Sizing A Solar Charge Controller, The charge controller Amp (A) rating should be 10 to 20% of the battery Amp/hour (Ah) rating. For example, a 100Ah 12V lead-acid battery will need a 10A to 20A solar charge controller. A 150W to 200W solar panel will be able to generate the 10A* charge current needed for a 100Ah battery to reach the adsorption charge voltage provided it is orientated correctly and not shaded. *Note: Always refer to the battery manufacturer’s specifications. Sizing A Solar Charge Controller
Advanced Guide To Off-Grid Solar Systems
Before selecting an MPPT solar charge controller and purchasing panels, batteries, or inverters, you should understand the basics of sizing an off-grid solar power system. The general steps are as follows:
Estimate the loads - how much energy you use per day in Ah or Wh
Battery capacity - determine the battery size needed in Ah or Wh
Solar size - determine how many solar panel/s you need to charge the battery (W)
Choose the MPPT Solar Charge Controller/s to suit the system (A)
Choose an appropriately sized inverter to suit the load.
- Sizing A Solar Charge Controller
1. Estimate the loads
The first step is to determine what loads or appliances you will be running and for how long? This is calculated by - the power rating of the appliance (W) multiplied by the average runtime (hr). Alternatively, use the average current draw (A) multiplied by the average runtime (hr).
Energy required in Watt hours (Wh) = Power (W) x Time (hrs)
Energy required in Amp-hours (Ah) = Amps (A) x Time (hrs)
Once this is calculated for each appliance or device, then the total energy requirement per day can be determined as shown in the attached load table.
Sizing A Solar Charge Controller
2. Sizing the Battery
The total load in Ah or Wh load is used to size the battery. Lead-acid batteries are sized in Ah while lithium batteries are sized in either Wh or Ah. The allowable daily depth of discharge (DOD) is very different for lead-acid and lithium, see more details about lead-acid Vs lithium batteries. In general, lead-acid batteries should not be discharged below 70% SoC (State of Charge) on a daily basis, while Lithium (LFP) batteries can be discharged down to 20% SoC on a daily basis. Note: Lead-acid (AGM or GEL) batteries can be deeply discharged, but this will severely reduce the life of the battery if done regularly. Sizing A Solar Charge Controller
For example: If you have a 30Ah daily load, you will need a minimum 100Ah lead-acid battery or a 40Ah lithium battery. However, taking into account poor weather, you will generally require at least two days of autonomy - so this equates to a 200Ah lead-acid battery or an 80Ah lithium. Depending on your application, location, and time of year, you may even require 3 or 4 days of autonomy. Sizing A Solar Charge Controller
3. Sizing the Solar
The solar size (W) should be large enough to fully charge the battery on a typical sunny day in your location. There are many variables to consider including panel orientation, time of year & shading issues. This is actually quite complex, but one way to simplify things it to roughly work out how many watts are required to produce 20% of the battery capacity in Amps. Oversizing the solar array is also allowed by some manufacturers to help overcome some of the losses. Note that you can use our free solar design calculator to help estimate the solar generation for different solar panel tilt angles and orientations. Sizing A Solar Charge Controller
Solar sizing Example: Based on the 20% rule, A 12V, 200Ah battery will need up to 40Amps of charge. If we are using a common 250W solar panel, then we can do a basic voltage and current conversion:
Using the equation (P/V = I) then 250W / 12V battery = 20.8A
In this case, to achieve a 40A charge we would need at least 2 x 250W panels. Remember there are several loss factors to take into account so slightly oversizing the solar is a common practice - See more about oversizing solar below. Sizing A Solar Charge Controller
4. Solar Charge controller Sizing (A)
The MPPT solar charge controller size should be roughly matched to the solar size. A simple way to work this out is using the power formula:
Power (W) = Voltage x Current or (P = V*I)
If we know the total solar power in watts (W) and the battery voltage (V), then to work out the maximum current (I) in Amps we re-arrange this to work out the current - so we use the rearranged formula:
Current (A) = Power (W) / Voltage or (I = P/V)
For example: if we have 2 x 200W solar panels and a 12V battery, then the maximum current = 400W/12V = 33Amps. In this example, we could use either a 30A or 35A MPPT solar charge controller.
5. Selecting a battery inverter
Battery inverters are available in a wide range of sizes determined by the inverter’s continuous power rating measured in kW (or kVA). More importantly, inverters are designed to operate with only one battery voltage which is typically 12V, 24V, or 48V. Note that you cannot use a 24V inverter with a lower 12V or higher 48V battery system. Pro-tip, it’s more efficient to use a higher battery voltage.
Besides the battery voltage, the next key criteria for selecting a battery inverter are the average continuous AC load (demand) and short-duration peak loads. Due to temperature de-rating in hot environments, the inverter should be sized slightly higher than the load or power demand of the appliances it will be powering. Whether the loads are inductive or resistive is also very important and must be taken into account. Resistive loads such as electric kettles or toasters are very simple to power, while inductive loads like water pumps and compressors put more stress on the inverter. In regards to peak loads, most battery inverters can handle surge loads up to 2 x the continuous rating.
Inverter sizing example:
Average continuous loads = 120W (fridge) + 40W (lights) + TV (150W) = 310W
High or surge loads = 2200W (electric kettle) + toaster (800W) = 3000W
Considering the above loads, a 2400W inverter (with 4800 peak output) would be adequate for the smaller continuous loads and easily power the short-duration peak loads.
MPPT Solar Oversizing
Due to the various losses in a solar system, it is common practice to oversize the solar array to enable the system to generate more power during bad weather and under different conditions such as high temperatures where power derating can occur. The main loss factors include - poor weather (low irradiation), dust and dirt, shading, poor orientation, and cell temperature de-rating (refer to the power temperature coefficient on the solar panel spec sheet for more details). Learn more about solar panel efficiency and cell temperature de-rating.
These various loss factors listed above can add up to as high as 20%. For example, a 300W solar panel will generally produce 240W to 270W in summer due to the temperature power de-rating, and in winter or due to lower irradiance levels, depending on your location. For these reasons, oversizing the solar array beyond the manufacturers ‘recommended or nominal value’ will help to generate more power. Oversizing by 150% or more is possible on some professional MPPT solar charge controllers. However, not all solar charge controllers are designed to handle the excess power when the solar is operating at full capacity and this can damage some controllers. Therefore, it is important to always check the manufacturer allows oversizing - Morningstar and Victron both allow oversizing beyond the nominal values listed on the datasheets but always double-check the manufacturer’s specifications.
Warning - you must NEVER exceed the maximum INPUT voltage (Voc) or maximum input current rating of the solar charge controller!
More About Solar Sizing
As previously mentioned, all solar charge controllers are limited by the maximum input voltage (V - Volts) and maximum charge current (A – Amps). The maximum voltage determines how many panels can be attached (in series), and the current rating will determine the maximum charge current and in turn what size battery can be charged.
As described in the guide above, the solar array should be able to generate close to the charge current of the controller, which should be sized correctly to match the battery. Another example: a 200Ah 12V battery would require a 20A solar charge controller, and a 250W solar panel to generate close to 20A. (Using the formula P/V = I, then we have 250W / 12V = 20A).
As shown above, a 20A Victron 100/20 MPPT solar charge controller together with a 12V battery can be charged with a 290W ‘nominal’ solar panel. Due to the losses described previously, it could also be used with a larger ‘oversized’ 300W to 330W panel. The same 20A Victron charge controller used with a 48V battery can be installed with a much larger solar array with a nominal size of 1160W.
In comparison to the Victron MPPT charge controller above, the Rover series from Renogy does not allow solar oversizing. The Rover spec sheet states the ‘Max. Solar input power’ as above (not the nominal input power). Oversizing the Rover series will void the warranty. Below is a simple guide to selecting a solar array to match various-size batteries using the Rover series MPPT charge controllers.
20A Solar Charge Controller - 50Ah to 150Ah battery
20A/100V MPPT - 12V battery = 250W Solar (1 x 260W panels)*
20A/100V MPPT - 24V battery = 520W Solar (2 x 260W panels)*
40A Solar Charge Controller - 150Ah to 300Ah battery
40A/100V MPPT - 12V battery = 520W Solar (2 x 260W panels)*
40A/100V MPPT - 24V battery = 1040W Solar (4 x 260W panels)*
* Remember only ‘some manufacturers’ allow the solar array to be oversized, as long as you do not exceed the max voltage or current rating of the charge controller - always refer to manufacturers' specifications and guidelines.