If you ask us whether to choose a 5 kWh solar battery or a 10 kWh solar battery, our first question is what you want to keep running, and for how long. A smaller system may be quite adequate for selected evening loads. An overnight supply that includes air conditioning is a different requirement.

At DW Solar, we assemble hybrid inverters, LiFePO4 batteries and integrated storage systems in-house. [7] When recommending a capacity, we consider how the battery will work with the inverter and the loads you actually intend to supply. More storage is useful only when the rest of that arrangement supports it.

Our advice is to settle the duty first. Then we can work through the capacity, power limits and charging conditions without making a larger battery the automatic answer.

Contents

Tell us what needs to stay on

The capacity we would quote for your system

How we work out backup time

Where we would use each capacity

Why we look at the inverter before adding storage

Can your solar system refill the battery

Choosing capacity for the years ahead

Frequently asked questions

Where we would start with your project

Tell us what needs to stay on

“Backup for the house” needs a little explanation before we can recommend either size. Does that mean a refrigerator, lights and internet? Or does it include cooking, hot water and cooling as well? That distinction can change the proposed system more than the number of rooms in the property.

For solar battery sizing for home use, we would ask for evening consumption or interval meter data. A monthly electricity total is a useful starting point, although it includes daytime demand that solar may already cover directly. The battery mainly needs to cover the gap between generation and use.

For an outage supply, we would work with you to define the backed-up circuits. If you are happy to leave water heating and some kitchen equipment off, a smaller capacity may remain practical. If those appliances must operate normally, we need to allow for them from the beginning.

It helps to separate energy from power here. A 1 kW load running for four hours uses 4 kWh. Kilowatts tell us what the system must deliver at one moment; kilowatt-hours tell us how much energy the operating period requires. Both matter when specifying storage. [3]

The capacity we would quote for your system

In our KNY range, the DW-KNY-E-5 is rated at 5.12 kWh, with a nominal voltage of 51.2 V and a capacity of 100 Ah. The DW-KNY-E-10 provides 10.24 kWh at the same nominal voltage and 200 Ah. These are the actual ratings behind the familiar 5 kWh and 10 kWh descriptions. [1][2]

For your proposal, we would distinguish that rated energy from the energy available for use. Solar battery usable capacity depends on the permitted operating window. Depth of discharge, or DoD, describes the proportion discharged. A reserve retained for outages may also reduce the energy available for normal evening use.

There is another detail we would clarify: whether an energy figure is measured at the battery or at the AC output. Energy is lost in conversion and operation. If a supplier already states usable AC energy, deducting the same losses again would understate it. [3][4]

Installation also enters the discussion. Our 5.12 kWh KNY model allows wall-mounted or floor installation, while the 10.24 kWh KNY model is specified for floor installation. [1][2] We would resolve the intended location with you before treating either model as an interchangeable addition to an existing system.

How we work out backup time

If you ask how long a 5 kWh solar battery will last, we need the average load over the required hours. Adding every appliance’s nameplate wattage can overstate continuous demand for equipment that cycles. Ignoring the hours of operation creates the opposite problem.

Here is a worked estimate using our two model ratings. Assume the battery starts fully charged, 80% of rated capacity is discharged, and 92% of that discharged energy reaches the AC loads. There is no solar charging during discharge. The percentages are calculation assumptions, not DW efficiency specifications or test results.

On that basis, the 5.12 kWh model supplies approximately 3.77 kWh to the loads: 5.12 multiplied by 0.80 and 0.92. The same calculation gives approximately 7.54 kWh for the 10.24 kWh model. Dividing those figures by average load gives the following runtime estimates.

Calculated backup time at different average loads

Calculated examples only. Actual runtime varies with starting charge, operating limits, load changes, temperature, battery condition and system losses. The loads must also remain within the battery and inverter power limits.

The useful point is how quickly the hours change with demand. At an average 0.5 kW, the smaller model gives around 7.5 hours under these assumptions. At 2 kW, it gives around 1.9 hours. We would establish which of those demands resembles your application before discussing an overnight supply.

Where we would use each capacity

A home that needs essential circuits overnight

Take a hypothetical home whose refrigerator, selected lights, router and small devices average 0.35 kW over ten hours. The energy requirement is 3.5 kWh. This is an assumed application, not a recorded DW customer installation.

A 5 kWh solar battery for home backup is worth considering here. Using the 5.12 kWh rating, our worked estimate covers the demand, but leaves a calculated energy margin of only about 0.27 kWh. We would be cautious about adding cooling or promising the same hours after the battery has aged.

Our recommendation would depend on how flexible those loads are. If the aim is simply to maintain essentials, controlling the backup circuits may make sense. If ten hours is a firm minimum with no reduction in demand, we would allow more capacity rather than design around such a narrow margin.

A home or small office using energy for longer

Suppose the average demand is 1 kW for eight hours. That requires 8 kWh. Under the same assumptions, the 10.24 kWh model supplies about 7.54 kWh, so we would not call it a comfortable fit for that duty.

For a 10 kWh solar battery for home use, we would discuss whether the operating period can be shortened, some demand can be shifted, or more storage is needed. A larger approved discharge window could change the calculation, but it needs confirmation for the selected system.

The same reasoning applies to a small office keeping selected equipment running after closing. A factory’s production loads need a separate design. A household-scale battery should not be proposed for those loads on capacity alone.

Why we look at the inverter before adding storage

If the concern is that a pump or compressor will not start, we would examine the power limits before recommending more stored energy. A battery can have charge remaining while the system is unable to deliver the required starting power.

Our DW-KNY-E-5 lists recommended and maximum discharge currents of 50 A and 100 A. The DW-KNY-E-10 lists 100 A and 200 A. [1][2] At the nominal 51.2 V, the recommended-current points correspond to approximately 2.56 kW and 5.12 kW on the DC side. Those are calculated electrical values; they are not guaranteed AC output ratings.

For the actual configuration, we need the inverter output rating, the battery management system limits and any permitted surge duration. That is why we would ask for the exact inverter model, especially if you are adding a battery to equipment already installed.

Solar battery inverter compatibility also includes communication. Our two models list CAN, RS485 and RS232 interfaces. [1][2] Matching an interface name does not establish protocol compatibility. The battery and inverter must support the intended combination and settings.

If you need a solar battery for power outages, tell us that explicitly. Backup operation requires suitable inverter and switching arrangements that isolate the supplied circuits from the grid. PNNL explains this requirement in its guidance on PV battery backup. [5] We would confirm that capability before discussing backup hours.

Can your solar system refill the battery

Before recommending the larger capacity, we would look at what is left from solar generation after daytime consumption. A battery that regularly finishes the day partly charged has less energy available for the night, regardless of its label.

For illustration, assume 2 kW remains available for charging after the building’s loads are supplied. Replacing 4 kWh takes two hours in a loss-free calculation; replacing 8 kWh takes four. Actual charging needs additional energy and may take longer as sunlight varies or charging power reduces.

Charging a solar battery from solar panels therefore depends on surplus generation at the right time. Panel count alone tells us too little. For your project, we would want to understand daytime demand and the less sunny season as well as the best summer days. [3]

Grid charging can provide another opportunity where the system supports it and local rules permit it. For an off-grid property, we would also discuss poor-generation days and the fallback supply. Increasing storage from one capacity class to the next does not by itself establish reliable off-grid operation. [6]

Choosing capacity for the years ahead

Our advice is to leave room for the duty to become harder. That may mean longer cooling hours, additional equipment or less available capacity as the battery ages. The appropriate allowance depends on how much flexibility you have during an outage.

Both KNY models list 6,000 cycles at 25°C and 80% DoD. [1][2] We keep those conditions with the figure. For a purchase decision, the warranty period, retained-capacity terms and any throughput limits need separate confirmation; the cycle figure alone does not establish them.

Placement matters as well. Both models specify charging at 0–55°C and discharging at −20–60°C. [1][2] If you are considering an unheated location, we would check the charging conditions there. The lower discharge-temperature limit is not permission to charge at the same temperature.

If you prefer to start smaller, we would settle the expansion arrangement before purchase. Battery combinations, inverter support and installation requirements need to be agreed. “Add another one later” is only useful advice when that later configuration is supported.

Frequently asked questions

Is a 10 kWh solar battery enough to run a house

It may be sufficient for selected loads or a defined evening period. We would need your consumption and required hours before recommending it. Continuous heating or cooling can change the result substantially.

Can a 5 kWh battery run air conditioning

Potentially, if the battery and inverter meet the unit’s running and starting demands. Give us the electrical input data or measured consumption. Cooling capacity is not the same as electrical input power.

Can two 5 kWh batteries replace one 10 kWh battery

Two compatible 5.12 kWh units provide 10.24 kWh of rated energy in an approved parallel arrangement. We would still confirm the inverter, protection and installation. Equal stored energy does not make every configuration equivalent.

Will the larger battery save more money

We would look at how much of the extra capacity you can charge and discharge usefully. Tariffs, displaced grid imports and installed cost determine the financial result. Capacity that is rarely used cannot guarantee a better return. [4]

Where we would start with your project

A 5 kWh solar battery is a sensible starting point for a modest, well-defined energy requirement. A 10 kWh solar battery gives more scope for longer use, provided the charging source and system power suit the duty.

Send us the equipment you need to supply, the operating hours and the inverter model if one is already installed. From there, we can discuss a DW configuration with a clear capacity basis. You should be able to see why it fits your use, rather than simply being offered the next size up.