Energy Budget Calculation
1. 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
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.
7. 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 5 Format
Adobe Acrobat Format