Solar yield vs. battery capacity

Reading time 4 minutes

How much solar output do you need from which battery capacity to cover your needs? This question is asked by many who either simply want to achieve self-sufficiency in their motorhome or who want to set up an island system in order to be independent of the public power grid, which may not exist, in remote areas.

Now you can of course plan for a lot of battery capacity, but space is at a premium, especially in mobile use scenarios, and the possible PV module output is limited to around 1,200 to 1,725 Wp, assuming 2 to 3 modules with 575 Wp. And the weight in terms of the number of batteries is - unfortunately - not negligible either, unless you have a vehicle with a permissible total weight of over 7.5 tons and the necessary driver's license.

So it is important to minimize consumption if you really want to be able to operate independently. But how long does the sun shine at different times of the day and month, at different geographical locations, and what yields result from the statistically average duration of sunshine? Can you always recharge the batteries?

High-voltage vs. low-voltage technology

12 V batteries are generally known. 24 V are used in trucks. 48 V can be found in mobile homes, boats and yachts. These are all low-voltage technologies.

High-voltage systems operate with voltages above 60 V DC, but usually between 100 and 200 V DC(!).

Why these differences? This is quickly explained when you look at the flowing currents: In inverter operation and 5,000 VA nominal AC power, 12 V batteries would have an impressive 400 A, which is 62 mm² thick, and therefore heavy cable is required, when using 48 V batteries and 104 A only 4 mm², at 200 V this still results in 25 A with a cable cross-section of only 0.25 mm².

However, the comparatively thin cables should not lead to carelessness when handling high DC voltages: all systems that are allowed to carry DC voltages greater than 60 V must – EXCLUSIVELY – be installed and maintained by trained specialists!

The data of the inverter to be used on the input side determines the battery configuration. The higher the input voltage, e.g. 48 V instead of 12 V, the lower the price.

A 24 V DC inverter with 5 kVA costs 1,500 euros, the 48 V version costs around 700 euros.

In high-voltage version, a three-phase 5 kW inverter with 150 V DC input costs around 1,200 euros, an 8 kW with 180 V DC costs around 1,400 euros.

The high-voltage variant is definitely more economical for stationary use.

Charging relativity in mobile use

Now a motorhome will not only stand still, but also drive. This means that the batteries are charged via the charging booster with electricity from the alternator. This is of course difficult to integrate into a calculation, as driving times can hardly be recorded statistically and therefore used in the calculation. But it is good to know that...

You will also occasionally have the opportunity to use a shore power connection to charge your batteries.

Calculable constancy in the stationary field

Calculable insofar as sufficient statistical data has now been collected worldwide which, taking all relevant factors into account, provide information about expected solar yields.

Despite all the statistics, experience shows that theory and practice differ, but it is helpful to get an idea of where you stand in your planning if you have space x and battery capacity y at location z.

This is where the internationally set up online tool helps PVGIS ( PhotoVoltaic Geographical Information System) that was developed by European Commission, Joint Research Centre, Energy Efficiency and Renewables Unit, via E. Fermi 2749, TP 450, I-21027 Ispra (VA).

The documentation The online tool, which is also intuitive to use, is very comprehensive and covers all questions, including those on the level of understanding and nomenclature.

Manufacturer assumptions

Manufacturers of batteries or battery systems want to present their systems in a favorable light and therefore provide prospective buyers with approximate comparative data that gives them an idea of the storage capacity. For example, the statement: Our 10 kW storage module is suitable for a four-person household, including operation of a heat pump and an electric vehicle.

The statement itself is actually quite low, as the annual consumption of such a household is stated by the electricity suppliers to be on average about 5 to 7 kW per year.

The only thing that may thwart this positive assumption is the fact that the stored and thus available energy must also be replenished: the sun naturally only shines for a fraction of the time in winter compared to summer, so the yield is far from equal to the consumption.

Some more clarity can be achieved by experimenting with different parameters in the PVGIS tool mentioned above, which below shows by way of example the influence of changing different parameters for an assumed location.

Example configurations and their results

The geo-location for all the following examples is assumed to be Düsseldorf-Volmerswerth with the coordinates (WGS84) 51.188 (N), 6.749 (E).

Mobile use

Due to the limited space available for PV modules, it is assumed that two 575 Wp modules will be used. The variable is the battery capacity, which allows longer periods of lower solar radiation as the size increases, but on the other hand also requires longer periods of sunshine to complete a full charging cycle.

The minimum possible consumption is considered to be a constant and mandatory quantity. Background: every consumer that MUST always be reliably supplied with sufficient power (e.g. medical equipment such as infusion pumps, ventilators, etc.), as well as lighting, routers and other consumers are added together, and the result is set as the minimum quantity that is reliably available at all times and under all conditions.

Stationary use

Here, both module area and battery capacity are considered as variables, only consumption is seen as static.

A rough guideline can be the min-max daily consumption during the winter months, determined by daily meter readings. In the minimum scenario, the operation of devices that are used permanently and more frequently during the day should be guaranteed, while particularly power-hungry devices are put into operation with caution. This saves financial resources in the storage design.

The maximum scenario allows all devices to operate at normal levels without any restrictions. This could be an optional objective, albeit requiring a larger amount of capital.

What if …?

Simulation – Mobile use

Assuming flat mounting of the PV modules (tilt angle 0°), the following data results:

500 Wh guaranteed yield in the winter months at 1,150 Wp and a battery capacity of 1,120 Ah, corresponding to 14,336 Wh, at a maximum discharge of 85 Wh.

If daily withdrawals exceed 500 Wh, there is a risk that the batteries will be completely discharged because the daily sunlight is no longer sufficient to fully charge them.

An increase in power to 850 Wh is only possible with a four-fold(!) higher battery capacity and results in a discharge of up to 71 percent.

Simulation – Stationary use

For stationary installations facing south, the aim is to optimise the angle of inclination: this will result in an increase in yield of up to 50%. A tilt angle of 35° is considered standard. As the sun is lower in the winter, a steeper angle of 39° will produce a higher yield in the winter months. Steeper angles, on the other hand, will result in a reduction in yield.

750 Wh daily withdrawal is possible at a 39° tilt angle, with otherwise identical data.

For comparison, Österby – Gotlands län, Sweden (51,188, 6,749) – here only an inclination angle of 69° results in a possible daily withdrawal of 500 Wh at an 85 percent discharge.
A reduction of the inclination angle to 39°, however, results in a reduction in yield of only 10%.
In areas with high snowfall, a steeper position makes sense simply because it reduces the amount of snow that accumulates on the modules.