Oasis Montana designs and sells residential solar electric power systems

Oasis Montana Inc.
Renewable Energy Supply and Design
Call us!  406-777-4321
Fax: 406-777-4309
M-F  8:00am-5:00pm MDT
Contact us: info@oasismontana.com
Home Page: www.oasismontana.com


Solar Home Introduction

How do you figure out your power requirements?

TO FIGURE YOUR POWER REQUIREMENTS, you need to do some homework for us--everyone's power consumption differs!  Without your load information, we cannot specify what components it will take for your system.

These items and appliances are listed with their hourly power requirements in terms of watts.  Figure out what you need, and multiply that by your hours of usage in a day. WE'RE TRYING TO COME UP WITH THE TOTAL WATT-HOURS PER DAY YOU NEED.  For example, a 300 watt (120V AC) load for ˝ hour consumes 150 watts--or for 3 hours would come to 900 watt-hours needed for this item.  If you cannot find your specific load requirement on this list, look for an electrical rating sticker on your unit, or contact the manufacturer.


For example, if it says 2 amps (@ 120V), the load is 240 watts per hour. Or, a 4 amp load for 8 hours a day would come to 3840 watt-hours needed per day.  If you already  know your daily power requirements, click on "Map & Insolation Data" to get an idea of the costs for your system.  If you need some assistance, click on the load evaluation button which gives you an outline/worksheet to help ascertain your power requirements.

YOU CAN USE YOUR UTILITY BILL AS A GUIDE--usually the number of kilowatt hours used per month is listed (a kilowatt-hour is 1000 watt-hours, a kilowatt is 1000 watts).  If you divide the number of kilowatt-hours used by the number of days of the billing period, then multiply that number by 1000, you'll come up with the average number of watt-hours per day you use.  The amount of power you use may shock you!  This is why, if you're considering going off-grid, it's crucial that you reduce your electrical loads as much as possible by using energy efficient lighting, refrigeration units, and by converting your heating loads to natural gas, propane or wood.  A photovoltaic (PV) powered home might look like a conventional home, but its electrical requirements are significantly reduced by the use of energy efficient appliances, and by using gas for all heating loads.

POWER USAGE:  add up your loads X the number of hours of usage

Unit/Loads                                                Watts used per hour (watt-hours)

Remember, if you're only using a given load for a part of an hour, divide that hourly total by the percentage of the hour you're using that load. A 300 watt per hour load for ten minutes equals 50 watt-hours total.  The electrical load information is most crucial;
without this information, there is a risk of under- or over-sizing the system requirements.  We don't want to sell you something that doesn't meet YOUR SPECIFIC power requirements.  If you have questions or need help figuring out your loads, please do not hesitate to e-mail us or call 406 777-4321, or use the load evaluation button and form to help figure your power usage.

After you have ascertained your load information, multiply the watt-hours needed per day by 1.3, to account for battery and inverter inefficiencies. THIS IS THE ACTUAL POWER YOUR SYSTEM NEEDS TO BE MAKING TO MEET YOUR LOAD REQUIREMENTS. Batteries and inverters are not 100% efficient, so we refer to this as the "load correction factor".  With this figure, we can determine what size system, and what components, are needed to meet your power requirements.   Click on the insolation map to determine the amount of sun-power available for your area. 


There are definitely electrical loads that are NOT at all cost-effective to power with solar electricity . These include: making heat in any way (space heating, water heating, clothes drying, cooking) and other large loads (air conditioning, refrigeration). It is generally MUCH more cost-effective to power these loads with wood, propane, or natural gas.  NOTE: there are some energy efficient refrigeration units available that are reasonably powered by photovoltaics. Be certain you know your manufacturer's (accurate) specifications.  The wattage demands for refrigerators and freezers may run from 540 watt-hours per day (for an efficient unit) to upwards of 8500 watt-hours per day. That's a BIG difference!  We have additional information on energy efficient and propane refrigeration units

SUGGESTED SYSTEMS - Map & Insolation Data

After you have ascertained your "adjusted watt-hours per day" required , using the power load information, you can get an idea of  what your power requirements will cost when you click on our Map & Insolation data in the  U.S.   By selecting  your zone location, we can determine the amount of sunlight that is provided to your area--and, with your load information, determine what components are necessary for your system.   

We've shown some suggested systems here, how much power they will provide, and their cost.  These are merely guidelines to assist you; we design power systems specifically to meet YOUR power needs.

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