Quick answer: Take your daily energy use in kWh, divide by 0.85 to account for depth of discharge, then round up to the nearest standard battery size — usually 5.12, 10.24 or 15.36 kWh. Most three-bedroom homes land on one 10.24 kWh battery. Below is how to get a number you can actually trust.
Start with what you actually use, not what you own
The most common mistake we see is sizing a battery around the house instead of the load. A five-bedroom house with efficient LED lighting and no AC can need less storage than a two-bedroom flat running a chest freezer and a split unit all night. Before you think about battery size, you need one number: how much energy you use in a typical day, measured in kilowatt-hours (kWh).
You can estimate this yourself with three steps:
- List every appliance you want backed up.
- Multiply each appliance's wattage by how many hours a day it runs.
- Add them all together, then divide by 1,000 to convert watt-hours into kWh.
A rough example for a modest household:
| Appliance | Watts | Hours/day | Watt-hours |
|---|---|---|---|
| Fridge | 150W | 24h | 3,600Wh |
| 4 LED bulbs | 40W | 6h | 240Wh |
| TV | 120W | 5h | 600Wh |
| 2 fans | 140W | 8h | 1,120Wh |
| Router | 15W | 24h | 360Wh |
| Total | ≈5,920Wh (5.9 kWh) |
That household is using just under 6 kWh a day. If you'd rather skip the arithmetic, our solar calculator does this automatically — tick what you want to power, and it estimates your daily use for you.
Why you can't use 100% of a battery's rated capacity
A "10.24 kWh" battery doesn't give you 10.24 kWh of usable power every day. Lithium batteries are rated with a depth of discharge (DoD) — the percentage of stored energy you can safely draw before recharging. Most LiFePO₄ batteries we install are rated for 85–95% DoD, which is one of the reasons lithium has replaced lead-acid in most new installations: lead-acid batteries are usually limited to 50% DoD, meaning you need double the rated capacity to store the same usable energy.
To size correctly:
Battery size (kWh) = Daily energy use (kWh) ÷ Depth of discharge
Using the 5.9 kWh household above, at 85% DoD: 5.9 ÷ 0.85 = 6.9 kWh needed. Since batteries come in standard sizes, that household would move up to a 10.24 kWh unit rather than run a battery close to empty every night, which shortens its lifespan.
Backup duration changes the answer
The calculation above sizes a battery to cover roughly one day of use. If you want the system to ride out a longer outage — say, 2–3 days of heavy cloud cover with no grid — multiply your daily figure by the number of days you want covered, then divide by DoD as before. This is the difference between our Essential package (built around 6–8 hours of backup for lighter loads) and Home Plus (10–14 hours, sized for a fridge and moderate AC use).
What actually changes battery count
Three things push people toward more storage, in order of how often we see them:
- Air conditioning. A single 1HP split unit can use as much energy in a night as everything else in the house combined.
- Refrigeration and freezers. They run 24 hours a day, so even a small load adds up over time.
- Wanting to go a full day without any grid or generator at all, rather than just covering evening hours.
If none of these apply to you, resist the temptation to over-buy. A bigger battery than you need doesn't hurt performance, but it does add unnecessary cost — money that's often better spent on a slightly larger solar array so the battery recharges fully each day.
The honest way to get this right
Every calculation above is an estimate built on typical usage patterns. Actual appliance draw varies by brand and age, and surge loads (the extra power a compressor or motor needs at start-up) aren't captured by a simple daily-use sum. That's what a proper energy assessment is for — one of our engineers reviews your actual appliances and usage pattern and confirms exact sizing before you commit to anything.
Related reading: How much solar power does a home need? and How lithium batteries work.
FAQs
Can I add a second battery later instead of buying big now?
Yes, most of our hybrid inverters support battery expansion, so it's reasonable to start with one battery and add a second if your usage grows.
Do batteries need replacing like a generator's alternator?
Not on the same schedule. A quality LiFePO₄ battery is typically rated for 6,000+ charge cycles — used daily, that's over 16 years before capacity meaningfully declines.
Is a bigger battery always better?
No. Battery size should match your daily use and desired backup duration. Oversizing mostly adds cost without adding meaningful backup, since the battery still recharges from the same solar array each day.