Hybrid systemsOff-grid systemsBatteries

How many batteries does it take to power a house?

Batteries are quite expensive to buy 'a couple more just in case'. We know the way to calculate how many of them you need exactly to power your house, depending on your system type. No more words. Let's dive into numbers!

How to read a battery spec sheet

Full PHI 3.8 Solar Battery specification

The number you see in the battery name is the maximum rated capacity under perfect conditions with 100% depth of discharge. To calculate the real battery capacity, you need to work with some basic battery characteristics, which can be found in the spec sheet.

Capacity shows how much energy a single battery can store. Usually, battery capacity is measured in Ah (ampere-hours), but, for your convenience, some manufacturers indicate capacity in Wh (watt-hours). It helps you compare your energy needs and the battery capacity to make the right choice. If the capacity is indicated in Ah, here is how to convert it to Wh:

battery size (Ah) * battery nominal voltage (V) = battery capacity (Wh)

Power rating shows how much electricity can be drawn from the battery to power your electrical devices, measured in kW. A battery with a high capacity and low power rating supplies a low amount of electricity for a long time. That energy would be enough to supply only a few devices. However, a low power rating is a good choice for backup generators. On the other hand, a battery with low capacity and a high power rating could run your entire home, but not for long.

Depth of discharge (DoD) shows to what extent a battery can be discharged without being harmed. For example, let's assume you have a solar battery with a 10 kWh capacity and a recommended DoD of 80%. This means you shouldn't use more than 8 kWh before you recharge your battery again.

Round-trip efficiency shows how much energy the battery loses while just storing it. The higher the round-trip efficiency is, the less energy you lose.

10 batteries for an off-grid system

Off-grid systems aren't connected to the grid, so they can't work without batteries at all. Choosing and sizing batteries for an off-grid system, you should follow a simple rule: the more, the better. For your battery powered home, they are the only source of electricity when the sun fails.

The main battery characteristics to take into account are its capacity, DoD and round-trip efficiency. When multiplied, they show a real battery capacity. For a PHI 3.8 battery, it is:

3.8 kWh * 80% * 98% = 2.98 kWh

Let's take an average American house with a daily energy consumption of about 30 kWh. So, the house batteries should provide those 30 kWh to ensure a one-day emergency backup. If we take that PHI 3.8 battery, we'll need at least 10 of them:

30 kWh ÷ 2.98 kWh ≈ 10 PHI 3.8 batteries

7 batteries for a hybrid system

Hybrid systems are connected to the utility grid, but they also have some extra battery storage as a backup. It's much less than an off-grid battery bank, and is used when the sun isn't active or the grid is down.

If you work a 9-5 job and are not home during the daytime, your house consumes only about 30% of the daily needs. So, the rest 70% are used at night. The primary task of the solar panel system is to generate and store those 70% during the day, so that you could economize and not to buy the electricity from the grid at the on-peak price. Given an average daily energy consumption of 30 kWh, you will need a 21 kWh battery array. It can be made of 7 PHI 3.8 batteries:

21 kWh ÷ 2.98 kWh ≈ 7 PHI 3.8 batteries

When the grid is down, 7 batteries would be enough to live through those blackout hours.

2 batteries for a grid-tied system

You can add solar batteries to a grid-tied system as well. In this case it will be more of an emergency backup for critical loads – the equipment and devices that must stay powered when the grid goes down. They are some medical devices, a fridge, a space heater, lightning etc.

The problem with a grid-tied system is that if the grid goes down, so does the PV array. That is why there is a sub electric panel connected to the essentials only. When the grid fails and your solar system gets disconnected, the sub-panel will switch on and you'll be able to draw the solar energy from your batteries to power your critical loads.

To size your battery bank, make a list of your critical loads and find out their peak power requirements – the maximum amount of energy those devices will use at one time. You can find this information on the label of the devices you've chosen, or check it online.

You'll need 2 numbers:

• Maximum running wattage – how much electricity the device needs to operate.

• Starting or surge wattage – additional power that items with an inductive motor require the first few seconds they are running.

For loads like a refrigerator, a washing machine, an electric stove etc., add their starting wattage to the running wattage. If there are things like light bulbs, a toaster or a microwave oven on your list, count their maximum running wattage only.

Critical LoadMax. Running WattageSurge WattagePeak Power Requirement
6 * 60 W light bulbs
360 W
360 W
200 W
1,100 W
1,300 W
Laptop Charger
60 W
60 W
620 W
1,100 W
1,720 W

When you sum everything up, you'll get the total peak power requirements, which are about 1.7 kW in our example. That is the most electricity you'll need at one time.

This is what your battery's maximum discharge rate should be. The power rating of PHI 3.8 batteries is 1.15 kW, so you'll need at least two of them to make it through the power outage:

1.7 kW ÷ 1.15 kW ≈ 2 batteries

Two PHI 3.8 batteries will be able to power the critical loads for almost 5 hours. To get that number, divide the battery's real capacity by the total running wattage from the list:

2,980 Wh ÷ 620 W ≈ 5 hours

Illustrations – Marina Fionova

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