Estimate off-grid daily energy in watt-hours with an appliance-by-appliance audit, then estimate battery-bank capacity and solar-array power under stated assumptions.
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Off-Grid Daily Load Formula
The energy audit starts with one formula applied to every appliance on your list, then summed:
E_appliance = W * Q * H E_load = SUM(E_appliance)
Raw appliance energy is not what your batteries deliver. AC loads pass through an inverter that wastes some power, and standby draws run around the clock, so the sizing number is the battery-side total:
E_battery = E_AC / n_inv + E_DC + 24 * P_standby
The second mode converts that daily load into hardware. The battery bank must hold enough usable energy to ride through cloudy days, and the solar array must replace a full day of use during your worst month:
Bank = (E_battery * D) / DoD Ah = Bank / V P_array = E_battery / (PSH * n_sys)
Variables:
- E_appliance is the daily energy of one line item in watt-hours (Wh)
- W is running watts, Q is quantity, and H is hours of use per day
- E_AC and E_DC are the summed watt-hours of your AC and DC line items
- n_inv is inverter efficiency as a decimal; the calculator assumes 0.90 by default. Use the manufacturer efficiency at your operating load
- P_standby is the always-on standby draw in watts, counted 24 hours per day
- E_battery is the battery-side daily load in Wh, the number every other sizing step uses
- D is days of autonomy, the cloudy days the bank must cover without charging
- DoD is the usable fraction of battery capacity: the default assumptions are 0.90 for LiFePO4 and 0.50 for lead-acid. Use the custom setting when manufacturer limits differ
- V is battery bank voltage (12, 24, or 48), and Ah is the bank size in amp-hours
- PSH is equivalent peak sun hours for the design season, and n_sys is PV-to-stored-energy efficiency, assumed to be 0.75 by default; inverter loss is already included in E_battery
- P_array is the required solar array size in watts
The audit mode applies the first two formulas. Choose 1–50 appliance rows, then enter watts, quantity, hours per day and AC or DC supply; names and illustrative presets are optional. Inverter losses apply only to AC loads. The result includes battery-side Wh/day and kWh/day, average draw, ranked appliance shares and combined running watts. Running watts do not include surge. When you first switch to sizing with an empty daily-energy field, the last successful audit total carries over without rounding. Sizing returns nominal bank kWh and Ah, 100 Ah equivalents at the selected complete bank voltage, array watts, rounded panel count and a rough array-power/nominal-voltage current comparison. Its separate 25% allowance is a planning comparison, not a universal controller or electrical-code minimum. Verify charging voltage, cold-weather PV open-circuit voltage, short-circuit current and manufacturer limits. The array estimate replaces one day of use and does not include recovery of a depleted multi-day reserve.
Typical Appliance Draws and the Standby Loads Most Audits Miss
Use measured average consumption for cycling appliances such as fridges. Nameplate power may describe peak operation. The following retained figures and presets are illustrative starting values, not manufacturer specifications or guaranteed daily consumption:
| Appliance | Running watts | Typical hours/day | Wh/day |
|---|---|---|---|
| LED light bulb | 10 | 5 | 50 |
| Wi-Fi router | 12 | 24 | 288 |
| Starlink (average) | 45 | 24 | 1,080 |
| Full-size refrigerator (average) | 50 | 24 | 1,200 |
| Chest freezer (average) | 40 | 24 | 960 |
| Laptop | 60 | 4 | 240 |
| LED TV (50 in) | 70 | 3 | 210 |
| Microwave | 1,000 | 0.25 | 250 |
| Induction burner | 1,400 | 0.5 | 700 |
| Water pressure pump | 500 | 0.3 | 150 |
| Ceiling fan | 50 | 6 | 300 |
| CPAP (no humidifier) | 40 | 8 | 320 |
| Mini-split AC (cooling, average) | 700 | 6 | 4,200 |
| Washing machine (per load) | 500 | 0.75 | 375 |
The loads that sink most first-time audits never appear on an appliance list at all. Standby draws look tiny in watts, but they run 24 hours a day, and the inverter’s own idle consumption is usually the largest of them. This table illustrates the energy of assumed always-on draws. Enter battery-side standby power in the calculator, and avoid counting an appliance twice:
| Always-on draw | Standby watts | Wh/day | Share of a 2,000 Wh/day budget |
|---|---|---|---|
| Inverter idling (1,000 to 3,000 W unit) | 10 to 25 | 240 to 600 | 12 to 30% |
| TV on standby | 0.5 to 3 | 12 to 72 | 1 to 4% |
| Microwave clock | 2 to 4 | 48 to 96 | 2 to 5% |
| Game console in rest mode | 1 to 10 | 24 to 240 | 1 to 12% |
| Smart speaker | 2 to 4 | 48 to 96 | 2 to 5% |
| Coffee maker with clock | 1 to 3 | 24 to 72 | 1 to 4% |
| Combined typical off-grid standby | 15 to 40 | 360 to 960 | 18 to 48% |
For the assumed 15–40 W combined draw, standby consumes 360–960 Wh/day, or 18–48% of a 2,000 Wh/day budget. Measure actual standby before changing the design. Only switch off an inverter or appliance when essential loads can safely remain supplied.
Off-Grid Daily Load Example Problems
Example 1: A weekend cabin runs four 10 W LED bulbs for 5 hours (200 Wh), a full-size fridge averaging 50 W around the clock (1,200 Wh), a 70 W TV for 3 hours (210 Wh), and a 60 W laptop for 4 hours (240 Wh), all AC. A 12 V water pump draws 60 W for 0.3 hours (18 Wh) direct from the battery. AC loads total 1,850 Wh. With 90 percent inverter efficiency, the battery must supply 1,850 / 0.90 = 2,056 Wh for them. Adding the DC 18 Wh and a 10 W standby draw (240 Wh) gives a battery-side load of about 2,314 Wh/day, or 2.31 kWh/day, with an average continuous draw of 96 W. Note the audit lists 1,868 Wh of appliance energy, but the system must be sized for 2,314 Wh.
Example 2: Size hardware for that 2,314 Wh/day load with 2 days of autonomy, LiFePO4 batteries at 90 percent usable capacity, a 24 V bank, 4 worst-month peak sun hours, and 75 percent system efficiency. Usable storage needed is 2,314 * 2 = 4,628 Wh. Nameplate bank capacity is 4,628 / 0.90 = 5,142 Wh, which is 5,142 / 24 = 214 Ah at 24 V, about three 100 Ah bank equivalents at 24 V; this is not a count of individual 12 V batteries. The array must be 2,314 / (4 * 0.75) = 771 W, so two 400 W panels. The rough nominal-voltage current comparison is 771 / 24 * 1.25 = 40.2 A with a 25% planning allowance. This does not establish a controller minimum: check the installed panels, actual charging voltage, PV cold-weather voltage, short-circuit current and controller manufacturer limits.
Frequently Asked Questions
How many watt-hours per day does an off-grid home use?
There is no single reliable daily allowance for an off-grid home. For scale, a hypothetical 50 W average load uses 1,200 Wh/day and a 500 W average load uses 12,000 Wh/day before additional losses. Audit your own appliances, season and operating schedule; heating, cooling, cooking and pumping can dominate consumption.
Why is my battery-side total higher than the sum of my appliances?
Inverter loss and battery-side standby increase the audit total. Under the default 90% inverter-efficiency assumption, 1,800 Wh of AC energy requires 2,000 Wh from the battery. A separate 15 W battery-side standby draw adds 360 Wh over 24 hours. Use actual efficiency and standby measurements and avoid double counting.
Should I audit for the average day or the worst day?
Size for the heaviest realistic day of the season you will actually use the system. A summer-only cabin should be audited with fans and fridge duty at their July peak, while a year-round home should be checked against winter, when lights run longer and peak sun hours are lowest. If your summer and winter routines differ a lot, run the audit twice and size from whichever combination of daily load and worst-month sun hours demands the larger array. Days of autonomy set a nominal energy reserve, but do not guarantee service through storms. This simple array model replaces one normal day of use; recovering a depleted reserve while supplying loads needs additional energy and site-specific design.
