Calculating watt-hours, amp-hours and battery sizing

One of the first items to take care of when designing an alternate energy system is correct sizing. Sizing takes into account all the ac and dc loads that are required to be powered. Properly estimating capacity needed will help stop batteries from wearing out before their time and ensure proper charging. You should be ruthless in your energy usage and practice energy efficiency and energy management.

After the appropriate amp hour capacity has been calculated, it is then easy to find the correct charge rate for the battery bank. Practice your calculations a few times and you will be able to easily work this out. To rapidly recharge the system, a generator should be able to supply a "c30" charge rate. This is equal to 30 percent of the battery banks rated amp-hour capacity as expressed in amperes. Solar(pv) systems will usually not deliver more than a 10-20 C rate of charge. Having a generator as part of the system is essential as it is needed for heavy loads, to top up the battery bank and every few weeks for equalization charging.

Wind energy can provide turbo charging, at times producing more power than the batteries can handle. Make sure that you have a proper shunt regulator to take care of situations like these. Excess wind energy can be shunted to an electric water heater. At other times the wind machine will not be contributing a lot to the system on calm days but that is what the battery bank and generator are for.

There are phantom loads from some appliances that remain on all the time. These loads come from standby modes, instant on features and power supplies (wall warts). Use power strips with a switch to shut off these devices when not being used. Motors and some other heavy duty appliances can draw many times their average usage when starting up. Calculate all the average on times of your various appliances and do the figures again. This will give you an accurate idea of what you true consumption really is. Add 20 percent more to the final figure as a safety factor.

Use these handy tools to help estimate your system sizing needs.

These calculators have been used in real time situations and have been shown to be accurate to within 1 percent, however you should always double check your calculations.

Equipment Averages

space heater 1200 watts

weed trimmer 850 watts

clothes dryer (gas) 720 watts

light bulb (100w) 100 watts

small radio am/fm 50 watts

radio, cb 50 watts

fan 200 watts

television 350 watts

microwave oven 1200 watts

air conditioner (12,000 btu) 3250 watts

furnace fan (1/3 hp blower motor) 600 watts

vacuum cleaner 600 watts

sump pump (1/3 hp) 700 watts

refrigerator/freezer 800 watts

deep freezer 500 watts

circular saw 6" 800 watts

floodlight 1000 watts

drill 1/2" electric 1000 watts

toaster 1200 watts

coffee maker 1200 watts

skillet 1200 watts

chain saw 14" electric 1200 watts

water well pump (1/2 hp) 1000 watts

hot plate/range (per burner) 1500 watts

table saw 10" 2000 watts

elect. water heater 3000 watts

12v dc battery charger 120 watts