How Long Will a Solar Power Backup Run? Watt-Hours Explained with Real Examples
A solar power backup runs for roughly its battery's usable watt-hours divided by the watts you are drawing. Multiply the battery voltage by its amp-hours to get watt-hours, take about 80 to 90 percent of that as usable, then divide by your load. Our 1000W box (25.6V 27Ah) stores about 691 Wh, so a router, four LED bulbs and a TV drawing around 80W would run for roughly seven hours; a router alone would run for more than two days. The "1000W" on the label is the most it can power at once, not how long it lasts.
The one sum you need
Every backup unit, from a twin-lamp emergency light to a home power system, comes down to the same arithmetic:
- Stored energy: battery volts x amp-hours = watt-hours (Wh).
- Usable energy: watt-hours x about 0.8 to 0.9. This allows for inverter losses when the box turns battery power into 220V AC, and for the reserve a battery keeps back. Treat it as general guidance; the exact figure varies by unit and by load.
- Runtime: usable watt-hours / load watts = hours.
The examples below use 0.85, the middle of that range. Add up the watts of everything you will run at the same time, and you have your load.
Inverter watts are not hours
The figure in a backup unit's name is usually its inverter rating: the most power it can deliver at one moment. It tells you what you can plug in together, not how long the battery will keep it going. That is why the eBean250, sold as a 250W unit, actually stores more energy than our 1000W solar power system:
| Unit | Inverter / rated output | Battery | Stored energy | Usable (x 0.85, approx.) | Price |
|---|---|---|---|---|---|
| 500W Solar Power System | 500W | LiFePO4 12.8V 35Ah | 448 Wh | 380 Wh | KSh 50,000 |
| 1000W Solar Power System | 1000W | LiFePO4 25.6V 27Ah | 691 Wh | 590 Wh | KSh 70,000 |
| eBean250 Energy Storage System | 250W | LiFePO4 3.2V 314Ah | 1,005 Wh | 855 Wh | KSh 40,000 |
So the choice is a trade-off. The 1000W box can run a heavier mix of appliances at the same time; the eBean250 runs a lighter mix for longer, because its output is capped at 250W. Its own listing describes it as sized for lighting, phones, routers, laptops and similar loads rather than heavy appliances.
Worked examples with real loads
These appliance figures are typical values for illustration only; check the label on your own equipment.
- Wi-Fi router: about 10W
- Four LED bulbs at 5W each: 20W
- LED TV (around 32 inches): about 50W
- Laptop while charging: about 60W
- Small fridge: often averages around 60W because the compressor cycles, but draws far more at start-up
Router, lights and TV, 80W
500W box: about 4.75 hours. 1000W box: about 7.4 hours. eBean250: about 10.7 hours.
Add a charging laptop (140W) and the hours fall to about 2.7, 4.2 and 6.1: runtime halves every time the load doubles.
A small fridge
A fridge is the hardest load to predict. As general guidance, its start-up surge can be several times its running draw, and that surge must fit within the inverter rating, so the eBean250's 250W cap is a real limit. If the unit can start it, a fridge averaging 60W would, by the same sum, run for about 6 hours on the 500W box, about 10 on the 1000W box and about 14 on the eBean250, before anything else is plugged in.
About the DQ1208 system
The DQ1208C1500-53 Solar Power System (KSh 90,000) is the largest model in its range. We confirm its battery figures with our supplier before quoting them, so ask us for the volts and amp-hours and run the same sum.
Why LiFePO4 for daily backup
The 500W box, the 1000W box and the eBean250 all use LiFePO4 (lithium iron phosphate) batteries. The 500W listing notes that this is the sensible chemistry because the pack is cycled daily. As general guidance, lead-acid batteries wear faster when they are deeply discharged day after day, so in practice you get less of their stated capacity if you want them to last.
Two of our smaller units use lead-acid. The 20W Portable Solar Home System (KSh 10,400) has a 12V 7Ah battery, about 84 Wh, for three 5W LED bulbs, a radio and USB. The 8W Solar Emergency Light (KSh 9,000) has a 6V 4.5Ah battery, about 27 Wh; its listing points out that lead-acid keeps the unit cheap to replace when the pack eventually tires. Both are good for lighting and small loads, not for running a TV or laptop through an outage.
The same logic applies to solar lights, whose "W" ratings are often marketing figures: compare battery size and panel watts instead, as our guide to solar flood light wattage explains.
What "10 to 24 hours discharge" means
The 500W, 1000W and 20W listings quote a discharging time of 10 to 24 hours. That range is not a promise for every use: it depends on what you draw. The 20W kit's own listing explains that the spread reflects how many bulbs are lit and whether the radio and USB are in use. On the bigger boxes, a router on its own runs past the long end, a router and a few bulbs lands within the range, and adding a TV, laptop or fridge brings you below 10 hours, as the examples above show.
Runtime at a glance
Approximate hours using 85 percent of stored energy. Real runtime also depends on your equipment, battery age and how fully the unit was charged.
| Load | 500W box (380 Wh) | 1000W box (590 Wh) | eBean250 (855 Wh) |
|---|---|---|---|
| Router, 10W | 38 h | 59 h | 85 h |
| Router + 4 LED bulbs, 30W | 12.7 h | 19.7 h | 28.5 h |
| Router + bulbs + TV, 80W | 4.75 h | 7.4 h | 10.7 h |
| Router + bulbs + TV + laptop, 140W | 2.7 h | 4.2 h | 6.1 h |
| Small fridge averaging 60W (if it starts) | 6.3 h | 9.8 h | 14.2 h |
If you are working out how big an inverter a whole house needs, that is a different question: see what size inverter for a house in Kenya. To decide between an all-in-one box and a separate inverter set-up, read solar home system vs inverter.
Where to buy
All of these units are in our solar home systems collection, with the wider range under solar. Collect from our Nairobi trade counter at Royal Business Park, opposite Complast, Old Mombasa Road, Monday to Saturday, 8:00 to 17:30, with free pickup, or order for countrywide delivery at a flat KSh 350, free on orders over KSh 15,000. Send us a list of what you want to run on WhatsApp and we will work through the sum with you. Installers and resellers can request a trade quote on our trade page.
Common questions
How long will a 1000W solar system last?
It depends on the battery, not the 1000W. Our 1000W box stores about 691 Wh, roughly 590 Wh usable, so it runs a 10W router for around 59 hours, an 80W mix of router, lights and TV for about 7 hours, and a 140W load for about 4 hours.
Is a bigger inverter rating better for blackouts?
Only if you need to run more at once. For a long blackout, watt-hours matter more, which is why the 250W eBean250 outlasts the 1000W box on light loads.
How long does it take to recharge?
The 500W and 1000W listings quote 6 to 8 hours of charging; solar charging depends on siting and season. The eBean250 charges from solar at up to 200W or from the mains at 250W.
Can these run a fridge?
Possibly, but check first. A fridge's start-up surge has to fit within the unit's rating, and the eBean250 is capped at 250W. Send us its rating first.