Generator Buying Guide

Author: Paul Napthali   Date Posted:12 February 2016 

There is a lot to consider when buying a generator that will suit your requirements. Sticking to trusted brands will help you safely navigate through the quality and performance issues of some of the lessor known, often cheaper brands...

How To Choose The Right Generator

 

Size does matter!  In a perfect world, when camping, the better you can match your generator output to your power requirement, the safer and more efficient all round it will be.

If you try to use a generator too small, the power supplied may be lower than specified by the manufacturer of your appliances.  This can lead to longer duty cycle times reducing the lifespan of some of your appliances and your generator itself by working harder and longer than it needs to.

By using a generator too big, you lose operating efficiency.  i.e., (extra fuel usage and not to mention the extra expense of the initial purchase of a bigger generator if it is not required). 

There is a lot to consider when buying a generator that will suit your requirements. Sticking to trusted brands will help you safely navigate through the quality and performance issues of some of the lessor known, often cheaper brands. If you’re happy with the brand you have chosen, the main thing to get right is matching the capabilities of the generator to the load drawn from your appliances.

 

How to determine the right size generator

 

Power output can be measured in watts or kVA.  Most appliances are rated in watts, this makes sense for generators to be rated in watts also, but unfortunately this is not always the case.

(Without getting deep into the mathematical theory of electricity generation and how it is measured, I’ll try explain the fundamental difference between the 2 power measurements in a practical way that would relate to choosing the right generator for your needs).

Watts can be converted to kVA and vice versa because they both measure power, but this is not the problem.  The problem is that there are 2 different ways of measuring power, basically total power supplied (generally measured in kVA) and total power actually used (generally measured in watts) are 2 different things.  

Generator manufacturers like to rate their products on what they can supply and appliance manufacturers like to rate their products on what they actually use.  No one wants to claim the approximately 20% ‘wasted power’ in a sense. So as a general rule for the caravan industry, if it is rated in kVA, convert this to watts and subtract approx 20% to show ‘real’ useable watts.  Conveniently on the other hand, if it is already rated in watts, this has already been done for you.  Also an important point to factor in, is that a generator is designed to run efficiently at no more than 80% of its maximum output or peak watts.

Watts: Is basically a measurement of what you would call ‘real’ power, it is the measured amount of useable power that an appliance draws. Most appliances are rated in watts, so if your generator power supply is rated in watts and you know your appliance wattage rating, it makes for an easy and somewhat accurate comparison – apples with apples.

kVA (kilo Volt Amperes):  A measurement of what we call ‘apparent’ power, which when converted to watts is generally a higher value to real useable power (apples and oranges). The useable ‘real’ power component is a percentage of the theoretical ‘apparent’ power.  What determines the percentage is the efficiency of your appliance, this is called the Power Factor (PF).     

Generally speaking, the ‘real’ power drawn by appliances in the caravan RV and camping industry are calculated on an average power factor of 0.8, which means on average the appliances will use 80% of the apparent (kVA) power supplied efficiently. (the other 20% is a byproduct in a sense and technically supplied but not used).

Putting this all into a practical sense it would look like this:

Watts and kVA can both measure power, people just choose to associate watts to ‘real’ power and kVA to ‘apparent’ power, although both can be used to measure either.

Example: If you want to know what a1 kVA generator can theoretically efficiently supply and run, you need to derive the ‘real’ rated watts from the kVA rating which can be worked out like this: kVA to VA, VA to ‘apparent’ watts, ‘apparent watts to ‘real’ watts, ‘real’ watts to rated ‘real’ watts.

1 KVA = 1000 VA. 1000VA = 1000 ‘apparent’ watts.

To convert 1000 ‘apparent’ watts to ‘real’ useable watts, multiply by the power factor. (Assume 0.8 if unknown)

(1000VA = 1000 watts) x (Power Factor of 0.8) = 800 ‘real’ watts (approx 20% less than ‘apparent’ watts)

800 ‘real’ watts would be the maximum or peak watts. Subtract a further 20% to find your rated wattage output.  This is the rated output a generator is designed to run at for best efficiency and lifespan of your generator and appliances.

800 watts less 20% = 640 ‘real’ continuous running useable rated watts

Therefore theoretically 1 kVA = 640 watts normal running output power.

There is 1 more important consideration: Start up power required.  A general rule of thumb is that the startup power required is 2-3 times the running power required of the appliance.

For example: A 400 watt appliance could potentially need 1200 watts start up power (400 watt x 3).  Best case scenario would be 800 watt start up power required (400 watt x 2).  Theoretically for best performance and longest life from your generator, the biggest single appliance you can run from a 1kVA/800 watt generator would be 400 watts at best. Of course if you have several smaller appliances you could use the majority of your capacity by staggering the startup times so as not to overload your generator.

 

 

Fuel type - petrol or diesel?

 

Generally diesels will last longer and are cheaper to run, but are more expensive to purchase. In smaller applications like the caravan industry, performance between petrol and diesel is comparative across the board.  It is really a matter of personal preference and how often you will use it. In a nutshell, both types will do the job required, but the more you use a generator, the more attractive a diesel type becomes.  For this reason, in general larger commercial applications, diesel powered generators far outweigh their petrol counterparts in nearly all aspects.

 

Traditional or Inverter?

 

Inverter generators are taking the Caravan RV and Camping industry by storm for many reasons, the main reason is for their pure sine wave technology.  This basically means an inverter generator can produce constant pure AC power which is suitable for sensitive electronics by eliminating the possibility of power surges. 

Other advantages that makes inverter generators a clear winner for our industry are that by comparison to traditional types, they are smaller and lighter making them more portable, fuel efficient, quieter and can be connected in parallel to double your power output if needed. The only real drawback is the higher price compared to the same output rated traditional type.  Although prices of inverters have come down considerably in the last few years, there complex design and sophisticated electronics will always demand a higher price point to their traditional counterparts.  

 

 

Basic small caravan/RV power usage

 

80 Litre fridge/freezer – 55 watts

2 x lights – 60 watts

Stereo or TV – 200 watts

Phone charger – 5 watts

Air conditioner (13,500 BTU) – 1300 watts

 

Total running watts = 1600 watts or (2 kVA applying a 0.8 average Power Factor)

A general rule of thumb is to allow 2-3 times the running wattage of your biggest appliance for startup.

In this case, for the 1300 watt air conditioner you would need to supply a minimum 2600 watts (3.2 kVA) to possibly 3900 watts (5 kVA) to start your air conditioner without anything else running at a minimum. Check the manufacturer’s specifications on rated running and startup watts required.

Note: Whilst smaller rated generators will possibly start and run your appliances, for longer life and best performance of appliance and generator, it is best to match your generator power output to your appliance power draw as close as possible.

Take the air conditioner out of this equation and the now 300 watts power required would be easily handled by a 1 kVA/800 watt generator

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