Part 2 – Sizing Your System

So how do you decide how big of generator or solar system to buy? The biggest question is what do you want to run off of it. It is unreasonable for most of us to think that life will not change in an emergency. The reason we are talking about portable backup power is that whole house solar systems are not in the financial reach of most of us. What essential electrical items do you need to run?

First, make a list of items you want to run. Second, determine how many watts each of those items takes to run.

KILL A WATT Meter – There is a great little meter called a “Kill A Watt” meter. You plug it into the electrical outlet and then plug the appliance into the meter. The meter will then measure volts and hertz of the electrical current and the watts and amps being used by the appliance. With the meter you can determine how much draw there will be for the appliances you want to run. The cost of the Kill A Watt meter is under $20.

Many electrical items do not run all the time. A refrigerator or deep freeze turns on and off when needed to keep a specific temperature. Same with a furnace blower motor; it will only turn on when the furnace needs to put out more heat. An energy saver refrigerator/freezer draws 360 watts per hour but only runs a total of 2 hours per day. A microwave uses 1200 watts of power per hour but only when it is turned on. A furnace blower fan uses 1200 watts per hour but only turns on when needed. Depending on how cold it is outside, the fan may run anywhere from 2 – 6 hours a day.

You get the idea. Use a Kill A Watt meter and determine the watt draw of the items you want to run and how many hours per day they need to run. This will be the basis for sizing your back up power.

How Big of Generator do I need?
Let’s say you want to run a 1200 watt microwave, a 1200 watt furnace fan, and a 1000 watt coffee maker. If you wanted to run all three at once a 4000 watt generator would do. If you only ran one item at a time, a 2000 watt generator would work.

A final word on generator ratings. Look for peak rating and constant rating. Many appliances require a power surge (peak) when started and then use a lower amount of power to run (continuous). Be sure to buy a generator based on continuous wattage not just peak wattage. Buy a generator with 25% more watts than you calculate. It is my experience that manufacturers tend to overstate the power of their generators

How Big of Solar System do I need?
Solar systems have four main components, Solar PV Panels, Charge Controller, Battery Bank, and Inverter. Any single component can be a limiting factor in how much power your system can supply. Let’s discuss each component in reverse order.

Inverters – Are sized just like generators. They have a peak watt rating and a continuous watt rating. Choose a Pure Sine or Modified Sine inverter depending if you plan to use appliances or tools that require a large amount of startup power. Decide how many electrical items you need to run at the same time and what items you can run separately.

Like generators, look for peak rating and constant rating. Be sure to buy an inverter based on continuous wattage not just peak wattage. Buy an inverter with 25% more watts than you calculate. It is also my experience that manufacturers tend to overstate the power of their inverters.

Here is my favorite mid-sized inverter.

Roadpro RPPI-1500W 1200/3000 Watt DC to AC Power Inverter
• 1200 watts continuous power, 1500 watts alternating current power output, 3000 watts peak power output with direct to battery hook-up
• 4 grounded 3-prong alternating current receptacle with dual internal cooling fans, enough power for multiple devices at one time
• Thermal protection shutdown at 140 degree Fahrenheit (60 degree Celsius) – 158 degree Fahrenheit (70 degree Celsius), short circuit protection shutdown and reverse polarity shutdown for internal protection
• Includes power cable with heavy duty battery terminal clips to connect directly to your battery
• High-power, affordable, and reliable method to transform direct current battery power into 115-volt household alternating current power

I have tried all of my kitchen appliances with this inverter and they all work, including a 1200 watt microwave, a 1000 watt coffee maker, and a 1000 watt electric tea pot.  Currently under $85.

Batteries – Make sure your batteries are deep cycle marine type batteries. Batteries have Amp Hour Ratings. I buy 125 amp hour batteries from Sam’s Club for $80. To calculate the watt storage of a battery, multiply the amp hours of the battery by the voltage of the battery. In my case, 125 ah x 12 volts = 1500 watts. To insure long battery life, don’t discharge your batteries more than 50% of their capacity. If we only use half of the battery power before recharging, then we have 750 watts of usable power in the battery.

You can use a 1200 watt inverter with the battery but if you were running a 1000 coffee maker, you would use all your stored power in 45 minutes. You can connect multiple batteries together. Just make sure to connect them in parallel. Two 125 ah 12 volt batteries would give you 250 ah and 1500 watts of usable storage. Three 125 ah 12 volt batteries would give you 375 ah and 2250 watts of usable storage,

Solar PV Panels
Solar panels have a watt rating and voltage rating. Most small panels are 12 volt. Many of the larger panels use voltages up to 48 volts since it allows for longer connection runs with smaller gauge wire. You just need to make sure you match the charge controller and panels.

The small Harbor Freight Kit has three 15 watt solar panels. In full sun, with the panels pointed directly at the sun, at the correct angle, the three panels will generate 45 watts of power each hour. If you have 6 hours of sun each day then the set of panels will collect 270 watts per day. If you have your panels on movable frames and you go out every 2-3 hours and reposition the panels to the sun then you could get up to 10 – 12 hours of sun in the summer or 450 watts – 540 watts per day.
In the winter the days are not as long. You may be lucky to get 6 – 8 hours of sun or 270 watts – 360 watts of power. On cloudy days you may get no sun or power. I would buy panels based on the minimum amount of power you need based on a 6 hour winter day.

Charge Controller
The Harbor Freight kit comes with a charge controller. If you want additional panels you will need to buy a different charge controller. Make sure the charge controller can handle the total watt and amp output of the panels. Also make sure it can handle the voltage of the panels. Some controllers can handle mixed panel sizes and voltages. Other controllers can only handle panels with the same watts and voltages.

If you just wanted to add another set of the Harbor Freight panels to double you collection capacity,
buy a small Sunforce or Kintrex controller from Amazon for only $18.

Small Basic System
A small basic solar system could consist of:
• Harbor Freight 45 watt solar kit. $150
• Energizer 125 ah battery form Sam’s Club $80
• 750 watt modified sine inverter from Sam’s Club $20
The solar panels can produce 270 watts in 6 hours, 360 watts in 8 hours, or 450 watts in 10 hours depending on how sunny the day is. Let’s go with the minimum of 270 watts per day.

The battery can hold 750 watts of usable power. It would actually take almost 3 days to recharge 750 watts with our solar panels at 270 watts per day. The fact that we have 3 times as much storage as we have ability to produce power would give us a power reserve to use on cloudy days.

The inverter can run appliances and tools up to 750 Watts.

Our limiting factor is only producing 270 watts of power a day. Let’s see what we could do over the course of a week on 270 watts of solar power.

One Week of Essential Power

Coffee Grinder – 2 watts/7 days 14 watts
Rice Cooker – 60 watts/7 days 420 watts
Wheat Grinder – 25 watts/3 days 75 watts
Bread Maker – 250 watts/3days 750 watts
13 watt Light – 13 watts/21 hours 273 watts
AA Battery Charger – 20 watts/7 days 140 watts
Lap Top – 25 watts/ 1 hour 25 watts
I-pod Charger – 5 watts/3days 15 watts
E-book Reader – 10 watts/2 days 20 watts
Sewing Machine – 20 watts/30 min 40 watts

Total 1772 watts/week or 253 watts/day. That even leaves some power to charge a cell phone daily if cell service is working.

If this seems too austere, add another set of panels for $150 and a new charge controller to handle the extra panels for $18 and you double your emergency power. Your current 125 ah battery will still be fine. If you want to run appliances with higher wattage, upgrade to the Road Pro 1500 inverter.

Other Components – There are just a couple other items you will need to finish your Battery Backup/Solar System. If you have multiple batteries, you’ll need connecting cables. If you have just 2 batteries, 4 gauge cables from Wal-Mart for $5 will work fine. If you have 3 or more batteries, use 00 or 0 gauge cables. If you have an inverter over 1500 watts you’ll need a 300 amp fuse and fuse holder between the battery and inverter. This will protect the batteries and inverter from overload.

The inverter manufacturer will tell you what size cables to use with the inverter, generally between 6 gauges and 0 gauges. Don’t use undersized cables to save a few dollars. Undersized cables will decrease the output of the inverter.

Great Setup for $1000
Since the Honda EU2000 generator is between $900 and $1000, I wanted to see what kind of solar setup I could put together for about the same money.

Sharp or SolarWorld 240 watt Solar PV Panel – $460
Xantrex C35 Charge Controller – $119
3 Energizer 125ah Batteries – $270
300 Watt Fuse and Fuse Holder – $10
2 sets 2 gauge battery Cables – $40
Whistler Pro-2000W 2000/4000 watt inverter – $150

Total $1049

Panels – 240 watts x 6 hours = 1440 watts
Batteries – 750 usable watts x 3 = 1500 watts

A 2000 watt continuous/4000 watt peak inverter would run pretty much anything around the house.

For $1000 this would be a great backup power setup.


Ed Rogers

Copyright “Keep It Simple” 2012. All rights reserved.

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