If you need to figure out how many hours a battery can keep your devices running, the short answer is: Backup Hours = (Battery Capacity (Wh) × Inverter Efficiency) ÷ Total Load (W). That simple formula gives you a baseline estimate, but real‑world performance depends on a handful of factors that can swing the result by ±20 % or more. Below is a step‑by‑step guide packed with data, tables, and practical tips so you can nail the numbers for any scenario—be it a home emergency kit, a cabin off‑grid, or a weekend camping rig.
Let’s break it down.
1. Identify What You Need to Power
First, list every device you plan to run and its typical power draw. Use the table below as a quick reference; values are rounded averages for typical North American/European appliances.
| Device | Typical Wattage (W) | Running Hours/Day (h) | Daily Energy (Wh) |
|---|---|---|---|
| LED Light (10 W equivalent) | 10 | 5 | 50 |
| Wi‑Fi Router | 12 | 24 | 288 |
| Laptop (charging) | 60 | 4 | 240 |
| Refrigerator (mini, 4 cu ft) | 80 | 10 | 800 |
| Full‑size fridge (Energy Star) | 150 | 12 | 1 800 |
| LED TV (42 in.) | 80 | 4 | 320 |
| CPAP machine | 50 | 8 | 400 |
| Ceiling fan (medium speed) | 75 | 6 | 450 |
| Microwave (1000 W microwave) | 1000 | 0.5 | 500 |
If your device isn’t listed, you can usually find the wattage on a label or in the user manual. Multiply the wattage by the number of hours you expect the device to run each day to get the daily energy demand (Wh).
2. Calculate Total Load
Add up the daily energy figures from the table. For example, a modest emergency setup might include:
- 3 LED lights (10 W each) → 3 × 50 Wh = 150 Wh
- 1 Wi‑Fi router → 288 Wh
- 1 laptop (charging) → 240 Wh
- 1 mini fridge → 800 Wh
Total = 1 478 Wh per day.
If you plan to run the system for multiple days without sunlight (e.g., two days), multiply the daily total by the number of days: 1 478 Wh × 2 = 2 956 Wh.
3. Know Your Battery Specifications
Battery capacity is expressed in amp‑hours (Ah) or watt‑hours (Wh). The relationship is simple:
Wh = Ah × Nominal Voltage (V)
Common battery voltages for backup systems are 12 V, 24 V, and 48 V. Choose a voltage that matches your inverter and wiring to minimize losses.
Also consider Depth of Discharge (DoD) – the percentage of capacity you can safely use without significantly shortening battery life. Typical DoDs:
| Battery Chemistry | Recommended DoD (%) | Round‑trip Efficiency (%) |
|---|---|---|
| Lead‑acid (Flooded) | 50 | 75‑80 |
| AGM | 50‑60 | 80‑85 |
| Gel | 60 | 80‑85 |
| Lithium‑ion (LiFePO4) | 80‑90 | 90‑95 |
| Nickel‑Manganese‑Cobalt (NMC) | 80 | 90‑95 |
If you’re using a lead‑acid pack, you’ll need more total capacity to achieve the same usable Wh because you can only discharge to ~50 % safely.
4. Apply the Formula – Step‑by‑Step
Now you have everything needed for the backup‑hours calculation. The steps are:
- Determine usable capacity: Usable Wh = Total Wh × DoD (as a decimal). For LiFePO4 at 80 % DoD: Usable Wh = Battery Wh × 0.80.
- Account for inverter efficiency: Inverters typically run at 85‑95 % efficiency. Multiply usable Wh by the inverter’s efficiency (e.g., 0.90 for 90 %).
- Compute backup hours: Backup Hours = (Usable Wh × Inverter Efficiency) ÷ Total Load (W).
Let’s plug in numbers for our example (2‑day backup, 3 kW inverter at 90 % efficiency, 24 V LiFePO4 battery):
- Required daily energy = 1 478 Wh
- Two‑day requirement = 2 956 Wh
- Choose a 48 V system with 100 Ah cells → Battery Wh = 48 V × 100 Ah = 4 800 Wh
- Usable capacity (80 % DoD) = 4 800 Wh × 0.80 = 3 840 Wh
- After inverter loss (90 %) = 3 840 Wh × 0.90 = 3 456 Wh
- Backup hours = 3 456 Wh ÷ (1 478 Wh / 24 h) = 3 456 Wh ÷ 61.58 W ≈ 56 hours
That’s roughly 2.3 days of continuous operation for the listed loads—more than enough for an emergency weekend.
5. Adjust for Real‑World Variables
- Temperature: Lead‑acid capacity drops ~1 % per °C below 25 °C. Lithium chemistries lose about 0.5 % per °C.
- Age and cycle count: After 500 full cycles, a lead‑acid battery may retain only 70‑80 % of original capacity; LiFePO4 can stay above 80 % after 3 000 cycles.
- Partial loads: If you don’t run every device at once, the effective load will be lower, extending backup time. Keep a “peak‑load” figure for worst‑case scenarios.
- Surge power: Appliances like microwaves or pumps draw 2‑3 × their running wattage for a few seconds. Make sure your inverter can handle the surge, otherwise you’ll need a larger model or a soft‑start controller.
6. Planning for Multiple Days
If you’re aiming for a multi‑day backup (e.g., 3‑day off‑grid weekend), you can either:
- Increase battery bank size (more Ah), or
- Add renewable charging (solar, wind) to replenish daily.
For a hybrid approach, consider a speicher für balkonkraftwerk which integrates a small battery with a balcony solar system—perfect for extending backup without a full‑scale installation.
7. Quick “Rule of Thumb” for Common Scenarios
| Scenario | Typical Load (W) | Desired Backup (h) | Recommended Battery (48 V LiFePO4) |
|---|---|---|---|
| Essential home lights + router | 30 | 12 | 2 kWh (≈40 Ah) |
| Small fridge + lights + Wi‑Fi | 150 | 24 | 5 kWh (≈100 Ah) |
| Full kitchen (microwave, fridge, TV) | 400 | 8 | 6 kWh (≈125 Ah) |
| Off‑grid cabin (lights, laptop, fan, fridge) | 250 | 48 | 10 kWh (≈200 Ah) |
These numbers assume 90 % inverter efficiency and an 80 % DoD. Adjust upward by ~20 % if you’re using lead‑acid chemistry.
8. Common Pitfalls and How to Avoid Them
- Ignoring the inverter’s own consumption: Many inverters draw 5‑15 W even when idle. Over a 24‑hour period that’s 120‑360 Wh, which can eat into your backup time.
- Neglecting voltage drop in wiring: Use appropriately sized cables for the current. A 2 % voltage drop can reduce effective usable capacity by a similar percentage.
- Over‑sizing the battery for a tiny load: If you only need 12 hours of backup for a 30 W load, a 4 kWh battery is overkill—choose a 0.8 kWh pack to save cost and space.
- Forgetting seasonal variation in solar input: If you rely on solar to recharge, model the worst‑case month (e.g., December in Northern Europe) to ensure you don’t run out.
9. Putting It All Together – A Real‑World Example
Imagine you live in a temperate climate and want a backup system for a weekend power outage. Your must‑have devices:
- 2 LED lights (10 W each)
- 1 Wi‑Fi router (12 W)
- 1 laptop (60 W charging)
- 1 mini fridge (80 W)
Total continuous load = 162 W. Daily energy needed ≈ 162 W × 24 h = 3 888 Wh. For two days, you need 7 776 Wh. Choose a 48 V LiFePO4 battery bank rated at 200 Ah → 9 600 Wh. Usable at 80 % DoD = 7 680 Wh. After inverter (90 % efficiency) → 6 912 Wh. Backup hours = 6 912 Wh ÷ 162 W = ≈ 42.7 hours, or roughly 1.8 days of full‑time use. If you add a 400 W solar panel that supplies ~2 kWh per day in winter, you can extend that to over 3 days.
That’s a solid, data