Power failures are a common problem across the US. So, what do you really need in order to sustain power to the essential appliances in your home?
First, you need to determine what items you need to power during a power failure and how long. Here is a brief example (watt requirements vary):
Lights – About 200 watts
Fridge – About 1000 watts
Radio – About 50 watts
Heater – About 1000 watts
Total wattage needed is 2250 watts. The fridge and heater have a start up power requirement so let’s allow 2x the continuous wattage for start up requirements. 2250 * 2 = 4500 watts
Second, select a power inverter. For this example, you will need a power inverter capable of handling 4500 watts. The continuous power requirement is actually 2250 but when sizing an inverter you have to plan for the start up so the inverter can handle it.
Third, you need to decide how long you want to run 2250 watts. Let’s say you would like to power these items for an 8 hour period. Well this can be tricky because heaters and fridges run intermittently. Let’s assume all of the appliances will run 40% of the 8 hr period which is 3.2 hours of actual run time. We need to convert the ac watts to dc amp hours because that’s how batteries are rated.
To convert ac watts to dc amps per hour you divide the watts by the DC voltage (usually 12v or 24volts). Let’s use 12volts since it is the most common.
2250 watts / 12 vdc = 187.50 dc amps per hour
187.50 is now your power requirement per hour
You have now determined that 187.50 is your power requirement per hour and now you need to multiply that by total hours of run time which is 3.2 in our example.
187.50 dc amps per hour 3.2 hours = 600 dc amps
Because you are using an inverter, you want to calculate the loss for converting the power which is usually around 5%.
(600 dc amps * 5%)+ 600 dc amps = 630 dc amps per hour (this is how much power you need in an 8 hour period running your appliances 40% of the time)
Fourth, now that you know your total power requirement is 630 dc amps we can select a battery source. Most typical deep cycle batteries are 6 volts or 12 volts. I will give you two examples using each voltage.
12 volt battery example:
If you select a 12 volt battery rated at 100 dc amps you will need 6 or 7 batteries in parallel (I will explain parallel vs. series later).
630 dc amps / 100 dc amp battery = 6.3 batteries
6 volt battery example:
If you select a 6 volt battery rated at 200 dc amps you will need 6 to 7 batteries in series.
3.15 * 2 = 6.3 batteries
No, I didn’t make a mistake. When you use 6 volt batteries, you have to connect them in series to reach 12 volts.
What is series and parallel you ask?
When you connect batteries is parallel you are increasing amps. When you connect batteries in series you increase voltage. In the battery world, it is better to limit your parallel strings. It is better for your power system. In this example, I would recommend using 6 volt batteries because of the number of batteries this example requires.
How do we charge these batteries? You will need a charger to charge the batteries when you have access to city power. Most deep cycle batteries need a “smart” charger so the charger doesn’t damage the batteries. In this example, you will need at least a 40 amp charger if not bigger. The bigger the charger, the faster the charge. Make sure your charger is for 12 volt batteries because the system we just identified is a 12 volt system.
You will also need cables. For this example, a 4 AWT (0000) cable is required to handle 4500 watts of start up power. That is huge cable. You may also want to consider an inline fuse. A 500 amp for this example is perfect. To figure out the size of fuse you divide your ac watts (start up) by dc voltage.
4500 watts / 12 vdc = 375 amps
You would need a 375 amp fuse or bigger. I recommened a 500 amp just incase you were to max out the 5000 watt inverter.
This is just a brief example. There are many different ways to set up your system. You can use solar panels, wind etc.
Thanks for the clarification. Nice to have a site that explains in detail
Hello,
I have to do something with my energy bills!!!! Solar power seems to be an option. However, all I know is how to spell solar power! I must turn to experts in this area, so Hi experts. Am I in trouble or what!
I will read up on this stuff so I will be a little more knowledgable than I am now.
Question! What is the difference between modified sine wave sine wave and a regular inverter? I already noticed that price was a factor!!
Thanks for any info.
Have a good day
Andy
Hello Andy,
The difference between a modified sine inverter and a pure sine inverter is the type of wave coming out of the inverter.
Pure Sine Wave
The sine wave produced by an analog pure sine wave inverter, is very similar to that of the digital pure sine wave inverter. The key difference is that the analog switching causes noise or static on the ac wave.
generally most appliances, motors, microwaves, chargers, and power tools will produce full power and not cause any buzzing or negative effects.
these types of pure sine inverters are not recommended for medical equipment unless manufacturer approved.
use this inverter for electric shavers and emergency flashlights, garage door openers, laser printers and large strobes used in photography
Modified Sine Wave (step sine)
A modified sine wave inverter actually has a waveform more like a square wave, but with an extra step. A modified sine wave inverter will work fine with most equipment, although the efficiency or power of the equipment will be reduced with some.
Motors, such as refrigerator motor, pumps, fans etc will use more power from the inverter due to lower efficiency. Most motors will use about 20% more power. This is because a fair percentage of a modified sine wave is higher frequencies – that is, not 60 Hz – so the motors cannot use it.
Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises.
Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the peak of the line power – basically, the modified sine has a flat top rather than a peak – this may cause the occasional double trigger. Because the modified sine wave is noisier and rougher than a digital pure sine wave, clocks and timers may run faster or not work at all.
Items such as bread makers and light dimmers may not work at all – in many cases appliances that use electronic temperature controls will not control. The most common is on such things as variable speed drills will only have two speeds – on and off.
most equipment will operate without any noticeable difference, and because the lower cost, makes this the most common inverter sold and generally the only type found at your local retailer.
Hope this helps!
I wish to power a 3/4 horsepower, 220v AC submersible water pump with a 12 DC battery, charged by solar panels. The pump is in operation today driven by the commercial electrical grid.
The pump does not run continuously, but only operates when the tanks go dry, which happens three or four times per 24 hour period.
When the pump operates, it runs for approximately 4 minutes.
I would appreciate your recommendation for equipment from your Firm sufficient to accomplish this task.
Douglas B. Kenney
dkenney@ix.netcom.com
(570)223-9320
71 White Birch Lane
East Stroudsburg PA 18302-8340
Hello Douglas,
1 HP motor equals about 750 watts so if you are running a 3/4 HP pump you will need about 560 watt inverter but pumps can take 2-3x the wattage just to start so I recommend an inverter that is around 1500-1800 watts to handle the start up of the pump. We have an 1800 watt on special for $199 with free shipping and cables. If you would like the to switch between city power and inverter power automatically you will need a transfer switch. We have a separate transfer switch or you can purchase our 1500 watt with a built in transfer switch and charger. It sounds like you have a battery and solar panels but you will need a charge controller between the battery and panels which we also sell.
See http://www.theinverterstore.com/the-inverter-store-category.php?cat=MSW.
Well put article. Will visit soon:
i wish to power a 1.5 hp air compressor that’s 10.5 amps @ 60hz i need it to pump my tires from 20 lbs back up to 40 lbs, (4×4 at beach),so it will only be running for 5 to 10 min. what size inverter would i need?
I am so glad I found you guys. This is the best info for the DIY er on the internet. You will be getting some of our business and we will add links for you at our website.
Keep up the good work!
in a whole house power back up system
(or a solar battery array type configuration, with multiple batteries for lengthened reserve times)
with all loads being the same,
would it be better to run, say 4 different loops with four different inverters at 12v, 24v, or 48v dc?
My charging circuit is versatile enough to be used for any of these voltages, so i’m interested in the efficiency aspect.
It’s stronger than a standard trickle charger, and can take load, which should help with run times. (i’ll find out when i throw the switch! LOL)
whats going to be most efficient, so as to consolidate space and not have my garage look like the battery rack at autozone?
thanks!
Chris
Hello Kenneth,
If you multiple 10.5 amp * 120 volts you get 1260 watts. However, compressors have very high start ups so you may want to plan for 3x that just to start the compressor. I would recommend our 5000 watt just to start the compressor. You don’t want to prematurely fry your inverter by not selecting one big enough. Your DC battery drain is 10.5 amps * 11 (converting ac amps to dc) which is 115 amps per hour. If you only need to run for 5-10 minutes your car battery should be fine.
Hello Chris,
This all depends on the total amps you plan on running and for how long. If you have that information, I can make a recommendation.
Powering the house by an inverter, will it take as much electricity to recharge the battery bank using a charger from the grid as it would powering the house directly from the grid?
Pete
have 1000 watt.wany to power rv ac/dc inverter to charge batt this well not do itmabe a 2500 what do we think
Can you recommend a book or reference site that can competely explains how Solar Panels and Inverters connected to the Grid could be done.
Grid Tie systems are very complex and the information changes from location to location. I would recommend you get in contact with a local expert or your power company.