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What factors go into sizing a solar and battery storage system?

While the benefits of solar-plus-storage – increased energy independence, reduced energy bills and lower emissions – are well-known, prices are finally reaching a point where systems are accessible for residential and commercial users.  

Whether installing a complete solar battery storage system or adding energy storage to an existing photovoltaic array, one of the first questions to come up is “How much capacity is required?”. We will take a look at the factors are taken into account when assessing what size solar array or battery capacity is best for an application.

This guide is intended as a starting point – individual factors such as equipment and installation costs, electricity rates and grid stability will vary depending between countries and depending on the year.

What is the goal of adding solar-plus-storage?

The ideal size of solar and storage will depend upon the purpose of the installation. The two most cited reasons behind adding solar battery storage capacity are:

• Increasing energy independence/self-sufficiency
• Reducing electricity costs long-term

While often these two factors go hand-in-hand, aiming for complete self-sufficiency from the grid requires a much higher investment in energy storage than merely reduce peak time use. This is related to the following section:

What level of energy independence is required?

To be completely “off-grid” and energy independent, both solar power generation and energy storage must be over-sized in case of a power outages. The upfront costs involved are expensive – but for most clients, this level of energy self-sufficiency is far beyond their requirements.

Decreasing the level of energy independence decreases the upfront installation costs. From highest to lowest:

• Winter self-sufficiency (without power cuts), summer surplus
• Summer self-sufficiency, grid reliance in winter
• Peak time self-sufficiency
• Reduced grid reliance during peak times

For example, for a business in a remote area with an unstable local power grid, complete or high self-sufficiency could be worth the investment, especially if the costs associated with an outage are high. On the other hand, an industrial property whose electricity costs are mainly related to time-of-use would see the fastest return by investing in enough energy storage to achieve peak time self-sufficiency.

What are the local electricity rates?

There are several factors to consider relating to local electricity rates.

• Overall Costs: High electricity rates mean a better return on increasing levels of self-consumption.
• Time-of-use Costs: Significant differences between peak and off-peak usage rates or high demand charges mean increasing both solar and battery storage capacity facilitates load-shifting.
• Feed-in tariffs (FITs): While FITs are not the lucrative prospect they were a decade ago, some power grids still incentivise feeding surplus power to the grid. High FITs support installing additional PV capacity to generate a greater surplus.

What are the energy requirements?

A rule of thumb for solar arrays without battery storage is to aim to consume 30% of the total solar power generated. The capacity that this translates to will vary wildly depending on location-based factors – what direction the PV panels are facing, local cloud coverage, daily hours of sunshine. To store energy, the solar capacity must be increased past this level.

The energy storage component is more complicated. There is no point in having more battery capacity than there is energy to store. The larger the available solar panel array and the greater the difference between peak and off-peak rates (allowing the cheaper rates to top up the battery levels), the better the economics of larger battery systems.

What is the pattern of electricity usage?

The electricity usage pattern will affect the optimum relative proportions of the solar-plus-storage system. There are five typical usage patterns:

Source: SolarChoice

A building with pattern 4, Day Focus, can adequately supply its energy needs from just self-generated solar power without the need for much additional storage, if any. The more energy is consumed outside of daylight hours, the more the building or development will rely upon stored energy.

If you want to know more about this and other topics directly from end users of energy storage technologies join us at one of these energy storage conference: The Energy Storage World Forum (Grid Scale Applications), or The Residential Energy Storage Forum, or one of our Training Courses.

Advantages and Disadvantages of Thermal Energy Storage

Storing heat for later use is not a new prospect, and has not created the same buzz as exciting new battery chemistries. Nevertheless, it offers many advantages to reduce pressure on power grids. For now, its usage for grid-scale applications has not yet taken off – but advances in the field could make this a thing of the past.

Advantages of Thermal Energy Storage

• Proven, safe technology: Whereas battery storage is a relatively recent technology for large-scale applications, ways of storing thermal energy – such as using cheap off-peak electricity to freeze ice for daytime air conditioning – have been around for decades. Making ice and heating water does not run the same risks as using toxic, potentially explosive battery materials.
• Makes use of waste or surplus heat: Many industrial processes produce large amounts of excess heat. Generally, this is a problem – hot water and air needs to be at a certain, cooler temperature, for the process in question. By redirecting this heat for other, useful, purposes, systems can be made vastly more efficient.
• Naturally occurring, cheap materials: In the vast majority of thermal storage applications, the storage medium consists of cheap, abundant materials such as water, rocks or air. Large installations can make use of naturally occurring boreholes and aquifers. Even with newer methods such as molten salt thermal storage, although the insulation facilities are expensive, the heat storage medium is cost-effective.
• Many possible sizes of installation: Stored heat and cold can be used at several different scales. From saving surplus solar heat for a few hours to heat a home through the night, to entire communities keeping warm through the winter from stored summer heat, the technology is eminently scalable. An additional benefit is that stored cold can easily be used for very low loads, which is problematic for industrial chillers.
• Reducing overall and peak electricity demand: Heating and cooling make up a large part of the total energy demands on the power grid. By directly using thermal energy for this – rather than converting electricity for the purpose on demand – pressure on the grid is reduced. This is especially relevant for peak-time air conditioning load, by far the greatest power consumer for most modern buildings.

Disadvantages of Thermal Energy Storage

• Energy losses over time: Stored heat or cold is naturally lost until the medium reaches the temperature of the surrounding material. When storing smaller amounts of thermal energy for a short period of time, this does not have a great effect on the efficiency of the system. The higher the temperature of the medium and the longer the heat must be stored, the greater the losses and the greater the need for expensive insulation materials to maintain system efficiency.
• Low energy density (in most cases): While it is easy to store thermal energy in materials such as water, air or concrete, the overall energy density is low. Molten salt and phase-change materials offer a much higher density solution.
• Geographical limitations: The effectiveness of stored thermal energy depends heavily on the local climate. Storing ice for air-conditioning is useful in sunny Spain, but little use in grey Britain. Unlike batteries, which can be used in similar ways no matter the location, local temperature matters.
• Limited grid-scale applications (for now): While thermal energy in the form of heat and cold are easy to store, transfer and use in the same form, turning heat to and from energy is a more complicated proposition. The majority of thermal storage mediums – not counting the case of molten salt – are not heated to high enough temperatures to produce steam and power turbines, generating electricity.

While thermal energy storage offers great advantages to make use of surplus heat and lower pressure on the grid, in its current form, it has limited suitability for grid-scale applications. However, this lowered electricity demand does increase grid stability overall. Additionally, the success of molten salt thermal storage, as well as research into the field of latent heat thermal storage are likely to make grid applications more viable in the future.

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