How many watts will you need in an emergency is the question and not easy to answer, you may find some suggestions very misleading.
It is not a simple calculation to make either with three different measurements needed;
- Continuous watts or when everything is running
- Peak or start up watts
- Surge current capabilities
Average is not enough
Looking at wattage charts for typical appliances is not really accurate enough, nor does it account for possible imbalance of loads.
Most US homes have 120/240 volt lines of wiring, so does a genset (remember a genset is standby or portable generator)
Sorry to say my friend if you thought you could measure your lump power consumption by measuring the rotation time of your leccie meter (advised by some bright sparks) because each of the outputs on your genset can only give you no more than half the total rated power.
So now you are thinking “How about finding the rated power of all my devices from the nameplates?”
My answer to you is “Sorry again, the rated power on your nameplates is not at all accurate, usually being around 50% above the actual operating level, so you could end up with an oversized over powerful unit!”
“So what can we do?” you ask me.
“I think I will write it all down for you.”
How to measure running wattage, step by step
- Decide which lights and equipment you will need in an emergency
- Ask your electrician to measure your electric consumption with necessary loads on
BTW: If you have electrical training and know your health and safety read on:
- You need a clamp-on amp meter with “peak” function for inrush currents
- You need line-workers rubber gloves
- Remove the front panel of the main disconnect box
- Enclose the line cable within the clamp-on amp meter
- Read the value of current on both “hot” wires
- Make several readings each within 10 minute intervals
- Use the largest reading and multiply by 240V
- So for example, if line 1 is 30A and line 2 is 20A, use 30A for measurement
- 30A x 240V = 7,200 volt-amps is your answer
- You may want to add 10 to 20% safety margin
- Adding a safety margin is for system derating will prevent false tripping of your generator circuit breaker
Did you say “What if I don’t want to do this measuring business?”
“Well, ok, fair enough, you can use this quick, easy guide as a benchmark, I will write it down for you”
For two or three essential loads such as;
- A fridge
- A small furnace
- Window A/C
You need 4,000 W 120V
For most items in a small home
You need 8,000 W portable 120/240V
Whole house with a central A/C
You need 15-20 kW standby
N.B. If you have a generator smaller than your requirements you can alternate the power by rotating the appliances. A fridge freezer will usually keep food frozen for some time, so may not be a priority in emergencies. The same goes if you just do not wish to purchase a larger more intelligent generator.
N.B Just to confuse you a bit more A/C in this case means Air Conditioning and not Alternating Current!
Because A/C units are a biggie, you may want a measurement for that alone
First you will need your;
- Clamp-on meter and gloves again
- Power off the unit
- Set for a peak reading
- You will need to clamp a single wire in the cable feed
- For cord-and-plug us an AC line splitter
- Take an extension cord and pull black lead out of the bundle
- Turn on the unit and take the reading
That’s it! Simple? No not at all.
And if that’s not bad enough, for a hardwired unit you would need to figure out how to reach a single string.
If I lost you on “you will need to clamp a single wire in the cable feed” then you definitely need an electrician!
Don’t create an emergency just for the sake of emergencies! You really do want to live to use your generator, not kill yourself in the process.
Now go check out these reviews on best rated portable generators to get yourself one for at home or at work!