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System Design


Energy Budget Calculation

. Write down all your loads and the rated watts of those loads in the energy budget on the bottom of this page.
2. Determine the number of hours per day and days per week you will operate each load and enter these values in the appropriate place in the energy budget.
3. Rated watts x hours per day x days per week equals the weekly watt hours of each load.
4. Add together the weekly watt hours for every load.
5. Divide the total weekly watt hours by seven days to obtain the average daily load in watt / hours.
6. Take the average daily load in watt / hours from the energy budget. Multiply the average daily load in watt / hours by 1.2 for DC only systems and 1.3 for AC only systems. This factor accounts for battery and inverter inefficiencies and gives the corrected average daily load in watt / hours.
. Divide the corrected average daily load in watt / hours by the system voltage. This gives the total daily load in amp / hours. The total daily load allows you to determine the size of your battery bank and charging system.
8. While the energy budget is a valuable tool in designing an energy system, accurate estimation of multiple variable loads is difficult. Avoid underestimating the energy budget as it will result in an undersized system. This is especially critical when choosing the battery. PV modules can always be added later, but a battery bank is much less flexible.

You can also download and use the Energy Budget files provided below for off-line calculations (more flexibility)

Microsoft Excel 2000 Format
Microsoft Excel 5 Format
Adobe Acrobat Format

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