A practical guide to sizing lithium batteries for 1HP, 1.5 HP, and 2HP air conditioners.
Imagine you have invested in a solar power system to keep your home comfortable during power outages.
But halfway through the night, your air conditioner switches off because the battery has run out of usable energy.
It’s a frustrating experience, particularly when the main reason for buying the system was to enjoy uninterrupted sleep.
For Nigerian homeowners considering solar power, battery capacity is one of the most important decisions.
Choose too little, and the system may not last through the night.
Buy more than you need, and you could spend unnecessarily on storage.
So, how many kilowatt-hours (kWh) of battery do you need to run an air conditioner overnight?
For many homes, a 10kWh lithium battery can be a reasonable starting point for running one efficient air conditioner for eight hours.
However, the right capacity depends on the AC’s actual electricity consumption, how long it operates, and what other appliances are connected.
Here’s how to calculate it.
How Much Battery Capacity Do You Need for 8 Hours of AC?
The table below provides estimated battery requirements for common air conditioner sizes used in Nigerian homes.
| AC size | Assumed average power consumption | Estimated battery capacity for 8 hours |
|---|---|---|
| 1HP | 600–900W | 6–9kWh |
| 1.5HP | 800–1,300W | 8–13kWh |
| 2HP | 1,100–1,800W | 11–18kWh |
These are illustrative estimates, not manufacturer specifications. .
An efficient inverter AC may consume less electricity after cooling the room. An older or less efficient model may use considerably more.
This is why horsepower alone cannot determine the correct battery size.
Understanding AC Power Consumption: HP, Watts and kWh
Three measurements matter when sizing a battery for air conditioning.
Horsepower (HP): Commonly used to describe air conditioner size in Nigeria. It is not a reliable measurement of the electricity an AC consumes.
Watts (W): Measure the electrical power an appliance draws at a particular moment.
Kilowatt-hours (kWh): Measure the amount of electricity consumed over time or stored in a battery.
For example, if an AC draws an average of 1,000 watts (1kW) over eight hours, it consumes:
1kW × 8 hours = 8kWh
However, an 8kWh battery would not necessarily provide eight hours of cooling because some stored energy is unavailable or lost during power conversion.
How to Calculate the Right Battery Size for Your AC
The basic calculation is straightforward:
Battery capacity (kWh) = (Average AC power in kW × Running hours) ÷ (Battery depth of discharge × Inverter efficiency)
The depth of discharge and inverter efficiency must be expressed as decimals.
Example: Running a 1.5HP AC for Eight Hours
Suppose your bedroom air conditioner draws an average of 1,000W and you want it to run from 10 p.m. to 6 a.m.
Your estimated overnight electricity consumption is:
1kW × 8 hours = 8kWh
Assuming 90% depth of discharge and 92% inverter efficiency:
8kWh ÷ (0.90 × 0.92) = 9.66kWh
A nominal 10kWh lithium battery would be close to the calculated requirement for the AC alone.
But this leaves very little allowance for other appliances, inverter standby consumption or unexpected increases in cooling demand.
In practice, your installer should include an appropriate reserve rather than designing the system to operate at its calculated limit every night.
Can a 5kWh or 10kWh Lithium Battery Run an AC Overnight?
The answer depends largely on the AC’s average power consumption.
For this comparison, we assume the battery starts fully charged, permits 90% depth of discharge and operates through an inverter at 92% efficiency.
Under these conditions, a nominal 5kWh battery supplies approximately 4.14kWh of usable AC electricity, while a nominal 10kWh battery supplies 8.28kWh.
| Average AC consumption | 5kWh battery runtime | 10kWh battery runtime |
|---|---|---|
| 500W | 8.3 hours | 16.6 hours |
| 750W | 5.5 hours | 11.0 hours |
| 1,000W | 4.1 hours | 8.3 hours |
| 1,500W | 2.8 hours | 5.5 hours |
Runtimes are theoretical estimates for the specified constant average loads. They exclude other connected appliances, standby consumption and additional energy reserves.
A 5kWh battery may support overnight cooling if the air conditioner is exceptionally efficient and the average load stays low.
A 10kWh battery offers more flexibility, but it is not a guarantee of eight hours of operation for every 1.5HP or 2HP AC.
Before buying either capacity, check the appliance’s electrical specifications and, where possible, measure its consumption during normal overnight operation.
What If You Want to Run Two or Three ACs Overnight?
Battery requirements increase significantly when several bedrooms need cooling.
Consider a four-bedroom duplex in Lagos where two air conditioners operate for eight hours every night.
If each AC consumes an average of 900W:
- Combined power consumption: 1,800W (1.8kW)
- Energy consumed over eight hours: 14.4kWh
- Calculated nominal battery capacity: 17.4kWh
A 20kWh battery could be a reasonable starting point for further assessment, but the final capacity must account for additional loads and an appropriate operating reserve.
If a third AC with the same average consumption is added, the calculated nominal capacity rises to approximately 26.1kWh before other household appliances.
The lesson is simple: the number of ACs matters, but their combined energy consumption matters more.
Does an Inverter AC Use Less Battery Power?
Generally, an efficient inverter AC can use less electricity than a comparable conventional fixed-speed model under suitable operating conditions.
A conventional AC typically switches its compressor on and off to maintain the selected room temperature.
An inverter AC can vary compressor speed, allowing it to reduce power consumption when less cooling is required.
However, the actual savings depend on factors such as equipment efficiency, room temperature, insulation, thermostat settings and outdoor conditions.
An inverter AC may still consume substantial electricity when cooling a hot room, particularly during the first few hours of operation.
When planning a solar system, use measured or reasonably estimated average consumption rather than assuming that every inverter AC has a low electricity demand.
Why Your AC Battery May Not Last Through the Night
If your battery runs out earlier than expected, there are several possible explanations.
1. The AC consumes more electricity than expected.
Its actual overnight energy demand may be higher than the figure used during system design.
2. Other appliances are using the battery.
Refrigerators, freezers, fans, televisions, security lights and internet equipment contribute to the household’s total electricity demand.
3. The battery has less usable capacity than its advertised nominal rating.
Manufacturers specify operating limits that determine how much stored energy can be discharged. Battery ageing, temperature and system settings may also affect performance.
4. The system has insufficient operating reserve.
A battery sized exactly to the expected load can fall short when consumption increases.
5. The battery was not fully charged before nightfall.
Even a large battery may run out early if it starts the evening at a low state of charge.
Identifying the actual cause is more useful than immediately purchasing another battery.
Can Solar Panels Recharge Your AC Battery Every Day?
Yes, but the solar array must produce enough usable electricity to support daytime consumption and replenish the energy used overnight.
Suppose your air conditioner consumes 8kWh during the night.
Your solar panels must generate enough energy to replace that consumption, accounting for charging and conversion losses, while also supporting any appliances that operate during the day.
Actual solar production depends on the location, weather, panel orientation, shading, system losses and available sunlight.
For Lagos homeowners, this is particularly important during periods of frequent cloud cover and rainfall.
A properly engineered solar solution should consider battery storage and daily energy generation together.
Related reading: How Many Solar Panels Do I Need for My House in Nigeria?
How to Reduce Battery Consumption While Running Your AC
You may be able to improve overnight battery runtime without adding more storage.
Start with the air conditioner itself. A correctly sized, energy-efficient AC can reduce unnecessary electricity consumption.
Keep doors and windows closed while cooling, maintain clean air filters and minimise unnecessary heat entering the bedroom.
Avoid setting the thermostat lower than required for comfortable sleep. Where suitable, a ceiling or standing fan can help circulate cooled air.
These steps may reduce energy consumption, although the actual savings will depend on your equipment and household conditions.
Why BOYLS Recommends an Energy Audit Before Battery Sizing
Two homes can have identical air conditioners and very different electricity requirements.
A well-insulated bedroom may retain cool air longer. Another room, exposed to intense afternoon sunshine, may require more cooling well into the night.
Battery capacity should reflect these differences.
At BOYLS Engineering Services Limited, we believe homeowners should understand their electricity consumption before investing in an expensive solar power system.
Our BOYLS Home Energy Confidence Audit helps eligible Lagos homeowners move from assumptions to measured energy data.
The service includes:
- Installation and subsequent removal of an energy-monitoring device
- 30 days of electricity consumption monitoring
- Analysis of the home’s energy-use patterns
- A detailed energy report
- Solar system design recommendations based on the findings
Current pilot audit fee: ₦225,000, subject to eligibility and applicable service terms.
The assessment helps establish how much electricity your home uses, when demand is highest and what battery and solar capacity may be appropriate.
It is particularly useful for homeowners who want reliable overnight air conditioning without spending unnecessarily on oversized equipment.
Frequently Asked Questions
Can a 5kWh battery run a 1HP AC overnight?
It depends on the air conditioner’s average power consumption. Under the assumptions used in this guide, a 5kWh battery could run an average load of approximately 518W for eight hours, excluding additional loads and reserves.
How long will a 10kWh lithium battery run a 1.5HP AC?
If the AC averages 1,000W, the estimated runtime is 8.3 hours. At 1,200W, it falls to approximately 6.9 hours. Actual runtime depends on the AC, battery specifications and connected loads.
Is a 15kWh battery enough for two ACs?
Possibly, if their combined average consumption is sufficiently low. Under our assumptions, a nominal 15kWh battery can provide approximately 12.42kWh of usable AC electricity before additional loads and reserves.
Can a 20kWh battery run three ACs overnight?
It may, but only if their combined electricity consumption is within the battery’s usable energy and the system’s power limits. Under our example assumptions, a nominal 20kWh battery can support an average AC load of approximately 2.07kW for eight hours, excluding other loads and reserves.
Do I need a 10kVA inverter to run a 2HP AC?
Not necessarily. The appropriate inverter depends on the air conditioner’s running power, starting characteristics, other connected appliances and the inverter’s actual power rating. A larger inverter does not automatically provide longer battery runtime.
Is a 10kWh or 15kWh battery better for overnight AC use?
A 15kWh battery stores more energy, but it is not automatically the better investment. The correct choice depends on the home’s electricity demand, desired operating hours, charging capacity and budget.
