POWER – calculation

In our last set of posts we wrote about the array and what effects there are on our solar panel array and why. Carrying on from the array, we now move to the power calculations on what your inverter, battery and solar panels need to provide you. You can catch up here.

According to the Department for Business, Energy & Industrial Strategy (BEIS), the average UK household uses 3,509 kWh of electricity per year. That’s 9,610 watt-hours per day, which can be divided by 24 hours to get an average of 400 watts (W) to power a home throughout the day.

We see that there are peaks of power during the day and night, and this will depend on what we are using.
The solar inverter plays a key roll in part of the system, but for a range of reasons.

A solar inverter is a device that converts the direct current (DC) electricity produced by solar panels into alternating current (AC) electricity that can be used by appliances or fed into the grid. A solar inverter is an essential component of a solar power system, as most devices and grids operate on AC electricity or they can be DC powered via a power brick. We use AC power as it can travel farther than DC without losses.

There are different types of solar inverters, such as microinverters, string inverters, and hybrid inverters, each with their own advantages and disadvantages. The choice of solar inverter depends on factors such as the size, design, and location of the solar array, the budget, and the preference of the user, but they should be considered on the peak power requirements of the devices that are attached (loads).
We will go more into this in following articles.

Peak Power:

Peak current or power use is the maximum amount of electricity that an appliance draws when it starts up or operates at full capacity. It is usually higher than the running or rated power, which is the average amount of electricity that an appliance uses when it operates normally.

An example of a fridge freezer and its startup and running power is:

• A typical modern upright freezer uses 1.2 amps when running, which is equivalent to 276 watts . The startup power is expected to be about 4 amps, which is equivalent to 840 watts.
  
• A typical modern refrigerator-freezer uses 500-750 watts when running, depending on the size and model. The startup power is expected to be about 2200 watts, which is equivalent to 9.5 amps

These values are only estimates, and the actual power consumption of your fridge freezer may vary depending on various factors. You can check the manufacturer’s label or use a wattmeter to measure the exact power usage of your appliance.

While we have looked at one item in the home, there are many, you may have a TV of 170w, a kettle which is 2.2kWh a hover 1kWh and so on. It is the combined total of the peak power ( the maximum of combined devices) that we need to know. This is OUR INVERTER POWER REQUIRED.

If you are off grid then the total power peak needs to be met by your inverter, however if you are grid tied or hybrid then you do not need to meet your peak power load requirements. but you will pay for anything over than of which the solar or inverter can provide.

How many solar panels do I need?

For grid tied, you would want to power your inverter at full power for as long as you can, while still meeting your loads. The excess goes to the grid normally and they sell your power you made.
We have to look at solar power generation around the year and how much we will want to off-set.
If you have a 3.6kW inverter you can meet loads up to 3.6Kw. therefore your solar panel array should make that power, however if you have a base load, for example 400W 24/7 then your minimum would be 400w.
You will always pay for what you cannot make and you will not save from what you cannot use. so getting this correct is important. You will have sun hours, therefore we can use 4 and 2 hours to calculate the power total from the array.

lets say we have a 3.6kw inverter. We have 8 435W solar panels in our array. This is a 3480W peak and our daily figures would look something like this:
4 Hours =13.92 kWh (9.1kWh normi)
2 Hours – 6.96 kWh (4.5kWh normi)

Normi is me average and real worlding the actual power over the day.

The key factor is that peak is the same as the solar peak, thus not peak for most of the day. you would need to change the array times and size to suit your installation and use, but as you read the pervious articles (about arrays) you would know all about this.

Hybrid Solar system arrays

Having a hybrid is a solar inverter with a battery. The battery can offer a buffer for clouds and the ups and downs of real work solar production. They also store power in a solar battery which can be used later.

While you can make more power and store that power to use, the key point is that you need to recharge the battery. But while that is clear you also need to meet the loads in the same time.
If we use again the 400w base load, we will add a 15kWh battery which will need to be charged.
15000(w) / 4 =375w
So we need to add the battery charge requirement to the base load, if we are only making the base load we would never have the spare power to charge the battery. So we now need a 775w array as a minimum but we want to meet our peak loads. which gives us a figure of 4375w

It is worth knowing that a lot of devices while they have a high peak do not run for an hour. ( the measure is per hour) a 2.2Kw kettle uses 2.2kWh only if it were on for an hour, its likely to be on for 3 minutes.
So we know that 2.2/60 is 36.66wh or (0.11kWh); equally the fridge freezer may have a 1.5kW start up and 230W run load, but be on for 15 minute in the hour. (0.057Wh)

In the above paragraph the bracketed figures would be calculated in a 24 hour period, the Freezer would be 1.36kWh a day. If we store this in the battery or have it as a live load on the array for the sunny part then this will change the power used and stored as well as the charge.
it is worth noting that most charges are around 2kWh max so you cannot exceed this amount to charge a single battery system.

Lets pretend your an average.

Using an average is never the right idea, I seldom find that family are “average” in their home and power use or that the panels would lay in the perfect placement. So with that in mind, lets do some averages.

You will use 9,610 watt-hours per day, so what are the calculations?

1. Number of Solar Panels:
   • To determine the number of solar panels you’ll need, we’ll first calculate the total power consumption in kilowatt-hours (kWh) per day.
   • Your daily power usage is 9610 watts, which is equivalent to 9.61 kilowatt-hours (since 1 kilowatt-hour = 1000 watts).
   • Given that your solar panels are 435 watts each, we can calculate the number of panels required:
     • Number of panels = Total daily power usage / Panel wattage
     • Number of panels = 9.61 kWh / 0.435 kW ≈ 22 panels (rounded up).
     • 22 Panels for winter ( 1 Sun hour).
     • Summer Total daily energy of 22 panels ≈ 2,000,000 watt-hours (Wh) or 2,000 kilowatt-hours (kWh).
     • Let’s focus on the summer value for our calculation.
       
   • Daily Solar Energy Generation:
     • Assuming an average daily sun exposure of 5 hours during summer ², we can estimate the daily energy generated by a single solar panel:
       • Energy per panel = Panel wattage × Sun hours
       • Energy per panel = 435 watts × 5 hours = 2175 watt-hours (Wh) or 2.175 kilowatt-hours (kWh).
   • To cover your daily power usage of 9610 watts, divide the total daily energy needed by the energy generated per panel:
     • Number of panels = Total daily power usage / Energy per panel
     • Number of panels = 9610 watts / 2175 watts ≈ 4.42 panels ( 5 rounded up).
       
2. Battery Capacity:
   • To determine the battery capacity, consider how much energy you want to store for nighttime or cloudy days.
   • Let’s assume you want to store enough energy for 2 days without sunlight.
   • Total energy needed for 2 days = 2 × 9.61 kWh = 19.22 kWh.
   • Battery capacity should be at least 19.22 kWh to cover your needs.
     
3. Sun Hours:
   • Sun hours refer to the number of hours during which your solar panels receive direct sunlight.
   • In the UK, the average daily sun hours vary based on location and season. As a rough estimate:
     • Summer: 4 to 6 hours
     • Winter: 1 to 2 hours
   • Let’s take an average of 5 sun hours per day for our calculation.
     
4. Daytime Loads and Charging:
   • During the day, your solar panels will generate electricity. Here’s how it works:
     • Solar panels generate power during daylight hours.
     • Excess power not used immediately charges the battery.
     • If the battery is fully charged, any additional power can be exported to the grid (if you have a Smart Export Guarantee).
   • Your daytime loads (appliances, lighting, etc.) will consume power directly from the solar panels.
   • Any surplus energy will charge the battery.
   • If your panels generate more power than your daytime loads and battery can absorb, the excess goes to the grid.

Remember that these calculations are approximate, and actual performance may vary based on factors like panel orientation, shading, and local weather conditions. It’s advisable to consult with us to tailor the system to your specific needs and location.