Off Grid Load Calculator

An off-grid power system starts with the energy demand that must be supplied each day and stored through periods without charging. The Off Grid Load Calculator converts an entered Average load and Hours per day into daily energy, then applies Days of autonomy to determine the energy reserve required.

The calculated values support preliminary battery-bank planning, DC feeder review, inverter loading review, and a first-pass check of the continuous current carried at the entered System voltage. They do not establish final battery capacity, conductor size, inverter rating, overcurrent protection, or photovoltaic-array production.

The calculator produces four values:

Result	Electrical use
Daily load energy	Daily energy consumption of the connected load in kWh/day
Autonomy energy	Energy required to carry the load for the selected number of days in kWh
Average DC current	Equivalent average DC current at the entered nominal system voltage
Autonomy amp-hours	Preliminary battery-bank capacity expressed in Ah at the entered system voltage

Load and Autonomy Inputs

Enter Average load (W) as the average real power demand expected while the off-grid load is operating. This is not necessarily the nameplate wattage of every connected device. A duty-cycled load, such as a pump, refrigerator, communications enclosure, or intermittent process equipment, should be represented by its expected average operating load for the period being evaluated.

Enter Hours per day (h/day) as the total daily operating time. A 600 W load running for 8 hours per day consumes the same daily energy whether it runs continuously for eight hours or in several scheduled intervals, provided the average load remains 600 W during those operating periods.

Enter Days of autonomy (days) as the number of days the system must support the stated load without relying on charging production. This value scales the battery energy requirement directly:

• One day of autonomy equals one day of calculated load energy.
• Two days of autonomy doubles the calculated energy reserve.
• Additional autonomy increases battery energy and amp-hour requirements in direct proportion.

Enter System voltage (V) as the nominal DC system voltage used for the screening calculation. The voltage does not change the calculated kWh requirement, but it changes the calculated DC current and amp-hour value. For a fixed energy requirement, a higher DC system voltage results in lower current and fewer amp-hours.

Energy and Amp-Hour Formula

The calculation uses average load energy arithmetic:

\(\displaystyle Daily load energy (kWh/day) = \frac{\text{Average load (W)} \times \text{Hours per day (h/day)}}{1000}\)

\(\displaystyle \text{Autonomy energy (kWh)} = \text{Daily load energy (kWh/day)} \times \text{Days of autonomy (days)}\)

\(\displaystyle Autonomy amp-hours (Ah) = \frac{\text{Autonomy energy (kWh)} \times 1000}{\text{System voltage (V)}}\)

Average DC current is calculated from the entered average load and system voltage:

\(\displaystyle \text{Average DC current (A)} = \frac{\text{Average load (W)}}{\text{System voltage (V)}}\)

The current output represents the DC current equivalent of the entered average load at the nominal system voltage. It is useful for early DC conductor and voltage-drop review, but it is not a final ampacity or overcurrent-protection value.

Calculation Example

For the following off-grid load:

Input	Value
Average load	600 W
Hours per day	8 h/day
Days of autonomy	2 days
System voltage	48 V

Daily load energy is:

\(\displaystyle \frac{600 \text{ W} \times 8 \text{ h/day}}{1000} = 4.8 kWh/day\)

Autonomy energy is:

\(\displaystyle 4.8 \text{ kWh/day} \times 2 \text{ days} = \text{9.6 kWh}\)

Average DC current is:

\(\displaystyle \frac{600 \text{ W}}{48 \text{ V}} = \text{12.5 A}\)

Autonomy amp-hours are:

\(\displaystyle \frac{9.6 \text{ kWh} \times 1000}{48 \text{ V}} = \text{200 Ah}\)

The resulting preliminary load profile is:

Result	Value
Daily load energy	4.8 kWh/day
Autonomy energy	9.6 kWh
Average DC current	12.5 A
Autonomy amp-hours	200 Ah

A 48 V battery system must therefore provide 9.6 kWh of load energy, expressed as 200 Ah at 48 V, before accounting for battery operating limits and system losses.

DC Distribution and Voltage Drop

The Average DC current output helps establish a starting point for the DC feeder between a battery bank, inverter, DC distribution equipment, or other off-grid equipment. It can be compared with preliminary conductor ampacity and used to begin a voltage-drop calculation.

Final conductor selection requires more than average current. The selected AWG or kcmil conductor must be evaluated for the actual circuit current, conductor length, installation method, terminal rating, insulation temperature rating, ambient temperature, bundling, and applicable adjustment factor or correction factor. Raceway fill and the number of current-carrying conductors can affect conductor selection where conductors share a raceway or cable assembly.

The average current result also does not represent inverter input current during high-demand periods. Motor starting, compressor startup, transformer inrush, inverter surge, battery charging current, and simultaneous branch-circuit loads can produce a materially higher current than the calculated average. Those conditions can control inverter selection, battery interconnect sizing, DC overcurrent protection, and voltage-drop performance.

Field Verification

The calculation boundary is limited to off-grid load arithmetic. It does not calculate solar production, charge-controller capacity, inverter efficiency, inverter surge, battery depth of discharge, battery temperature effects, battery aging, charging limits, conductor ampacity, voltage drop, overcurrent protection, code compliance, or product approval.

Use the energy and amp-hour results as a load-screening value, then verify the final installation using the actual equipment data and the applicable electrical requirements. Final design decisions should address battery chemistry and manufacturer limits, inverter continuous and surge ratings, PV and charging conditions, branch-circuit and feeder loads, conductor ampacity, terminal temperature limitations, disconnecting means, grounding and bonding, available fault conditions, and the requirements of the local AHJ.