Volt Battery Amphours

circuits, and the current used by each, to be supplied from the EPS. Once the number and size of the circuits is determined then the sub-panel and the appropriate breakers may be chosen. During normal operation from the utility the total load supplied by the EPS sub-panel may not exceed the 30 amp current capacity of the inverter's internal transfer switch unless an external transfer switch is used.

If the normal load on the EPS exceeds 30 amps when utility power is available an optional 50 amp external transfer switch may be used. With this option the EPS Sub-Panel may supply 50 Amps during normal utility service.

Inverter/Battery Charger

The Inverter/Charger actually does three jobs. Its most important job is to convert energy stored in the battery into 120 vac power for the EPS Sub-Panel during a power outage. Its second job is to charge the battery and maintain it in a fully-charged state, whenever 120 vac power is available from the utility or an auxiliary generator. Its third job is to automatically supply utility power through its internal transfer switch to the EPS Sub-Panel. The power is supplied from the utility, if it is available, or from the inverter if it is not. The EPS should use an 1800 Watt inverter that is capable of supplying 15 Amps of 120 vac power when utility power is not available. This inverter should also have a built in automatic 65 to 110 Ampere battery charger with electronic regulation.

A system that requires more capacity would utilize more powerful inverter that is capable of supplying a maximum of 25 Amps of power when utility is not available.


The EPS may use any typical 200 Amp-hour deep cycle battery purchased locally. The battery is the energy storage reservoir that supplies the inverter with power during a blackout. The larger the battery the longer it can supply energy without recharging. Watt-Hours of use is how the utility determines your bill. If you use a 60 Watt light bulb for 1 hour you have used 60 Watt-Hours.

Unfortunately batteries are typically specified by the voltage and the number of Ampere-hours (Ahr) that it can supply. To convert your Watt-Hour requirement to Amp-Hours use the following formula: Amp-Hour Battery Capacity Required = Watt-Hours Required /Battery Voltage

If you want to know the Watt-Hour capacity of a battery use the following formula: Watt-Hour Battery Capacity = Amp-Hour Capacity x Battery Voltage

If a battery is routinely fully discharged, it shortens its life.

If the EPS is to be used only during power outages, this is not a critical design factor. However, if the system is to be expanded for use with renewable energy then the battery size should be increased by a factor of two or more to avoid damaging discharges.

A 200 Ahr, 12 Volt battery will supply about 2,400 Watt-Hours. If discharge were limited to 80% there would only be about 1,900 usable Watt-Hours. Battery systems can be designed with capacities of thousands of Ahrs. To do so requires careful engineering judgment to make sure the system components are properly matched. The Standard EPS is intended to provide a limited amount of power for a limited time. If you need a larger system, consult an experienced renewable energy system designer, or learn the design process yourself.

How to Make a Load List

List the actual or estimated power consumption (watts) of each load. Sometimes the number of Amps a load uses is given instead of its Wattage. To find the number of Watts a load uses, multiply the number of Amps it uses by its Voltage. If a load uses 1 Amp when supplied from 120 Volts then it is a 120 Watt load.

To determine your daily Watt-Hour requirement use the following formula: Daily Watt-hours = Number of Watts the load uses X Hours of use per day

Making a load list is the only way to determine exactly how much battery capacity is required for your EPS. An EPS with a 200 Ahr, 12 Volt battery (2,400 Watt-hours) will last four to twenty-four hours, depending on the loads. In the two examples that follow, the significant impact of using energy saving lighting, appliances, and energy conserving tactics, is clear. Use these examples to make a load list for your particular requirements.

Load List for 24 hour blackout without conservation





Living Room incandescent light

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