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Home Generator Question

Home Generator Question

Home Generator Question

I am new to this home generator concept. After monitoring my KWH usage on my "smart meter" on my home, I see that even with spikes, I do not go above 3KWh, I stay around 1.6KWh all the time in my home. Why is everyone telling me I need a 20KWh generator??? Shouldnt I be fine with a 5KW generator at most? I cannot find any information on this, Thank you

RE: Home Generator Question

Probably, you meant "kW" not "kWh".

RE: Home Generator Question

The units you're using are a bit confused. kW is a measure of power at an instant in time. kWh is the energy consumed over a period of an hour. The size of the generator is determined by the required kW, while the volume of the fuel supply is determined by the kWh.

Generators are sized based on both kW (power) and kVA (apparent power). The engine is rated in kW, while the alternator is rated in kVA and the two quantities are typically in a ratio of 1:1.25 or thereabouts: the chances are the generator you're being advised to get is 20kVA which will have an engine of about 16kW or so.

You need to consider your peak power requirements when sizing the generator, not the energy consumption. Some peaks don't last long enough for most meters to register them, for example when a motor starts. Knowing more about the loads you need to supply would be helpful. Loads like electric cooking appliances take a lot of power but not on a continuous basis because they cycle the heating element on and off. Washing machines need a lot of power and have a powerful motor so they are unfriendly loads for a small generator.

All other things being equal, the bigger generator will do a better job of controlling the frequency - i.e. the speed the engine runs at - and at keeping the voltage stable as loads cycle on and off.

One word of caution - don't grossly oversize a diesel generator so that it runs at very light load. Diesel engines don't like idling and you'll damage the engine if you run it with very light load for long periods. Petrol (gasoline) or LPG generators don't suffer in the same way. At this power level you could be offered either, although I suspect the petrol type will be the cheapest to buy.

RE: Home Generator Question

A couple of things to get the discussion rolling.
First of all you need to make sure that you have all your terminology correct as you seem to be mixing kwh, kw and kva interchangeably.

Your smart meter will probably be measuring kwh and able to give you a peak demand in kw.
KWH is a measure of energy
KW is a measure of power.
A lot of gensets are rated in terms of KVA, which is kilo-volt amps. In a DC system KVA and kilowatts are interchangeable but in AC there is another parameter called power factor which is influenced by the inductance or capacitance of the load that it is connected to. A fully resistive load has a power factor 1.0. A highly inductive load such as a lightly loaded electric induction motor will have a power factor of 0.5 or less.
I am thinking that the 20kwh that you have been given is a solar panel rating for how much power you need to generate to offset your day time consumption.
A 5kw generator may well meet your need but it will depend on how big your peak load is and what causes that peak. I am not sure that a smart meter measures the instantaneous peak but average that peak over maybe 10 secs or so. However that generator has to deal with that peak. If for example the peak was caused by a refrigeration unit starting up then you may need something much bigger than 5kw. However if the peak was caused by you progressively turning on 30x 100watt incandescent light bulbs over several minutes 5kw would be plenty of capacity

"Any water can be made potable if you filter it through enough money"

RE: Home Generator Question

For what its worth my off grid house has a 1.5 kW solar array (50V at 30A) and as a backup I run a 4 kW diesel generator. I can trip the overload on my generator if I run the 700W immersion heater plus my 1800 W electric chainsaw, plus my fridge, 250 W, while charging the batteries at ~1000W. Of course the toaster, microwave and vacuum cleaner, and any number of pumps, could also kill it.

The simple answer is to run only one high power device at once, or buy a bigger generator.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Home Generator Question

What is the intent of the generator, is it meant as an emergency back-up or are you going off-grid?

If it's an emergency back-up you can switch off everything non-essential and you can switch between larger loads (e.g. don't use the oven and the air conditioning at the same time) in the interest of cutting down your generator size.

I have a 2 kW Yamaha inverter generator. If there's a major power failure I'll run an extension cord from the generator outside on the deck to the furnace and to the refrigerator and call it good enough.

If you are going off grid, or if you insist on trying to live a completely normal and uninterrupted life in the event of a power failure, you are not going to want to do that.

RE: Home Generator Question

I have done this a number of times.
The following comments apply mainly to standby or back-up generators. A prime power or "Off the grid" installation may have greater capacity.

I ran my own home on a 6 KVA generator for several months. We had to watch our loads and we couldn't use our clothes drier. We don't have A/C.
I installed six 15 KVA sets for backup in houses with A/C. I did not size the sets. All were too small. Within a few years I replaced all six of the sets with larger sets. Depending on the number and size of the A/C units and the owner's willingness to restrict his loading, the replacements ranged from 20 KVA to 50 KVA. On all but the largest set I had to disable automatic restart on the A/C units so that they would not all start at once.
If you are peaking at 3 kilo-Watts that may be a motor running. To start a motor drawing 3 kW you will need between 9 and 18 KVA capacity.
If you are peaking at 3 Kilo-Watt-hours, that information is not really helpful.
"I have a pile of bricks to move. I think that each brick weighs 4 pounds. How big a truck do I need?"
"Correction. My friend just checked the weight of a brick and they are only 3 pounds each. Will I need a bigger truck or a smaller truck?"
Back to reality;
Take the sum of all your loads in KVA. Now add twice the KVA of the largest motor or motors that will start simultaneously.
That is all other loads PLUS 3 times the KVA of the largest motor.
That is the minimum size.
If you have voltage sensitive equipment you may have to increase the motor factor from 3 times to 5 or 6 times.

A small standby diesel generator has a rough life. It may be called upon to start cold and within less than 10 seconds be hit with a transient overload of 500%. Then it may spend extended periods of time running at a very low load level.
I have had a couple of new sets glaze the cylinder walls and within an hour or so pump out enough oil into the exhaust as to shut down on low oil pressure.
Both of these sets were later replaced with larger sets as both were too small to start the A/C loads even though they often ran for hours at very low load levels.
The repair was to "Load bank" the sets for about five days to wear in the rings. After that the sets would take light loading without pumping oil. (Sometimes called "Slobbering")
The best solution for a residential installation is a natural gas engine or a propane engine running of the heating supply of gas.
The worst solution may be a gasoline engine, despite the cost difference.
But sometimes "You gotta do what you gotta do!

"Why not the best?"
Jimmy Carter

RE: Home Generator Question

Waross post above hits the key point that motor starting loads. They are one of the more difficult loads to compute for generator sizing. Depending on the motor design and attached inertial load, anything less than motor rated locked rotor current draw is risky. As already stated can range from 3 to 6 times the run current.. Failure of a motor to come up to speed quickly can burn out the motor start windings.

For residential environments, if submersible well pumps are used, they can be one of the more difficult loads to start (ref: Franklin submersible motor technical literature) for the following reasons. Narrow diameter motor design (to fit in the well), the inertial and pressure load of a head of water sitting on top of the pump, typical considerable long wiring from service distribution panel to well head to motor depth in well all add up to the upper end current starting requirements. Other motors driving pumps (e.g. compressors in heat pumps/ air conditioning, refrigerators, etc) need to be addressed..


Investigate your motor loads carefully, and perhaps put some extra capital in up front for reserve generator capacity or keep it handy to replace burned out motors.

RE: Home Generator Question

A note about manufacturers sizing charts. Bigger means more profit.
I sized and installed the replacement sets that I mentioned earlier. Years later I came across a manufacturers sizing program.
The program told me that all of the sets were undersized, despite that the customers had been happy with the performance for years.

There are always exceptions for special cases and larger systems, but for the greatest majority of residential installations the main concern is getting the largest motor started.
In a typical installation, if you can start the largest motor, the set is easily large enough and will run most of the time at less than full load.
1. Add up your total load in Amps.
2. Multiply this by the voltage you will use and divide by 1000. The answer will be in KVA.
3. Multiply the Amps for the largest motor by 3.
4. Multiply this by the voltage you will use and divide by 1000. The answer will be in KVA.

Add the KVAs
That is the size of generator you need in KVA.
Total load without largest motor = 18 Amps @ 120 Volts = 2.6 KVA
The largest motor draws 7 Amps @ 240 Volts.
7 Amps x 3 = 21 Amps
21 Amps times 240 Volts = 5 KVA
2.6 KVA + 5 KVA = 7.6 KVA
I would look for an 8 KVA Generator. (Or an 8 kW generator. In that size KVA often equals kW.)
The maximum running load will be the base load of 2.6 KVA plus 1.7 KVA for the motor = 4.3 KVA.
4.3 KVA on an 8 KVA set is a maximum continuous load of 53.75%
That is with everything turned on.
Much of the time the load will be less than 40% and with the motor off the load may be less than 20% of maximum load.

"Why not the best?"
Jimmy Carter

RE: Home Generator Question

Waross is the pro at this.
For example, I have a well. The steady power is only 2hp.
I couldn't start is with a 5kVA generator. The voltage dropped too much.
My 7.5kVA (steady rating, allegedly it will do 8.5 peak) will start the well pump.
All that I power is my well, the fridge, freezer, furnace blower, and some lights.
I figured that to run the AC also would require a 25-30kVA nat gas unit. I'll do it at my next house.

I should also mention that the larger 'portable' units 5kVA-12.5kVA are not inverter units, they will run at full speed at all times.
This makes them noisy and they suck a lot of gasoline, a real lot.
I can manage a couple of 36hrs with the 20 gal that I have on hand.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Home Generator Question

Speaking of inverters, it should be possible to size a house generator to run at near peak/ideal engine load just to meet the steady state needs of the house, and cover motor starting and other peaks with a couple of large truck batteries and a serious inverter.

I am not aware of anyone attempting to sell such a system, because of course it could not be cheap enough to compete with simpler systems at home supply stores, and nobody thinks about fuel consumption (or oil consumption) when considering their first generator.

Mike Halloran
Pembroke Pines, FL, USA

RE: Home Generator Question

Agreed Mike and I wonder why no one has made a much LARGER inverter generator. A whole house inverter generator could be a real boon for efficiently running a home during multi day power failures.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Home Generator Question

itsmoked, its probably due to cost, perceived or otherwise.

Its entirely possible here in Australia to purchase a generic small petrol generator for a few hundred dollars, a better quality synchronous machine with a brand name engine can still be had for well under a thousand, for a 2kVA unit. The equivalent brand name inverter unit is at least two thousand.

For equivalency's sake, to allow for the big motor starting, a house that could use a 2kVA unit would probably still need at least 6kVA worth of inverter capability, possibly more. That's a lot of extra semiconductor to pay for, when the equivalent spend on a synchronous unit would get you about 6kVA.

Of course, the market seems to be bypassing some of the argument and looking at battery storage, with larger inverters, but that's a whole different cost level.

Mike's right, of course. For domestic units no one wants to spend that much.

RE: Home Generator Question

When I had to run my home on a generator, I started with a small gasoline unit.
Two problems;
1> It would not run all night on a tank of gasoline. Someone had to start it every morning at about 6 AM, in the dark, at minus 30 C.
2> It quickly became apparent that gasoline costs would be in excess of $1000 per month.
The diesel unit would run 3 days on a tank of fuel and could be refuelled while running.
It soon became apparent that the greatest fuel consumption was during the night time hours when nothing was turned on but the set was left running in the event that one of the furnaces cycled on. Also, in my case, there was the issue of starting a cold set in very cold weather.
Running costs:
The fuel to keep an unloaded or lightly loaded engine running for a lot of hours is often greater than the fuel to run a loaded engine for a few hours.
For continuous running the fuel cost is important.
A standby set typically runs a few hours a year. Fuel cost is much less important.

"Why not the best?"
Jimmy Carter

RE: Home Generator Question

Hi Mike:
When I read your post, I immediately saw some problems.
Then I saw some solutions.
Your description sounds like peak shaving.
The generator supplies the load and the inverter supports overloads or peaks.
The frequency of small generators is not all that stable. In a good quality diesel set the frequency drops (droops) 3% from no load to full load by design. Most users never know and this makes a very stable governor control system.
Under block loading there may be a transient excursion of more than 3%.
When the maximum size motor is started, the voltage may drop 15% or 20% or more. Some systems are designed for as much as 30% voltage drop on motor starting.
Frequency drop is a lot more than 3%.
To effectively share motor starting peaks between an inverter and a small generator the inverter may have to be both frequency agile and voltage agile.
Anecdote time.
I was consulting for a sawmill in Central America. At times the mill would be running on diesel power. There were two large sets that could be run in parallel.
Both sets were needed to start a 300 HP motor.
One set had an electronic governor and one set had a hydraulic governor.
When the hydraulic set was cold, the electronic set would react to the motor starting surge faster and hog the load. The breaker would trip. That would dump the load on the other set and that breaker would trip.
If the hydraulic set was run for 20 to 30 minutes to warm up the oil the governor response would be fast enough that the sets woud share enough load to start the 300 HP motor.
Anecdote off.
I can't see an off the shelf inverter being able to share large fast transient loads with a small generator. Yes a suitable inverter could be designed but given the relatively tiny market the development costs may be prohibitive.
BUT, a possible solution.
Use an inverter to carry the entire load including motor starting.
Use the generator to charge the batteries.
The load profile may be broken into several groups.
1. Light loads.
2. Normal loads.
3. Full loads without motor starting.
4. Full loads plus motor starting.
The generator may be AC or DC and will be used to maintain the battery charge.
1. Light loads; The generator may be off and be started as needed based on battery voltage every hour or so.
2. Normal loads; The generator may be run at the most economical speed at which it is capable of maintaining the battery charge.
3. Full loads; The generator may be run at full speed.
4. Motor starting; This is a transient event that lasts possibly less than a second, (Peak starting draw) and may be ignored.
This should work, and for standby use, can be in use while on the grid (The grid will keep the batteries charged without the generator) and give a seamless transfer to standby.
Good idea Mike, we can make this work.

"Why not the best?"
Jimmy Carter

RE: Home Generator Question

Best Power Products UBS product line, marketed many years ago, was a version of the concept described above. We had several customers using these systems. The generator model in the pic was a 120VDC 17.2kW natural gas powered engine for charging a 120VDC UPS battery bank and carrying the UPS load.

It was designed to be used in conjunction with their UPS product line. I no longer have access to the documentation on this unit and web searching did not find any details to quote. If I remember correctly, our customer installations simply had the generator start after some delay when the loss of commercial power was detected and the engine would throttle according to charging/load requirements.

Later sold and installed Alpha Technologies UPS systems. Never used any of their generators. Their 7.5kW AlphaGen is a variable speed generator system https://atl.app.box.com/v/alphagen for 48 or 96 volt battery systems powering a suitable UPS or inverter.

My home solution uses a commercial Best Power UPS that powers a standby branch circuit (as defined by NEC) with around 700AH of battery power to run from, that carries continuous (light duty) essential lighting, TV, computers, network communications and security system loads. I have run it close to 4 days 24x7 before recharging batteries during a major power outage. Its ferro-resonant design offering 2000:1 voltage spike protection is the second reason for using this unit.

With the above system during a week long power outage from an ice storm with 10deg F daily temperatures, and with a neighbor family staying with us during the outage, I ran the 8.5kW LP powered generator only about 4 hours total a day.. 2 hours in the morning for electric cooking & water heating and well pump (one at a time) , and refrigerator and battery charging.

Heat was by wood stove.

The problem as mentioned several times is cost.. Battery costs even at wholesale pricing has grown really high, not to mention the higher capability UPS and inverter systems. I put what I have together when I had access to good used commercial equipment and used 10 year design life sealed AGMs that were being switched out of critical accounts on 3 year intervals.

Scotty I agree with you on the cost/repairability. Ran into that problem on several major name brands of commercial systems, trying other major brand name systems that turned out to be overseas-outsourced design and build, and not well understood by their own product support. .. and then found Alpha Technologies out of Vancouver, BC They make great products using current IGBT transistor technology, well supported, but expensive.

RE: Home Generator Question

On the subject of inverter v's rotating generators - most rotating types can be repaired given a bit of effort, even if the machine is damaged to the point of rewind. Inverter types aren't, or at least the ones I've looked at as repair prospects had all had a Very Bad Day event and were write-offs. The engine-generator were operable, it's the power output stage that dies and spares are either prohibitively expensive or unavailable and designed to be non-repairable. I suspect the inverter types are much less tolerant of abuse and load-side faults than the iron & copper rotating types although I can't present hard evidence to support this gut feeling.

I would think carefully about investing serious money into an inverter type unless I was confident about repair support and costs.

RE: Home Generator Question

Nice presentation Dan.
Thanks for sharing.

"Why not the best?"
Jimmy Carter

RE: Home Generator Question

Yes Dan! Sounds exactly 'right'.

Bill; Nice succinct statement of operation. Thanks.

Scotty; Reliability point taken.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Home Generator Question

For micro-grid applications, Xantrex makes battery inverters that can be set to share with DG supplemental generation. They simulate the frequency and voltage droop of the DG set so they will help share startup surges with the DG.

We're talking about big units though, I doubt anyone isaking them for home size systems.

RE: Home Generator Question

angelarr - see if your smart meter can give out the instantaneous usage figures which will be much more spiky than the smoothed KWH reading. As noted here many times on a great discussion forum, the peak load can be several times higher than the steady state load. Large motors ( vacuum cleaners, saws etc) can require a lot of power to get them going.

Think of it like a car - If you've ever been able to get data on instantaneous fuel consumption you find accelerating, especially quite hard, gives fuel economy figures of < 10 MPG and lots of KW. However quite soon you stop accelerating and cruise at a steady speed. fuel economy figures leap up to say 40MPG (less KW). Depending how much stop start you're doing your overall fuel economy figures (KWH) will either be a bit less than 40 or maybe only say 25.

Hence unless you can live with accelerating at the scale 10 times less than you want to, you need a motor capable of much more power than you need for steady state cruising at low to medium speeds.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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