None, if the system is grid-tied and you only wanted to lower a bill. All of them, if you are off-grid in December.
The array size is not the input. A 10 kW array producing about 45 kWh on a good day tells you what is available to charge with; it says nothing about how much you need to store, because storage is sized by what runs when the sun is not shining.
Three different questions, three different answers
Essentials backup — 10 to 15 kWh. Refrigerator, some lights, internet, phone charging, a furnace fan. Carries an evening and a night comfortably, and most of a day-long outage if you are careful. This is what one typical home battery unit provides.
Whole-home overnight — 20 to 30 kWh. Everything, including HVAC, from sunset to sunrise without thinking about it. Usually two battery units.
Off-grid autonomy — 60 to 90 kWh. Two to three days of no meaningful sun, which is the standard design target for off-grid. The jump is large because autonomy multiplies daily consumption directly.
Start from your own daily consumption — the utility bill gives it — and multiply by the fraction of the day and the number of days you want covered. That is the whole method.
Nameplate is not usable
This is where sizing goes wrong most often, and it goes wrong in the direction of buying too little.
Depth of discharge is the fraction of a battery you may actually use before damaging it or shortening its life badly:
| Chemistry | Usable DoD | 10 kWh nameplate gives |
|---|---|---|
| LiFePO4 | 80–100% | 8–10 kWh |
| Flooded lead-acid | ~50% | 5 kWh |
So a lead-acid bank needs roughly twice the nameplate to deliver the same usable energy. The comparison between chemistries is covered in LiFePO4 or lead-acid; for sizing purposes the point is simply that the number on the box is not the number you get.
Round-trip efficiency takes another slice. Energy in does not equal energy out — roughly 90–95% survives the round trip in lithium, less in lead-acid. Add a margin rather than sizing to the exact figure.
Charging a large bank needs enough array
Storage and generation have to be balanced in the other direction too. A very large battery bank behind a modest array charges slowly, and in winter may never reach full.
A 10 kW array yields roughly 30–50 kWh on a clear day depending on region and season, and considerably less under overcast conditions. If a bank needs 60 kWh to refill from empty, a run of dull days leaves it partially charged indefinitely — which is exactly when you wanted it.
This is why off-grid designs oversize the array relative to what annual consumption suggests: the array must be sized for the worst month, not the average one. See peak sun hours for how large that seasonal swing is.
Power and energy are separate limits
A battery has two ratings and both can constrain you.
Energy (kWh) is how long it runs. Power (kW) is how much it can deliver at once. A 13.5 kWh battery rated for 5 kW continuous will not start a large air conditioner regardless of how full it is — the surge exceeds what the unit can supply.
For backup, list the loads that must run simultaneously and check the continuous and surge power ratings against that total. A battery sized correctly on energy and undersized on power fails at the moment it is needed, and the fault looks like a dead battery rather than what it is.
A worked example
A household using 30 kWh a day, wanting whole-home backup overnight:
- Overnight is roughly 40% of daily use → 12 kWh needed at the meter.
- Round-trip losses, add ~10% → 13.2 kWh.
- With LiFePO4 at 90% usable → 14.7 kWh nameplate.
- Peak simultaneous load ~6 kW → the unit must be rated at least that continuous.
One large home battery, or two smaller ones. The 10 kW array is comfortably able to refill that in a day, which is the check that closes the loop.
FAQ
How many batteries do I need for a 10 kW solar system?
There is no fixed number — it depends on what you want to run and for how long. Essentials backup is 10–15 kWh, whole-home overnight 20–30 kWh, and off-grid autonomy 60–90 kWh.
Can a 10 kW array charge a large battery bank?
It produces roughly 30–50 kWh on a clear day, so it can refill a moderate bank easily. A very large bank may not fully recharge during winter or a run of overcast days.
What is depth of discharge?
The share of a battery's nameplate capacity you can use without damaging it. Lithium allows 80–100%; flooded lead-acid should not go below about 50%.
Do I need batteries with grid-tied solar?
No. Batteries add backup during outages and let you use your own evening production, but a grid-tied array reduces bills without them.
Why won't my battery start the air conditioner?
Almost certainly a power limit rather than an energy limit. Check the continuous and surge kW ratings, not the kWh.