Planning solar backup power for home appliances starts with two questions: how much power each appliance uses and how long your solar battery can deliver it. This guide provides a practical wattage reference, a repeatable runtime formula, battery and inverter assumptions, worked examples, and a safety-focused setup checklist for outages.
Overview
A home solar backup system can keep essential loads operating when the grid fails, but its performance depends on more than the battery’s advertised capacity. The inverter must supply enough continuous power and handle each appliance’s starting surge. The battery must provide enough usable energy. Your wiring, transfer equipment, charging source, and backup priorities also affect the result.
For many households, the most useful backup loads are a refrigerator, internet equipment, selected lights, phone chargers, a small pump, and medically necessary equipment. Running every appliance at once usually requires a substantially larger system than maintaining a carefully selected essential-load circuit.
Solar batteries are commonly described in kilowatt-hours (kWh), while appliances are usually labeled in watts (W). One kilowatt-hour equals 1,000 watt-hours (Wh). A battery listed at 5 kWh therefore has 5,000 Wh of nominal storage, although not all of that energy may be available for use. A LiFePO4 solar battery buying guide can help you compare capacity, compatibility, and battery-management features before choosing equipment.
How to estimate
Use this basic formula for an appliance connected to an AC inverter:
Estimated runtime in hours = battery capacity in Wh × usable-capacity fraction × inverter efficiency ÷ appliance load in W
For example, a 5,000 Wh battery with an 80% usable-capacity assumption and a 90% inverter-efficiency assumption provides approximately 3,600 Wh for AC loads:
5,000 × 0.80 × 0.90 = 3,600 Wh
If the connected appliance averages 300 W, the simple estimate is:
3,600 ÷ 300 = 12 hours
This is an estimate, not a guarantee. Inverter standby consumption, battery temperature, battery age, cable losses, changing appliance demand, and the system’s low-voltage shutdown setting can reduce the actual result. For a more conservative plan, use the usable-capacity and efficiency figures stated by the equipment manufacturer. If those figures are unavailable, label your assumptions clearly and leave a margin rather than planning to use every advertised watt-hour.
For multiple appliances, add their average wattages first, then apply the same formula:
Total load = appliance 1 average W + appliance 2 average W + appliance 3 average W
Remember that runtime and inverter sizing are different calculations. Runtime uses average operating watts. Inverter sizing must also consider the highest simultaneous load and the starting surge of motors and compressors. A refrigerator may use modest power while running but briefly demand more when its compressor starts. Pumps, freezers, and some power tools can have similar behavior.
Inputs and assumptions
Appliance wattage reference
The following ranges are planning estimates only. Check the rating plate, user manual, or a plug-in power meter for your specific model. Devices with thermostats, heating elements, compressors, or motors can cycle between low and high demand.
| Appliance or load | Planning range | Important consideration |
|---|---|---|
| Refrigerator | 100–300 W while running | Compressor starting surge; cycles on and off |
| Internet router and modem | 10–30 W | Usually a steady, small load |
| LED light | 5–15 W per lamp | Add only the lights needed during an outage |
| Phone or tablet charging | 5–30 W per charger | Intermittent rather than continuous demand |
| Laptop | 30–100 W | Demand varies with workload and charging |
| Small television | 50–150 W | Check the label for screen size and mode |
| Circulation or sump pump | 300–1,000 W while running | Starting surge and run cycle matter |
| Medical device | Use the manufacturer’s stated value | Confirm backup requirements with a qualified provider |
Do not estimate a medically necessary device from a general table. Record its rated watts, expected hours of use, battery or backup instructions, and any required alarm or humidifier accessories. If an outage could create a health risk, arrange a tested backup plan with the device provider and an appropriate professional.
Battery and inverter assumptions
- Nominal capacity: Convert the battery’s kWh rating to Wh by multiplying by 1,000.
- Usable capacity: Use the manufacturer’s recommended usable percentage or depth-of-discharge limit. Avoid assuming that the full nominal capacity is available.
- Inverter efficiency: Apply the published efficiency where available. A planning assumption such as 85%–90% may illustrate losses, but it should not replace the product specification.
- Continuous output: Make sure the inverter can support the combined running load.
- Surge output: Check the inverter’s surge rating for motors and compressors, and consider which appliances may start at the same time.
- System compatibility: Confirm that the battery, inverter, battery-management system, charge controller, and solar panels are designed to work together. Do not mix components solely because their voltage labels appear similar.
A battery can have sufficient energy for an appliance yet still fail to run it if the inverter cannot supply the required starting surge. Conversely, a large inverter does not create more energy; it only allows larger loads to operate when the battery and wiring can support them.
Worked examples
Example 1: Refrigerator and connectivity
Assume a refrigerator averages 150 W while operating and a router uses 20 W. If the refrigerator operates for an estimated 10 hours during a 24-hour period, its energy use is approximately 1,500 Wh. If the router runs continuously, it uses 480 Wh per day:
150 W × 10 hours = 1,500 Wh
20 W × 24 hours = 480 Wh
The combined estimate is 1,980 Wh before inverter losses and other system consumption. A practical plan should also account for compressor starting demand and variation in room temperature. This illustrates why calculating daily energy use can be more useful than multiplying a refrigerator’s running wattage by every hour of the day.
Example 2: Essential outage circuit
Suppose a household chooses a 120 W refrigerator average load, a 20 W router, three 10 W LED lamps, and 30 W of charging. The simultaneous planning load is:
120 + 20 + 30 + 30 = 200 W
With a 5,000 Wh battery, an 80% usable-capacity assumption, and 90% inverter efficiency, the simple runtime estimate is:
5,000 × 0.80 × 0.90 ÷ 200 = 18 hours
Because the refrigerator cycles rather than drawing 120 W continuously, this calculation is intentionally conservative if 120 W represents its average load during operation. The actual outcome will depend on the refrigerator’s duty cycle, inverter idle draw, charging during daylight, and whether all lights and chargers operate continuously.
Prioritization template
- Tier 1—health and safety: medically necessary equipment, emergency communication, and required water or sump pumping.
- Tier 2—food and information: refrigerator or freezer, router, phone charging, and selected lighting.
- Tier 3—comfort: television, laptop use, fans, and other optional equipment.
- Tier 4—high-demand appliances: kettles, ovens, space heaters, hair dryers, and other heating loads unless the system was specifically sized for them.
During an outage, use one tier at a time if the inverter is close to its limit. Turn off chargers and lights when they are not needed, and record actual battery percentage and runtime during a planned test.
Safe setup checklist
- List essential appliances and record their rated or measured watts.
- Identify motor, compressor, and heating loads that may require higher surge or continuous output.
- Calculate both the expected simultaneous load and the estimated daily energy requirement.
- Confirm battery voltage, usable capacity, inverter output, surge rating, and manufacturer compatibility.
- Use suitable cables, overcurrent protection, ventilation, and mounting as specified for the equipment.
- Do not plug a portable inverter into a wall outlet or backfeed household wiring. A permanent home solar backup system requires correctly installed transfer equipment and qualified electrical work.
- Keep portable batteries and inverters dry, protected from excessive heat, and accessible for inspection.
- Test the system with the intended loads before an outage, including refrigerator starts and any essential medical equipment procedure.
For a broader equipment list, see the home backup power checklist. If solar panels will recharge the battery, estimate the panel array separately using a solar panel sizing calculator. Battery runtime and solar charging speed are related, but they are not the same calculation.
When to recalculate
Revisit your solar battery runtime estimate whenever an input changes. Recalculate after buying a new refrigerator, pump, medical device, inverter, or battery; after changing the essential-load list; or after adding solar panels or another battery. Seasonal conditions can also change results: heating and cooling loads, refrigerator cycling, battery temperature, and available sunlight may all differ across the year.
Update the worksheet when manufacturer specifications change or when your measured runtime differs materially from the estimate. During a controlled test, note the starting battery state, connected appliances, operating time, solar input, and ending state. Use those observations to replace assumptions with household-specific values.
Finally, review the plan before periods when outages are more likely in your area. Keep a printed load list, appliance manuals, charging cables, and emergency contacts with the system. A short, tested list of essential loads is generally more useful than an untested promise to power the entire home. For battery selection and capacity comparisons, consult the best solar battery for home backup guide, then verify every final specification against the product documentation and local installation requirements.
