How we calculate
The calculator turns a list of appliances and outage hours into four numbers: energy, start-up peak, battery size and generator size. The same formulas run on every product page and list on this site.
1. Energy
Energy (Wh) = watts × hours × duty cycle, summed over the appliances. The duty cycle is the share of time a motor or heater actually runs: a refrigerator compressor runs about a third of the time, electronics run all the time.
2. Start-up peak
Peak (W) = all running watts + the single largest start-up surge. Motors (fridges, pumps, compressors) draw 3–5 times their running watts for a moment. The inverter or generator must cover that moment.
3. Battery size
Battery (Wh) = energy ÷ (inverter efficiency × usable depth of discharge) × reserve.
4. Solar and generator
Solar (W) = daily energy ÷ (peak sun hours × 0.75). We show winter first (2 peak sun hours in the northern US, 5.5 in summer). Generator (W) ≥ peak × 1.25.
The coefficients
| Factor | Value | Why |
|---|---|---|
| Inverter efficiency | 88 % | A pure-sine inverter loses 8–15 % turning battery DC into 120 V AC. |
| Usable depth of discharge | 90 % | LiFePO4 stations stop before the cells are fully empty. |
| Reserve | × 1.2 | Capacity fades with age and in the cold; the reserve keeps the plan honest in year five and in winter. |
| Solar losses | 25 % | Panel angle, heat, wiring and the charge controller; we use winter sun hours as the main case. |
| Generator headroom | × 1.25 | Generators should not run at their limit; the headroom covers the start-up peak. |
Worked example: 24-hour outage
| Appliance | Running W | Start W | Hours | Duty | Wh |
|---|---|---|---|---|---|
| Refrigerator (with freezer) | 120 | 600 | 24 | 35 % | 1,008 |
| Wi-Fi router + modem/ONU | 12 | 12 | 24 | 100 % | 288 |
| LED lighting (5 bulbs) | 40 | 40 | 5 | 100 % | 200 |
| CPAP (no humidifier) | 35 | 40 | 8 | 100 % | 280 |
| Sump pump 1/2 HP | 920 | 2,150 | 1 | 100 % | 920 |
- Energy: 2,696 Wh for 24 hours.
- Battery: 2,696 ÷ (0.88 × 0.9) × 1.2 = 4,085 Wh.
- Start-up peak: 1,127 W running + the sump pump 1/2 hp surge (1,230 W extra) = 2,357 W.
- Generator: 2,357 × 1.25 = 2,947 W.
- Solar to refill a day's use: 1,798 W of panels in winter, 654 W in summer.
Run it with your own appliances →
Where the inputs come from
Appliance watts are typical values with a source where one exists (see each appliance page); your own label beats ours. Product ratings come from the manufacturer's page, linked on each model page. Outage hours come from EIA Form 861 (outages by state). More on sources: about the data.
Limits
This is a sizing estimate, not an engineering design. Real duty cycles depend on the weather and on how you use the appliances. Connecting a generator or battery to a home panel needs a transfer switch or interlock and a licensed electrician.
FAQ
Why is the battery bigger than the energy I use?
The inverter loses about 12 %, only 90 % of a LiFePO4 battery is usable, and we add a 20 % reserve for ageing and cold. 2,696 Wh of use becomes 4,085 Wh of battery in the example.
Why do you add only one start-up surge?
Motors start for a fraction of a second and rarely at the same moment. Adding every surge would oversize the inverter several times; we add the single largest one to everything that runs together.
What generator size do I need for this example?
At least 2,947 W: the start-up peak of 2,357 W plus 25 % headroom.