Complete System Specification
Energy Balance
Full System Summary
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How to Use the Solar System Sizer
Select grid-tied PV for electricity generation, solar thermal DHW for hot water, solar space heating, or a combi system for both DHW and space heating. Each system type shows the relevant load inputs and produces a different specification output.
Each city loads its peak sun hours, latitude, and cold-water inlet temperature. These drive monthly production profiles and collector sizing for your climate. For cities not listed, pick the closest match by latitude and climate type.
For PV, enter your annual electricity use from your utility bill. For thermal, enter occupants for DHW sizing and your annual heating load for space heating. The target offset or solar fraction sets how large the system will be.
The results panel delivers a full system spec: panel or collector count, array or collector area, storage tank size, backup capacity, annual energy output, annual savings, and simple payback. Use this as a starting point for quotes from licensed installers.
How to Size a Solar System for a Canadian Building
Solar system sizing starts with three inputs that never change regardless of system type: how much energy you need to supply (the annual load), how much solar energy is available at your location (the solar resource), and how efficiently your system converts sunlight into usable energy (the system efficiency). Everything else - panel count, collector area, storage volume, backup capacity - follows from these three numbers.
Grid-tied PV: match the annual load
For a grid-tied PV system, the sizing target is your annual electricity consumption in kWh. The array size in kilowatts equals the annual target divided by peak sun hours times 365 times the performance ratio. In Toronto with 3.9 PSH and a PR of 0.78, each kW of PV produces 3.9 × 365 × 0.78 = 1,110 kWh per year. A 10,000 kWh/year home needs a 9.0 kW array - about 22 to 23 standard 400 W panels. Sizing to 100% of annual consumption and relying on net metering to smooth seasonal imbalances is the most financially efficient approach for most Canadian homeowners. The net metering calculator models the monthly credit flow in detail.
Solar thermal DHW: 1 to 1.5 m² per person
For domestic hot water systems, the practical starting rule is 1.0 to 1.5 m² of flat-plate collector per person to achieve a 55 to 65% annual solar fraction. For evacuated tube collectors, that drops to 0.7 to 1.1 m² per person for the same solar fraction. Storage tank size follows the collector area at 50 to 75 litres per square metre. A 4-person household in Toronto with 4 m² of flat-plate collector and 240 L of storage achieves roughly 60% annual solar fraction, displacing 60% of the backup heater's annual energy. The backup heater remains the same size as before - solar reduces how often it runs, not its rated capacity.
Solar space heating: seasonal mismatch is the challenge
Space heating solar fraction is fundamentally constrained by the seasonal mismatch between peak solar supply (summer) and peak heating demand (winter). Even with a large collector array, annual solar fractions above 40% for space heating alone require large thermal storage to carry summer surplus into autumn. Most Canadian solar space heating systems target 25 to 35% annual solar fraction, which represents the economic optimum where collector cost is balanced against energy savings. Combi systems that serve both DHW and space heating improve the economics significantly because the DHW load provides year-round solar revenue that helps justify the collector capital cost. Use the solar fraction calculator to explore the diminishing-returns curve for your specific load and location.
Storage sizing for thermal systems
Thermal storage bridges the gap between when solar energy is available (daytime) and when it's needed (evenings and nights). The standard sizing rule for Canadian solar DHW systems is 50 to 75 litres per square metre of collector. For a 4 m² flat-plate system, that's 200 to 300 L of storage, typically a single dedicated solar pre-heat tank in series with the existing water heater. Space heating storage is sized differently: the goal is to hold enough hot water to supply heat for 6 to 12 hours at peak load without solar input. For a 10 kW peak heating load, 6 hours of storage at a 20°C temperature swing needs roughly 2,600 L, which is unusually large for residential - this is why whole-season solar storage is mostly commercial or high-performance building territory in Canada.
Backup capacity sizing
The backup heater or boiler must be sized for 100% of peak load, independent of the solar system size, because there will always be cold, cloudy days when solar contribution is zero. Solar reduces runtime and fuel cost but does not reduce the required backup capacity rating. When replacing an existing water heater as part of a solar DHW retrofit, match the backup to the original or current peak load rather than downsizing based on solar contribution.
Frequently Asked Questions
To size any solar system accurately you need three things: your annual energy load in kWh, your location's peak sun hours or daily solar irradiance, and the type of system you want. For a grid-tied PV system you also need your electricity rate, panel wattage, and performance ratio. For a solar thermal system you additionally need the thermal load profile by month and the collector type and efficiency. This sizer collects all of these in one place and produces a complete system specification. Use the solar irradiance calculator to verify your location's PSH if needed.
Grid-tied PV is sized to match annual energy consumption, because surplus is exported to the grid under net metering and deficit is imported. Off-grid PV must be sized to meet peak daily demand during the worst solar month, with enough battery storage to bridge multi-day cloudy periods. Off-grid systems are therefore much larger and more expensive for the same annual energy because they can't rely on the grid as backup. In Canada, the vast majority of residential and commercial solar installations are grid-tied. This calculator focuses on grid-tied PV and solar thermal systems where the grid or a backup heater covers gaps. The solar payback calculator adds incentive modelling to the financial results.
The standard Canadian sizing rule for solar thermal storage is 50 to 75 litres of storage per square metre of collector area for domestic hot water applications. For space heating systems, the storage volume depends on how many hours of heat supply you want to buffer - a common target is 6 to 12 hours of peak load capacity. This calculator applies the 60 L/m² rule for DHW and a load-based approach for space heating. Use the thermal storage calculator for a more detailed tank sizing analysis.
A solar thermal system always needs a backup heater sized for 100% of peak load, because the system must maintain comfort on the coldest, cloudiest days when solar contribution approaches zero. The backup heater is typically an existing gas or electric water heater for DHW, or the existing boiler or heat pump for space heating. It does not need to be upsized compared to the non-solar system - solar reduces how often it runs, not how large it needs to be. The only exception is if you're replacing an undersized backup as part of the solar retrofit.
Dedicated Sizing Tools
Detailed PV array sizing for HVAC loads
Full thermal collector sizing with tilt
DHW system sizing with monthly profile
Storage tank sizing and stratification
Find the economic optimum collector area
Full financial model with incentives