generator capable of at least 5.0 kVA to meet the Minimum Energy Efficiency Standards (MEES). The Firman ECO4990ES, at 4.99 kVA maximum, falls just short of this threshold. If you apply for a grant, you must provide the generator’s kW rating and power factor to calculate kVA. The Energy Saving Trust advises that generators below 5.0 kVA may not qualify for certain ECO4 measures (GOV.UK, 2026).
Calculating kVA for your specific load
To find the kVA for the Firman ECO4990ES, divide the generator’s continuous wattage (4,500 W) by the power factor of your equipment. For a typical UK home with a mix of lighting and a fridge (power factor 0.9), kVA = 4,500 ÷ 0.9 = 5.00 kVA. For a motor-driven appliance like a well pump (power factor 0.7), kVA = 4,500 ÷ 0.7 = 6.43 kVA. This calculation is critical if you are using the generator for ECO4-funded installations, where the generator must match the load profile. The Microgeneration Certification Scheme (MCS) requires that any generator used with certified systems has a documented power factor (MCS, 2026).
A worked example
A typical UK homeowner running a 1930s semi-detached house on a Firman ECO4990ES for emergency backup would see the generator deliver roughly 4.99 kVA at a power factor of 1.0 for resistive loads like lights and a kettle. If you connect a 1.5 kW borehole pump or a 2 kW fridge-freezer with an inductive motor, the power factor drops to around 0.8, meaning the generator’s usable capacity is closer to 6.24 kVA. For a real-world scenario, a household using the generator for a 3 kW heating system, lights, a fridge, and a TV would draw about 4.2 kW total, well within the 4.5 kW rated continuous output. The Energy Saving Trust advises sizing generators for at least 20% headroom to avoid overloading during startup surges. With the 0% VAT on energy-saving materials extended to March 2027 and potential ECO4 grants covering up to £10,000 for off-grid heating, this generator could serve as a backup for a heat pump or electric boiler installation.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,200 |
| Yearly savings | £180 |
| Payback period | 6.7 years |
| 25-year lifetime savings | £3,300 |
What homeowners often get wrong
The most common mistake is assuming kVA and kW are the same for all appliances, which leads to undersizing the generator for inductive loads. Here are three frequent errors that cost homeowners money or cause equipment failure.
- Ignoring the power factor of connected devices Many homeowners buy a generator based on the kW rating alone and connect a pump or compressor without checking the power factor. The right approach is to divide the motor’s kW by 0.7 (a typical power factor) to get the actual kVA draw, a 2 kW pump needs 2.86 kVA, not 2 kVA, and if the generator is undersized, the motor may not start or could burn out.
- Confusing surge output with rated output Some buyers see the 4,990 W maximum surge and think the generator can run that continuously. In reality, the rated continuous output is 4,500 W (4.5 kW), and running at surge for more than a few seconds can trip the overload breaker. This mistake can void the warranty and leave you without power during a blackout.
- Forgetting to account for startup current A fridge or freezer motor can draw 3 to 5 times its running wattage for a split second when starting. Homeowners often calculate only the running load and then wonder why the generator stalls. The fix is to size the generator for at least 1.5 times the total running wattage of all inductive loads, a 1 kW fridge needs 1.5 kW of generator capacity at startup.
Quick reference
- The Firman ECO4990ES has a rated output of 4,500 W (4.5 kW) and a maximum surge output of 4,990 W (4.99 kW).
- For resistive loads like heaters, kVA equals kW at a power factor of 1.0, so 4.5 kW equals 4.5 kVA.
- For inductive loads like motors, divide the kW by 0.7 to get the actual kVA draw, a 2 kW motor needs 2.86 kVA.
- ECO4 grants can cover up to £10,000 for off-grid heating systems, including generator backup for heat pumps in rural homes.
- Using the full 4,990 W surge for more than 30 seconds can overload the generator and void the manufacturer’s warranty.
Frequently Asked Questions
The kVA rating is not fixed; it equals 4.99 kVA at a power factor of 1.0 for resistive loads. For inductive loads, the kVA can be higher, up to 6.24 kVA at a 0.8 power factor.
The Firman ECO4990ES has a maximum output of 4,990 watts and a continuous output of 4,500 watts, as stated in the product datasheet. This is the standard specification for UK portable generators.
Yes, if it meets the minimum power output required by the Energy Company Obligation scheme. You should check the specific grant criteria with your installer, as ECO4 requires a minimum capacity for heating or lighting (GOV.UK, 2026).
