Electrical Energy Consumption

Electrical energy consumption can be calculated by first looking at the electrical power and then looking at the time over which that power is used.

Power (P) = Voltage (V) × Current (I)

where:

I is current (Amperes)
P is power (Watts)
V is voltage (Volts)

Then energy used by an electrical appliance, we can use the formula:

Energy (E) = Power (P) × Time (t)

where:

P is power (Watts)
t is time (seconds)
E is energy (Joules)

In the case of some systems it is useful to look at Energy Demand over a day (useful when looking at Solar System sizing). The longer term energy demand is then useful when a system is designed to operate over many years. The study by Perriment et al [1] shows:

Analysing load demand data from 1,000 systems, the study found that 77% of households reduced their daily electricity consumption after the first year, with average demand falling by about one third by year two, even when payments were not missed.
The findings challenge common assumptions about energy access and highlight the importance of long-term demand data when designing off-grid systems. Without accounting for these changes, systems risk being oversized, increasing costs and reducing efficiency.

References

R. Perriment, V. Mergulhão, V. Kumtepeli, P. Parikh, M.D. McCulloch, D.A. Howey, Understanding long-term energy use in off-grid solar home systems in sub-Saharan Africa, Energy for Sustainable Development, Volume 90, 2026

References

See also