KW or KVA: What is the difference?
kW = Kilowatts
1 kW = 1,000 Watts
In the section just below, we’re going to talk Watts and Kilowatts, because people are generally more familiar with these ratings. Then further down well talk about current spikes, kVA and generator capacity (output).
Typical domestic electrical loads in Kilowatts
In New Zealand, because our domestic socket power outlets typically have a 10 Amp rating …
- electric kettle/jug is probably about 2 kW
- electric hob in the kitchen can be up to 6 kW, if all the rings are on
- electric immersion heater in the hot water cylinder will be either 2 kW or maybe 3 kW
- Washing machine ~2 kW
- dishwasher ~2 kW
- large heat pump for your living spaces are usually about 3kW
- electric light bulbs may be 20W each for older fluorescent types or 14W each for LED. A typical house might have 30-50 of these which will total about 0.7 kW, if they were all on at once
- fridges and freezers – usually quite small at about 75W (0.075 kW)
Rural properties typically have pumps for clean water supply and septic tank waste disposal
- clean water supply pump is about 1 kW
- septic tank pumps can vary from small continuous-flow aeration pumps to larger high pressure water pumps. So between 0.5 kW and 1.5 kW depending on technology and conditions / events.
How much do you need? Lets talk loads …
In normal life we generally don’t switch everything on all at once. Some of these electrical loads we have a high degree of manual control over. Other things are semi-automatic and we only have partial control over. Others are generally fully automated.
Manual
These are the electrical loads we control directly by switching them on and off as required. For example the kettle, lighting or the cooker hob.
Semi-Automatic
Our dishwashers and washing machines have internal water heaters that consume up to 2KW, but they don’t heat the water all the time the washing machine is running. These internal heater elements only come on intermittently during the programme as required. We start the machine manually, then it does it’s own thing.
Automatic
Hot water cylinder immersion heaters come on for a while to reheat the water when the thermostat detects a drop in water temperature (usually after you use some hot water), then it clicks off again when it reaches the target temperature. These consume a lot of electricity for a short time.
Septic tanks often have small aerator pumps that run 100% of the time, and/or they have large water pumps to push a batch of treated water from one chamber to the next, or out to the field. These consume a lot of electricity for a short time.
Fridge and freezer motors run a high percentage of the time, but this will depend on the insulation, how often you open the door, how much food is inside, environmental conditions etc. These are pretty small electrical loads, but they are on quite frequently.
So basically you add up all the known kW for your Automatic devices (including spikes – see below) and then add the kW for the semi-automatic or manual devices which you might want to use simultaneously. For example if you can remember not to turn on the kettle at the same time as the dishwasher, then you only need to add one of them, not both.
Spikes !!
We need to cover off spikes – huge jumps in the electrical load (current). Urban dwellings will generally not have equipment that causes significant spikes, but in a rural environment we certainly do. The problem is that large electric motors (eg water pumps, septic tanks, compressors, macerators etc.) have what is termed an ‘inrush current’. Every time these motors start, they suck in huge amounts of power; often around four times their normal running current, or more. However, this only happens for a fraction of a second and then it drops back to normal, but your generator has to be able to deal with this. So if you add up the kW of the items you might be running during an emergency, you then need to factor in the spikes. For example a 750W pump motor might very briefly try to pull in 4 kW to get started.
You might have two or three large pumps. You probably wont normally be starting them all simultaneously … but it could happen. After the power has been off for a while, the water pressure drops because someone opened a tap and/or a septic tank chamber fills up and needs to pump out. As soon as the power comes back (perhaps from your generator) they might all try to start at once, creating a massive load spike. This can overload a generator that is too small for the job.
Generators usually have a safety cut-out built-in (miniature circuit breaker); so an overload situation will simply flip the MCB out and the generator is not damaged, but you will be unable to reconnect your property to the generator until you resolve the overload situation – or else the MCB just flips out each time.
In this situation you’d need to identify which devices are likely to be causing the problem and manually turn them off at the main fuse board then try reconnecting the generator. Once the generator is running and connected OK, you can try turning these things back on, one at a time to reduce the size of the spike.
Power factor
This can get complex …so we’ll keep it simple.
At school you probably learned that Watts = Amps x Volts. This is true for simple direct current (DC) loads. But for alternating current (AC) and especially those pesky large motors again, it is no longer true. In these situations the current gets out of sync with voltage, and we have to bring in a thing called the Power Factor.
The problem is those large electric motors again. Basic urban type (resistive) electrical loads, are simple and have a power-factor of one. This means that 1 kilowatt of power is the same as 1 kilovolt-Ampere. For things like lighting, electric hobs and immersion heaters the power factor will be close to one. But those motors (inductive loads) drag it down to about 0.8.
This means that when if you’ve calculated your load by adding up the kW for each device, and for you this was 5kW, you need to multiply that by 1/0.8 or 1.25 to get the kVA. For example 5kW x 1.25 = 6.25 kVA
kVA (kilo Volt-Ampere)
The kilo prefix here means 1,000 (written in lower case). The V for Volts, A for Amps, and W for Watts are capitalised because they are named after people.
For alternating current – like most mains electrical supplies.
- Watts = Amps x Volts x Power Factor
- kVA = Kilovolt Amps = Kilowatts x Power Factor
Generator capacity is often expressed in kVA, because its a bigger number than the kW capacity and makes it seem like you’re getting more for your money.
However, it is generally safe to assume that your Power Factor will be about 0.8 for a rural load. So your 8 kVA generator can only really supply about 8 x 0.8 = 6.4 kW
Matching engine size to alternator requirements
Start by calculating how many kilowatts or kVA you need to power your necessary equipment. Then add maybe 20 or 30 percent as contingency. That gives you the alternator capacity you need.
From that, you can calculate the prime mover (engine) output you need drive that. Engines outputs are often specified in horsepower (hp). There are old imperial and newer metric versions of this unit but the different is relatively small (746 vs 735 kilowatts respectively).
So divide your required kW value for your alternator by 0.735 to convert to horsepower.
For example:
- You determine you need about 5 kW to run your electrical stuff.
- Multiply 5kW by 1.2 to give 20% contingency = 6kW.
- Then to get horsepower: 6kW x 0.735 = 8.1 hp.
- So you need a generator head (alternator) that provides at least 6kW
- Once you have selected the alternator you need an engine which provides just a bit more power than the alternator requires.
Remember that with a Power Factor for your load of 0.8, that 6kw of power can also be stated as 7.5 kVA.
You should have a fairly closely matched engine and alternator. If your engine is larger than necessary for your alternator, you’ll just burn more fuel than you should, with a loss of efficiency. If your alternator is too large for the engine, the engine will be overloaded and unable to deliver the power you need.