Alternate Energy, Part II -- Sizing the Inverter and Battery : LUSENET : TimeBomb 2000 (Y2000) Preparation Forum : One Thread

So, youre going to use a generator or some solar power, eh? How much can you use?

This is basic stuff, a primer. It is intended to let you surf commercial sites with a little background and confidence. Youll need to learn to read specification sheets and to tie things together.

Consider going to the following web sites for further information. Roy, please come in and leave your site address as well -- Roy Butler has helped a lot of people prepare for Y2k.....hes a very knowledable professional, and no, I dont own a piece of his company -- Four Winds -- but wish I did. :) Sorry, I lost your address when my puter bombed a while back:

Four Winds (Roy Butler will provide the addy:


Jade Mountain

Mr. Solar

{well see if I can fake my Word out and get these links in this time. If they dont go that scream will be me}

Dont think that you will be able to put in a few solar panels or plug in a generator and go through an extended power outage as if nothing happened -- unless you have a bank account large enough to consider buying Bill Gates island.

In this thread, well look at sizing a system. Well do it by seeing what it takes to provide the following electrical power:

1. Some lights. We will assume youve done the homework and replaced your incandescent bulbs with low energy fluorescent lights. Well let you light 4 of these, each using 20 watts. Youll have the lights on for 5 hours a day (its winter and the nights are long).

2. A microwave. Youll use this for heating food, 3 times a day, for 1/2 hour per shot. [Thats a lot]

3. A small pump to pump water from a cistern to pressurize the house water supply system.

4. A heater (furnace) fan.

We will also look at putting together a system that is designed to last us only a few (3-4?) years, so well look at using golf cart batteries [Trojan T-105, about $80 each], rather than heavier duty batteries such as the Trojan L-16, at about $180 each, which might give us 8 to 10 years life if treated well.

First Step: Determine how much power each load will take, and the number of hours the load will be used. You know the size of light bulbs, go to the name plates of the various appliances to determine power consumption. Were going to work in watts, a unit of power right now. Hint, you may find units rated in amperes (amps), which the current drawn by the device. Now, Power = Volts x Current (P=EI), so if your appliance nameplate gives the ratings in amps, simply multiply the current by 120 volts to get the power consumption. Example, a motor may be rated at 1 amp run, 4 amps start. That motor would be rated at 120 watts running, but takes 480 watts to get it started (1 x 120 = 120, 4 x 120 = 480).

We will use the following table of power required to start and run motors, published by the Virginia Cooperative Extension, Select the Right Portable Generator after a DisasterPub No. 490--303: You can look this up -- do a search on VA Coop. and then that pub.

Motor hp--------------1/6----1/4---1/2----1 Power to Start-------1000---1500--2300---4000 Power to Run----------215----300---575---1000


Lights, incandescent------75-100W

Lights, Fluorscent--------20-28W



Electric Skillet--------1150-1500W

Electric Stove----------3000-4000W

Washing Machine-----------400W

Water Heater------------1000-5000W

Electric Fan--------------75-300W

1. Lights: 4 lights, at 20 watts each, for 5 hours a day = 80 watts for 5 hours = 160 W-Hr.

2. Microwave: Well assume a moderate sized unit, at 1000 watts, used for 1 1/2 hours. This is equivalent to 1500 W-Hr.

3. Our pump. We will assume that youre using a small, 1/2 horse pump since this is a ground level cistern, and that you pump water for a total of 1 hour per day (a lot). That 1/2 horse pump will require 2300 watts, peak, to start up, and 575 watts to run for an hour.

4. Furnace fans. We will assume that the furnace fan is 1/4 horsepower. The furnace will both operate about 1/8 of the time, or 3 hours a day, total. Thats a total of 515 watts used for 3 hours, or 1545 W-Hr of total energy. The furnace will require 1500 watts at start up.

Now, add all of these watt-hour numbers up. This will determine the total energy use:

1. Lights = 160 W-Hr

2. Microwave = 1500 W-Hr

3. Pump = 575 W-Hr.

4. Furnace = 1545 W-Hr.

The total = 3870 W-Hr.

Multiply this by a factor of 1.2. This covers the losses youll experience in the inverter, the loss of power in cables, etc. Your new total is 4536 W-Hr.

This is the amount of energy that youll use in one day. This is the amount of energy that will have to be supplied by the batteries you have hooked up to that inverter.

Sizing the Inverter

Next, how big does the inverter have to be? Well, if you were going to power everything at one time, youd need:

1. Lights = 80 watts

2. Microwave = 1000 watts

3. Pump (startup) = 2300 watts

4. Furnace/ = 2500 watts

Total = 5880 watts.

Thats if you let every motor start up at one time. But, youre not going to do that, because its expensive. Youre going to shut down the pump and furnace motors when you want to use the microwave, and youre going to make sure only the furnace and lights are running if you want to heat the house.

Certainly you could do this job with a 3600 watt inverter, such as the Trace DR 3612 or DR 3624. You might squeak by with a 2400 watt unit (DR 2412 or 2424). It would save a few hundred dollars.

Sizing the Batteries

Now, how much battery do we have to provide? Each 2 batteries give us 12 volts. We will have to draw 4536 W-Hr. out of the batteries over the course of 24 hours. Plugging this into our equation P=EI, (4536=12xI), or I is approximately 378 amp-hrs. Thats the current drawn per day in order to supply our little power house, from two batteries, stacked in series.

Deep discharge batteries are rated in many amps can be discharged over an hour, with the battery assumed to be discharged in an hour. Ratings are to full discharge.

Deep doesnt mean full.

The rule of thumb is that the batteries should only be discharged to 50% of their capacity. Recent studies have shown that limiting discharge to only 30% of rating more than doubles battery life. But, you need twice as many to begin with. This becomes a question of when you want to buy batteries. If you believe that Y2K effects will last for years and years, you might want to load up on batteries for your system now and double or triple their normal life.

The Trojan T-105 batteries are rated at 235 amp-hr. [This is an old, conservative rating]. However, we only want to discharge the battery a maximum of 50%, so that we only want to draw 117 amps out of the batteries. That means that we need 378/117 = 3.2 sets of batteries. OK, lets round down, and we need 3 sets of batteries, with each set being 2 batteries. So, we need 6 batteries to do the job for one day without over discharging the batteries. We could last for 2 days by using 6 sets of batteries (12 batteries), which would make a nice package for routine power outages due to winter storms. :)

Now, the bigger Trojan L-16 batteries are rated at 360 amp-hours, and can supply 180 amp-hours if we discharge to the 50% point. So, we would need 378/180 = 2.1 sets.......we can get by with only 23 sets of 2 batteries. However, the L-16s are much more expensive than the T-105s to begin with. They will typically last twice as long as the T-105s.


There you have it, sizing the system.

1. We decided on the items we wanted powered.

2. We computed the amount of power used daily for each item -- watts times number of hours used.

3. We also computed the peak loads for our motors........typically 4 times the running load, more in the case of small motors.

4. We used the total peak load to determine the size of the inverter we need.

5. We used the total daily draw to determine the number of batteries we have to provide.

6. We briefly looked at the tradeoffs involved in two different battery types.

-- de (, July 09, 1999


Let's reformat that motor power table, shall we?

Motor hp--------------1/6----1/4---1/2----1

Power to Start-------1000---1500--2300---4000

Power to Run----------215----300---575---1000

-- de (, July 09, 1999.

Anyone who would like a transcript of a recent online chat session which focused on selecting and installing generators should give me an email holler.

-- BigDog (, July 09, 1999.

Kudos definitely have all your ducks in a row on this de! And since you's my site link (let's hope this works!) Roy Four Winds Renewable Energy Co.

-- Roy Butler (, July 09, 1999.

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