Energy Consumption Calculator
Enter watts and hours for five load groups to get your daily kWh — the load-list number every off-grid or backup system design starts from, translated to battery and array sizes.
Last updated: 2026-09-28
How the calculation works
- Each group contributes watts × hours; the sum is the daily energy the system must supply.
- Refrigeration should use average draw and compressor-run hours, not the 24 h nameplate — the honest way to count cycling loads.
- The battery row applies 50% DoD (lead-acid convention) at 12 V; lithium at 80-90% DoD needs less.
- The array row uses a 4 PSH / 0.8 derate screening figure; the off-grid calculator refines it with autonomy and worst-month sun.
Formula
kWh/day = Σ (watts × hours) ÷ 1000 Battery Ah = Wh ÷ DoD ÷ system V Array Wp = Wh ÷ (PSH × derate)
| Symbol | Meaning | Unit |
|---|---|---|
Wh | Daily energy per load group | Wh |
DoD | Depth of discharge | — |
Worked example
Interpreting the result
The load list is where solar designs succeed or fail: optimism here multiplies through every later component. Measure what you can — a plug-in power meter on the fridge beats any table. Loads that run 24/7 (fridge, network gear) dominate; occasional high-watt loads (kettle, microwave) matter less for energy but set the inverter size, which the inverter calculator handles.
Assumptions
- Average watts × hours for each group — cycling loads need compressor-run hours, not clock hours.
- Battery and array rows are screening figures (50% DoD, 4 PSH, 0.8 derate).
Limitations
- Does not capture seasonal variation (pumps in summer, heating in winter) — audit per season if loads vary.
- Peak simultaneous load is not computed — enter it in the inverter or off-grid calculators.
Frequently asked questions
How many kWh does a cabin use per day?
A modest off-grid cabin runs 1.5-3 kWh/day (lights, fridge, water pump, electronics); a full-size off-grid home typically needs 8-15 kWh/day plus backup for winter. The honest number comes from the load list, not a rule of thumb.
How do I count a fridge's consumption?
Use average watts while running (often 40-60% of nameplate) times actual compressor-run hours (8-12 h/day typical). A '120 W' fridge averaging 55 W for 10 h is 0.55 kWh/day.